On the right is the largest helix of human DNA, built from people on the beach in Varna (Bulgaria), included in the Guinness Book of Records on April 23, 2016

Deoxyribonucleic acid. General information

DNA (deoxyribonucleic acid) is a kind of blueprint for life, a complex code that contains data on hereditary information. This complex macromolecule is capable of storing and transmitting hereditary genetic information from generation to generation. DNA determines such properties of any living organism as heredity and variability. The information encoded in it sets the entire development program of any living organism. Genetically determined factors predetermine the entire course of life of both a person and any other organism. Artificial or natural influences of the external environment can only slightly affect the overall expression of individual genetic traits or affect the development of programmed processes.

Deoxyribonucleic acid(DNA) is a macromolecule (one of the three main ones, the other two are RNA and proteins) that ensures storage, transmission from generation to generation and implementation of the genetic program for the development and functioning of living organisms. DNA contains structural information various types RNA and proteins.

In eukaryotic cells (animals, plants and fungi), DNA is found in the cell nucleus as part of chromosomes, as well as in some cellular organelles (mitochondria and plastids). In the cells of prokaryotic organisms (bacteria and archaea), a circular or linear DNA molecule, the so-called nucleoid, is attached from the inside to cell membrane. In them and in lower eukaryotes (for example, yeast), small autonomous, predominantly circular DNA molecules called plasmids are also found.

From a chemical point of view, DNA is a long polymer molecule consisting of repeating blocks called nucleotides. Each nucleotide consists of a nitrogenous base, a sugar (deoxyribose) and a phosphate group. The bonds between nucleotides in the chain are formed by deoxyribose ( WITH) and phosphate ( F) groups (phosphodiester bonds).

Rice. 2. A nucleotide consists of a nitrogenous base, a sugar (deoxyribose) and a phosphate group

In the vast majority of cases (except for some viruses containing single-stranded DNA), the DNA macromolecule consists of two chains oriented with nitrogenous bases towards each other. This double-stranded molecule is twisted along a helix.

There are four types of nitrogenous bases found in DNA (adenine, guanine, thymine and cytosine). The nitrogenous bases of one of the chains are connected to the nitrogenous bases of the other chain hydrogen bonds according to the principle of complementarity: adenine combines only with thymine ( A-T), guanine - only with cytosine ( G-C). It is these pairs that make up the “rungs” of the DNA spiral “staircase” (see: Fig. 2, 3 and 4).

Rice. 2. Nitrogenous bases

The sequence of nucleotides allows you to “encode” information about various types of RNA, the most important of which are messenger or template (mRNA), ribosomal (rRNA) and transport (tRNA). All these types of RNA are synthesized on a DNA template by copying a DNA sequence into an RNA sequence synthesized during transcription, and take part in protein biosynthesis (the translation process). In addition to coding sequences, cell DNA contains sequences that perform regulatory and structural functions.

Rice. 3. DNA replication

Location of basic combinations chemical compounds DNA and the quantitative relationships between these combinations provide the coding of hereditary information.

Education new DNA (replication)

1. Replication process: unwinding of the DNA double helix - synthesis of complementary strands by DNA polymerase - formation of two DNA molecules from one.
2. The double helix "unzips" into two branches when enzymes break the bond between the base pairs of chemical compounds.
3. Each branch is an element of new DNA. New base pairs are connected in the same sequence as in the parent branch.

Upon completion of duplication, two independent helices are formed, created from chemical compounds of the parent DNA and having the same genetic code. In this way, DNA is able to pass information from cell to cell.

More details:

STRUCTURE OF NUCLEIC ACIDS

Rice. 4. Nitrogen bases: adenine, guanine, cytosine, thymine

Deoxyribonucleic acid(DNA) refers to nucleic acids. Nucleic acids are a class of irregular biopolymers whose monomers are nucleotides.

NUCLEOTIDES consist of nitrogenous base, connected to a five-carbon carbohydrate (pentose) - deoxyribose(in case of DNA) or ribose(in the case of RNA), which combines with a phosphoric acid residue (H 2 PO 3 -).

Nitrogenous bases There are two types: pyrimidine bases - uracil (only in RNA), cytosine and thymine, purine bases - adenine and guanine.

Rice. 5. Structure of nucleotides (left), location of the nucleotide in DNA (bottom) and types of nitrogenous bases (right): pyrimidine and purine

The carbon atoms in the pentose molecule are numbered from 1 to 5. The phosphate combines with the third and fifth carbon atoms. This is how nucleinotides are combined into a nucleic acid chain. Thus, we can distinguish the 3' and 5' ends of the DNA strand:

Rice. 6. Isolation of the 3' and 5' ends of the DNA chain

Two strands of DNA form double helix. These chains in the spiral are oriented in opposite directions. In different strands of DNA, nitrogenous bases are connected to each other by hydrogen bonds. Adenine always pairs with thymine, and cytosine always pairs with guanine. It's called complementarity rule.

Complementarity rule:

A-T G-C

For example, if we are given a DNA strand with the sequence

3’- ATGTCCTAGCTGCTCG - 5’,

then the second chain will be complementary to it and directed in the opposite direction - from the 5’ end to the 3’ end:

5'- TACAGGATCGACGAGC- 3'.

Rice. 7. Direction of the chains of the DNA molecule and the connection of nitrogenous bases using hydrogen bonds

DNA REPLICATION

DNA replication is the process of doubling a DNA molecule through template synthesis. In most cases of natural DNA replicationprimerfor DNA synthesis is short fragment (recreated). Such a ribonucleotide primer is created by the enzyme primase (DNA primase in prokaryotes, DNA polymerase in eukaryotes), and is subsequently replaced by deoxyribonucleotide polymerase, which normally performs repair functions (correcting chemical damage and breaks in the DNA molecule).

Replication occurs according to a semi-conservative mechanism. This means that the double helix of DNA unwinds and a new chain is built on each of its chains according to the principle of complementarity. The daughter DNA molecule thus contains one strand from the parent molecule and one newly synthesized one. Replication occurs in the direction from the 3' to the 5' end of the mother strand.

Rice. 8. Replication (doubling) of a DNA molecule

DNA synthesis- this is not as complicated a process as it might seem at first glance. If you think about it, first you need to figure out what synthesis is. This is the process of combining something into one whole. The formation of a new DNA molecule occurs in several stages:

1) DNA topoisomerase, located in front of the replication fork, cuts the DNA in order to facilitate its unwinding and unwinding.
2) DNA helicase, following topoisomerase, influences the process of “unbraiding” of the DNA helix.
3) DNA-binding proteins bind DNA strands and also stabilize them, preventing them from sticking to each other.
4) DNA polymerase δ(delta) , coordinated with the speed of movement of the replication fork, carries out synthesisleadingchains subsidiary DNA in the 5"→3" direction on the matrix maternal DNA strands in the direction from its 3" end to the 5" end (speed up to 100 nucleotide pairs per second). These events at this maternal DNA strands are limited.

Rice. 9. Schematic representation of the DNA replication process: (1) Lagging strand (lagging strand), (2) Leading strand (leading strand), (3) DNA polymerase α (Polα), (4) DNA ligase, (5) RNA -primer, (6) Primase, (7) Okazaki fragment, (8) DNA polymerase δ (Polδ), (9) Helicase, (10) Single-stranded DNA-binding proteins, (11) Topoisomerase.

The synthesis of the lagging strand of daughter DNA is described below (see. Scheme replication fork and functions of replication enzymes)

For more information about DNA replication, see

5) Immediately after the other strand of the mother molecule is unraveled and stabilized, it is attached to itDNA polymerase α(alpha)and in the 5"→3" direction it synthesizes a primer (RNA primer) - an RNA sequence on a DNA template with a length of 10 to 200 nucleotides. After this the enzymeremoved from the DNA strand.

Instead of DNA polymerasesα is attached to the 3" end of the primer DNA polymeraseε .

6) DNA polymeraseε (epsilon) seems to continue to extend the primer, but inserts it as a substratedeoxyribonucleotides(in the amount of 150-200 nucleotides). As a result, a single thread is formed from two parts -RNA(i.e. primer) and DNA. DNA polymerase εruns until it encounters the previous primerfragment of Okazaki(synthesized a little earlier). After this, this enzyme is removed from the chain.

7) DNA polymerase β(beta) stands insteadDNA polymerase ε,moves in the same direction (5"→3") and removes the primer ribonucleotides while simultaneously inserting deoxyribonucleotides in their place. The enzyme works until the primer is completely removed, i.e. until a deoxyribonucleotide (an even earlier synthesizedDNA polymerase ε). The enzyme is not able to connect the result of its work with the DNA in front, so it goes off the chain.

As a result, a fragment of daughter DNA “lies” on the matrix of the mother strand. It's calledfragment of Okazaki.

8) DNA ligase crosslinks two adjacent fragments of Okazaki , i.e. 5" end of the segment synthesizedDNA polymerase ε,and 3"-end chain built-inDNA polymeraseβ .

STRUCTURE OF RNA

Ribonucleic acid(RNA) is one of the three main macromolecules (the other two are DNA and proteins) that are found in the cells of all living organisms.

Just like DNA, RNA consists of a long chain in which each link is called nucleotide. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate group. However, unlike DNA, RNA usually has one strand rather than two. The pentose in RNA is ribose, not deoxyribose (ribose has an additional hydroxyl group on the second carbohydrate atom). Finally, DNA differs from RNA in the composition of nitrogenous bases: instead of thymine ( T) RNA contains uracil ( U) , which is also complementary to adenine.

The sequence of nucleotides allows RNA to encode genetic information. All cellular organisms use RNA (mRNA) to program protein synthesis.

Cellular RNA is produced through a process called transcription , that is, the synthesis of RNA on a DNA matrix, carried out by special enzymes - RNA polymerases.

Messenger RNAs (mRNAs) then take part in a process called broadcast, those. protein synthesis on an mRNA matrix with the participation of ribosomes. Other RNAs undergo chemical modifications after transcription, and after the formation of secondary and tertiary structures, they perform functions depending on the type of RNA.

Rice. 10. The difference between DNA and RNA in the nitrogenous base: instead of thymine (T), RNA contains uracil (U), which is also complementary to adenine.

TRANSCRIPTION

This is the process of RNA synthesis on a DNA template. The DNA unwinds at one of the sites. One of the strands contains information that needs to be copied onto an RNA molecule - this strand is called the coding strand. The second strand of DNA, complementary to the coding one, is called the template. During transcription, a complementary RNA chain is synthesized on the template strand in the 3’ - 5’ direction (along the DNA chain). This creates an RNA copy of the coding strand.

Rice. 11. Schematic representation of the transcription

For example, if we are given the sequence of the coding chain

3’- ATGTCCTAGCTGCTCG - 5’,

then, according to the complementarity rule, the matrix chain will carry the sequence

5’- TACAGGATCGACGAGC- 3’,

and the RNA synthesized from it is the sequence

BROADCAST

Let's consider the mechanism protein synthesis on the RNA matrix, as well as the genetic code and its properties. Also, for clarity, at the link below, we recommend watching a short video about the processes of transcription and translation occurring in a living cell:

Rice. 12. Protein synthesis process: DNA codes for RNA, RNA codes for protein

GENETIC CODE

Genetic code- a method of encoding the amino acid sequence of proteins using a sequence of nucleotides. Each amino acid is encoded by a sequence of three nucleotides - a codon or triplet.

Genetic code common to most pro- and eukaryotes. The table shows all 64 codons and the corresponding amino acids. The base order is from the 5" to the 3" end of the mRNA.

Table 1. Standard genetic code

1st warp tion	2nd base								3rd warp tion
	U		C		A		G
U	U U U	(Phe/F)	U C U	(Ser/S)	U A U	(Tyr/Y)	U G U	(Cys/C)	U
	U U C		U C C		U A C		U G C		C
	U U A	(Leu/L)	U C A		U A A	Stop codon**	U G A	Stop codon**	A
	U U G		U C G		U A G	Stop codon**	U G G	(Trp/W)	G
C	C U U		C C U	(Pro/P)	C A U	(His/H)	C G U	(Arg/R)	U
	C U C		C C C		C A C		C G C		C
	C U A		C C A		C A A	(Gln/Q)	C GA		A
	C U G		C C G		C A G		C G G		G
A	A U U	(Ile/I)	A C U	(Thr/T)	A A U	(Asn/N)	A G U	(Ser/S)	U
	A U C		A C C		A A C		A G C		C
	A U A		A C A		A A A	(Lys/K)	A G A		A
	A U G	(Met/M)	A C G		A A G		A G G		G
G	G U U	(Val/V)	G C U	(Ala/A)	G A U	(Asp/D)	G G U	(Gly/G)	U
	G U C		G C C		G A C		G G C		C
	G U A		G C A		G A A	(Glu/E)	G G A		A
	G U G		G C G		G A G		G G G		G

Among the triplets, there are 4 special sequences that serve as “punctuation marks”:

• *Triplet AUG, also encoding methionine, is called start codon. The synthesis of a protein molecule begins with this codon. Thus, during protein synthesis, the first amino acid in the sequence will always be methionine.

• **Triplets UAA, UAG And U.G.A. are called stop codons and do not code for a single amino acid. At these sequences, protein synthesis stops.

Properties of the genetic code

1. Triplety. Each amino acid is encoded by a sequence of three nucleotides - a triplet or codon.

2. Continuity. There are no additional nucleotides between the triplets; the information is read continuously.

3. Non-overlapping. One nucleotide cannot be included in two triplets at the same time.

4. Unambiguity. One codon can code for only one amino acid.

5. Degeneracy. One amino acid can be encoded by several different codons.

6. Versatility. The genetic code is the same for all living organisms.

Example. We are given the sequence of the coding chain:

3’- CCGATTGCACGTCGATCGTATA- 5’.

The matrix chain will have the sequence:

5’- GGCTAACGTGCAGCTAGCATAT- 3’.

Now we “synthesize” information RNA from this chain:

3’- CCGAUUGCACGUCGAUCGUAUA- 5’.

Protein synthesis proceeds in the direction 5’ → 3’, therefore, we need to reverse the sequence to “read” the genetic code:

5’- AUAUGCUAGCUGCACGUUAGCC- 3’.

Now let's find the start codon AUG:

5’- AU AUG CUAGCUGCACGUUAGCC- 3’.

Let's divide the sequence into triplets:

sounds like this: information is transferred from DNA to RNA (transcription), from RNA to protein (translation). DNA can also be duplicated by replication, and the process of reverse transcription is also possible, when DNA is synthesized from an RNA template, but this process is mainly characteristic of viruses.

Rice. 13. Central Dogma of Molecular Biology

GENOME: GENES and CHROMOSOMES

(general concepts)

Genome - the totality of all the genes of an organism; its complete chromosome set.

The term “genome” was proposed by G. Winkler in 1920 to describe the set of genes contained in the haploid set of chromosomes of organisms of the same biological species. The original meaning of this term indicated that the concept of genome, in contrast to genotype, is genetic characteristics the species as a whole, rather than an individual. With the development of molecular genetics, the meaning of this term has changed. It is known that DNA, which is the carrier of genetic information in most organisms and, therefore, forms the basis of the genome, includes not only genes in the modern sense of the word. Most of the DNA of eukaryotic cells is represented by non-coding (“redundant”) nucleotide sequences that do not contain information about proteins and nucleic acids Oh. Thus, the main part of the genome of any organism is the entire DNA of its haploid set of chromosomes.

Genes are sections of DNA molecules that encode polypeptides and RNA molecules

Over the last century, our understanding of genes has changed significantly. Previously, a genome was a region of a chromosome that encodes or defines one characteristic or phenotypic(visible) property, such as eye color.

In 1940, George Beadle and Edward Tatham proposed a molecular definition of the gene. Scientists processed fungal spores Neurospora crassa X-rays and other agents that cause changes in the DNA sequence ( mutations), and discovered mutant strains of the fungus that had lost some specific enzymes, which in some cases led to disruption of the entire metabolic pathway. Beadle and Tatem came to the conclusion that a gene is a region genetic material, which defines or codes for a single enzyme. This is how the hypothesis appeared "one gene - one enzyme". This concept was later expanded to define "one gene - one polypeptide", since many genes encode proteins that are not enzymes, and the polypeptide may be a subunit of a complex protein complex.

In Fig. Figure 14 shows a diagram of how triplets of nucleotides in DNA determine a polypeptide, the amino acid sequence of a protein, through the mediation of mRNA. One of the DNA chains plays the role of a template for the synthesis of mRNA, the nucleotide triplets (codons) of which are complementary to the DNA triplets. In some bacteria and many eukaryotes, coding sequences are interrupted by non-coding regions (called introns).

Modern biochemical determination of the gene even more specific. Genes are all sections of DNA that encode the primary sequence of end products, which include polypeptides or RNA that have a structural or catalytic function.

Along with genes, DNA also contains other sequences that perform exclusively a regulatory function. Regulatory sequences may mark the beginning or end of genes, influence transcription, or indicate the site of initiation of replication or recombination. Some genes can be expressed in different ways, with the same DNA region serving as a template for the formation of different products.

We can roughly calculate minimum gene size, encoding the middle protein. Each amino acid in a polypeptide chain is encoded by a sequence of three nucleotides; the sequences of these triplets (codons) correspond to the chain of amino acids in the polypeptide that is encoded by this gene. A polypeptide chain of 350 amino acid residues (medium length chain) corresponds to a sequence of 1050 bp. ( base pairs). However, many eukaryotic genes and some prokaryotic genes are interrupted by DNA segments that do not carry protein information, and therefore turn out to be much longer than a simple calculation shows.

How many genes are on one chromosome?

Rice. 15. View of chromosomes in prokaryotic (left) and eukaryotic cells. Histones are a large class of nuclear proteins that perform two main functions: they participate in the packaging of DNA strands in the nucleus and in the epigenetic regulation of nuclear processes such as transcription, replication and repair.

As is known, bacterial cells have a chromosome in the form of a DNA strand arranged in a compact structure - a nucleoid. Prokaryotic chromosome Escherichia coli, whose genome has been completely deciphered, is a circular DNA molecule (in fact, it is not a perfect circle, but rather a loop without a beginning or end), consisting of 4,639,675 bp. This sequence contains approximately 4,300 protein genes and another 157 genes for stable RNA molecules. IN human genome approximately 3.1 billion base pairs corresponding to nearly 29,000 genes located on 24 different chromosomes.

Prokaryotes (Bacteria).

Bacterium E. coli has one double-stranded circular DNA molecule. It consists of 4,639,675 bp. and reaches a length of approximately 1.7 mm, which exceeds the length of the cell itself E. coli approximately 850 times. In addition to the large circular chromosome as part of the nucleoid, many bacteria contain one or several small circular DNA molecules that are freely located in the cytosol. These extrachromosomal elements are called plasmids(Fig. 16).

Most plasmids consist of only a few thousand base pairs, some contain more than 10,000 bp. They carry genetic information and replicate to form daughter plasmids, which enter the daughter cells during the division of the parent cell. Plasmids are found not only in bacteria, but also in yeast and other fungi. In many cases, plasmids provide no benefit to the host cells and their sole purpose is to reproduce independently. However, some plasmids carry genes beneficial to the host. For example, genes contained in plasmids can make bacterial cells resistant to antibacterial agents. Plasmids carrying the β-lactamase gene provide resistance to β-lactam antibiotics such as penicillin and amoxicillin. Plasmids can pass from cells that are resistant to antibiotics to other cells of the same or a different species of bacteria, causing those cells to also become resistant. Intensive use of antibiotics is a powerful selective factor contributing to the spread of plasmids encoding antibiotic resistance (as well as transposons that encode similar genes) among pathogenic bacteria, and leads to the emergence of bacterial strains resistant to several antibiotics. Doctors are beginning to understand the dangers of widespread use of antibiotics and prescribe them only in cases of urgent need. For similar reasons, the widespread use of antibiotics to treat farm animals is limited.

See also: Ravin N.V., Shestakov S.V. Genome of prokaryotes // Vavilov Journal of Genetics and Breeding, 2013. T. 17. No. 4/2. pp. 972-984.

Eukaryotes.

Table 2. DNA, genes and chromosomes of some organisms

	Shared DNA p.n.	Number of chromosomes*	Approximate number of genes
Escherichia coli(bacterium)	4 639 675		4 435
Saccharomyces cerevisiae(yeast)	12 080 000	16**	5 860
Caenorhabditis elegans(nematode)	90 269 800	12***	23 000
Arabidopsis thaliana(plant)	119 186 200		33 000
Drosophila melanogaster(fruit fly)	120 367 260		20 000
Oryza sativa(rice)	480 000 000		57 000
Mus musculus(mouse)	2 634 266 500		27 000
Homo sapiens(Human)	3 070 128 600		29 000

Note. Information is constantly updated; For more up-to-date information, refer to individual genomics project websites

* For all eukaryotes, except yeast, the diploid set of chromosomes is given. Diploid kit chromosomes (from the Greek diploos - double and eidos - species) - a double set of chromosomes (2n), each of which has a homologous one.
**Haploid set. Wild yeast strains typically have eight (octaploid) or more sets of these chromosomes.
***For females with two X chromosomes. Males have an X chromosome, but no Y, i.e. only 11 chromosomes.

Yeast, one of the smallest eukaryotes, has 2.6 times more DNA than E. coli(Table 2). Fruit fly cells Drosophila, a classic subject of genetic research, contain 35 times more DNA, and human cells contain approximately 700 times more DNA than E. coli. Many plants and amphibians contain even more DNA. The genetic material of eukaryotic cells is organized in the form of chromosomes. Diploid set of chromosomes (2 n) depends on the type of organism (Table 2).

For example, in a human somatic cell there are 46 chromosomes ( rice. 17). Each chromosome of a eukaryotic cell, as shown in Fig. 17, A, contains one very large double-stranded DNA molecule. Twenty-four human chromosomes (22 paired chromosomes and two sex chromosomes X and Y) vary in length by more than 25 times. Each eukaryotic chromosome contains a specific set of genes.

Rice. 17. Chromosomes of eukaryotes.A- a pair of linked and condensed sister chromatids from the human chromosome. In this form, eukaryotic chromosomes remain after replication and in metaphase during mitosis. b- a complete set of chromosomes from a leukocyte of one of the authors of the book. Each normal human somatic cell contains 46 chromosomes.

If you connect the DNA molecules of the human genome (22 chromosomes and chromosomes X and Y or X and X), you get a sequence about one meter long. Note: In all mammals and other heterogametic male organisms, females have two X chromosomes (XX) and males have one X chromosome and one Y chromosome (XY).

Most human cells, so the total DNA length of such cells is about 2m. An adult human has approximately 10 14 cells, so the total length of all DNA molecules is 2・10 11 km. For comparison, the circumference of the Earth is 4・10 4 km, and the distance from the Earth to the Sun is 1.5・10 8 km. This is how amazingly compact DNA is packed in our cells!

In eukaryotic cells there are other organelles containing DNA - mitochondria and chloroplasts. Many hypotheses have been put forward regarding the origin of mitochondrial and chloroplast DNA. The generally accepted point of view today is that they represent the rudiments of the chromosomes of ancient bacteria, which penetrated the cytoplasm of the host cells and became the precursors of these organelles. Mitochondrial DNA encodes mitochondrial tRNAs and rRNAs, as well as several mitochondrial proteins. More than 95% of mitochondrial proteins are encoded by nuclear DNA.

STRUCTURE OF GENES

Let's consider the structure of the gene in prokaryotes and eukaryotes, their similarities and differences. Despite the fact that a gene is a section of DNA that encodes only one protein or RNA, in addition to the immediate coding part, it also includes regulatory and other structural elements that have different structures in prokaryotes and eukaryotes.

Coding sequence- the main structural and functional unit of a gene, it is in it that triplets of nucleotides encoding are locatedamino acid sequence. It begins with a start codon and ends with a stop codon.

Before and after the coding sequence there are untranslated 5' and 3' sequences. They perform regulatory and auxiliary functions, for example, ensuring the landing of the ribosome on mRNA.

Untranslated and coding sequences make up the transcription unit - the transcribed section of DNA, that is, the section of DNA from which mRNA synthesis occurs.

Terminator- a non-transcribed section of DNA at the end of a gene where RNA synthesis stops.

At the beginning of the gene is regulatory region, which includes promoter And operator.

Promoter- the sequence to which the polymerase binds during transcription initiation. Operator- this is an area that special proteins can bind to - repressors, which can reduce the activity of RNA synthesis from this gene - in other words, reduce it expression.

Gene structure in prokaryotes

The general plan of gene structure in prokaryotes and eukaryotes is no different - both contain a regulatory region with a promoter and operator, a transcription unit with coding and untranslated sequences, and a terminator. However, the organization of genes in prokaryotes and eukaryotes is different.

Rice. 18. Scheme of gene structure in prokaryotes (bacteria) -the image is enlarged

At the beginning and end of the operon there are common regulatory regions for several structural genes. From the transcribed region of the operon, one mRNA molecule is read, which contains several coding sequences, each of which has its own start and stop codon. From each of these areas withone protein is synthesized. Thus, Several protein molecules are synthesized from one mRNA molecule.

Prokaryotes are characterized by the combination of several genes into a single functional unit - operon. The operation of the operon can be regulated by other genes, which can be noticeably distant from the operon itself - regulators. The protein translated from this gene is called repressor. It binds to the operator of the operon, regulating the expression of all genes contained in it at once.

Prokaryotes are also characterized by the phenomenon Transcription-translation interfaces.

Rice. 19 The phenomenon of coupling of transcription and translation in prokaryotes - the image is enlarged

Such coupling does not occur in eukaryotes due to the presence of a nuclear envelope that separates the cytoplasm, where translation occurs, from the genetic material on which transcription occurs. In prokaryotes, during RNA synthesis on a DNA template, a ribosome can immediately bind to the synthesized RNA molecule. Thus, translation begins even before transcription is completed. Moreover, several ribosomes can simultaneously bind to one RNA molecule, synthesizing several molecules of one protein at once.

Gene structure in eukaryotes

The genes and chromosomes of eukaryotes are very complexly organized

Many species of bacteria have only one chromosome, and in almost all cases there is one copy of each gene on each chromosome. Only a few genes, such as rRNA genes, are found in multiple copies. Genes and regulatory sequences make up almost the entire prokaryotic genome. Moreover, almost every gene strictly corresponds to the amino acid sequence (or RNA sequence) it encodes (Fig. 14).

The structural and functional organization of eukaryotic genes is much more complex. The study of eukaryotic chromosomes, and later the sequencing of complete eukaryotic genome sequences, brought many surprises. Many, if not most, eukaryotic genes have interesting feature: their nucleotide sequences contain one or more DNA sections that do not encode the amino acid sequence of the polypeptide product. Such untranslated insertions disrupt the direct correspondence between the nucleotide sequence of the gene and the amino acid sequence of the encoded polypeptide. These untranslated segments within genes are called introns, or built-in sequences, and the coding segments are exons. In prokaryotes, only a few genes contain introns.

So, in eukaryotes, the combination of genes into operons practically does not occur, and the coding sequence of a eukaryotic gene is most often divided into translated regions - exons, and untranslated sections - introns.

In most cases, the function of introns is not established. In general, only about 1.5% of human DNA is “coding,” that is, it carries information about proteins or RNA. However, taking into account large introns, it turns out that human DNA is 30% genes. Because genes make up a relatively small proportion of the human genome, a significant portion of DNA remains unaccounted for.

Rice. 16. Scheme of gene structure in eukaryotes - the image is enlarged

From each gene, immature or pre-RNA is first synthesized, which contains both introns and exons.

After this, the splicing process takes place, as a result of which the intronic regions are excised, and a mature mRNA is formed, from which protein can be synthesized.

Rice. 20. Alternative splicing process - the image is enlarged

This organization of genes allows, for example, when different forms of a protein can be synthesized from one gene, due to the fact that during the splicing process, exons can be stitched together in different sequences.

Rice. 21. Differences in the structure of genes of prokaryotes and eukaryotes - the image is enlarged

MUTATIONS AND MUTAGENESIS

Mutation is called a persistent change in the genotype, that is, a change in the nucleotide sequence.

The process that leads to mutations is called mutagenesis, and the body All whose cells carry the same mutation - mutant.

Mutation theory was first formulated by Hugo de Vries in 1903. Its modern version includes the following provisions:

1. Mutations occur suddenly, spasmodically.

2. Mutations are passed on from generation to generation.

3. Mutations can be beneficial, harmful or neutral, dominant or recessive.

4. The probability of detecting mutations depends on the number of individuals studied.

5. Similar mutations can occur repeatedly.

6. Mutations are not directed.

Mutations can occur under the influence of various factors. There are mutations that arise under the influence of mutagenic impacts: physical (for example, ultraviolet or radiation), chemical (for example, colchicine or reactive oxygen species) and biological (for example, viruses). Mutations can also be caused replication errors.

Depending on the conditions under which mutations appear, mutations are divided into spontaneous- that is, mutations that arose in normal conditions, And induced- that is, mutations that arose under special conditions.

Mutations can occur not only in nuclear DNA, but also, for example, in the DNA of mitochondria or plastids. Accordingly, we can distinguish nuclear And cytoplasmic mutations.

As a result of mutations, new alleles can often appear. If a mutant allele suppresses the action of a normal one, the mutation is called dominant. If a normal allele suppresses a mutant one, this mutation is called recessive. Most mutations that lead to the emergence of new alleles are recessive.

Mutations are distinguished by effect adaptive leading to increased adaptability of the organism to the environment, neutral that do not affect survival, harmful, reducing the adaptability of organisms to environmental conditions and lethal, leading to the death of the organism in the early stages of development.

According to the consequences, mutations leading to loss of protein function, mutations leading to emergence the squirrel new feature , as well as mutations that change gene dosage, and, accordingly, the dose of protein synthesized from it.

A mutation can occur in any cell of the body. If a mutation occurs in a germ cell, it is called germinal(germinal or generative). Such mutations do not appear in the organism in which they appeared, but lead to the appearance of mutants in the offspring and are inherited, so they are important for genetics and evolution. If a mutation occurs in any other cell, it is called somatic. Such a mutation can manifest itself to one degree or another in the organism in which it arose, for example, leading to the formation of cancerous tumors. However, such a mutation is not inherited and does not affect descendants.

Mutations can affect regions of the genome of different sizes. Highlight genetic, chromosomal And genomic mutations.

Gene mutations

Mutations that occur on a scale smaller than one gene are called genetic, or point (point). Such mutations lead to changes in one or several nucleotides in the sequence. Among gene mutations there arereplacements, leading to the replacement of one nucleotide with another,deletions, leading to the loss of one of the nucleotides,insertions, leading to the addition of an extra nucleotide to the sequence.

Rice. 23. Gene (point) mutations

According to the mechanism of action on the protein, gene mutations are divided into:synonymous, which (as a result of the degeneracy of the genetic code) do not lead to a change in the amino acid composition of the protein product,missense mutations, which lead to the replacement of one amino acid with another and can affect the structure of the synthesized protein, although they are often insignificant,nonsense mutations, leading to the replacement of the coding codon with a stop codon,mutations leading to splicing disorder:

Rice. 24. Mutation patterns

Also, according to the mechanism of action on the protein, mutations are distinguished that lead to frame shift reading, such as insertions and deletions. Such mutations, like nonsense mutations, although they occur at one point in the gene, often affect the entire structure of the protein, which can lead to a complete change in its structure.

Rice. 29. Chromosome before and after duplication

Genomic mutations

Finally, genomic mutations affect the entire genome, that is, the number of chromosomes changes. There are polyploidies - an increase in the ploidy of the cell, and aneuploidies, that is, a change in the number of chromosomes, for example, trisomy (the presence of an additional homologue on one of the chromosomes) and monosomy (the absence of a homolog on a chromosome).

Video on DNA

DNA REPLICATION, RNA CODING, PROTEIN SYNTHESIS

The functions of RNA vary depending on the type of ribonucleic acid.

1) Messenger RNA (i-RNA).

2) Ribosomal RNA (r-RNA).

3) Transfer RNA (tRNA).

4) Minor (small) RNAs. These are RNA molecules, most often with a small molecular weight, located in various parts of the cell (membrane, cytoplasm, organelles, nucleus, etc.). Their role is not fully understood. It has been proven that they can help the maturation of ribosomal RNA, participate in the transfer of proteins across the cell membrane, promote the reduplication of DNA molecules, etc.

5) Ribozymes. A recently identified type of RNA that takes an active part in cellular enzymatic processes as an enzyme (catalyst).

6) Viral RNA. Any virus can contain only one type of nucleic acid: either DNA or RNA. Accordingly, viruses containing an RNA molecule are called RNA-containing viruses. When a virus of this type enters a cell, the process of reverse transcription (the formation of new DNA based on RNA) can occur, and the newly formed DNA of the virus is integrated into the genome of the cell and ensures the existence and reproduction of the pathogen. The second scenario is the formation of complementary RNA on the matrix of the incoming viral RNA. In this case, the formation of new viral proteins, the vital activity and reproduction of the virus occurs without the participation of deoxyribonucleic acid only on the basis of the genetic information recorded on the viral RNA. Ribonucleic acids. RNA, structure, structures, types, role. Genetic code. Mechanisms of transmission of genetic information. Replication. Transcription

Ribosomal RNA.

rRNA accounts for 90% of the total RNA in a cell and is characterized by metabolic stability. In prokaryotes, there are three different types of rRNA with sedimentation coefficients of 23S, 16S and 5S; Eukaryotes have four types: -28S, 18S,5S and 5,8S.

RNAs of this type are localized in ribosomes and participate in specific interactions with ribosomal proteins.

Ribosomal RNAs have the form of a secondary structure in the form of double-stranded regions connected by a curved single strand. Ribosomal proteins are associated predominantly with single-stranded regions of the molecule.

rRNA is characterized by the presence of modified bases, but in significantly smaller quantities than in tRNA. rRNA contains mainly methylated nucleotides, with methyl groups attached either to the base or to the 2/-OH group of ribose.

Transfer RNA.

tRNA molecules are a single chain consisting of 70-90 nucleotides, with a molecular weight of 23000-28000 and a sedimentation constant of 4S. In cellular RNA, transfer RNA makes up 10-20%. tRNA molecules have the ability to covalently bind to a specific amino acid and connect through a system of hydrogen bonds with one of the nucleotide triplets of the mRNA molecule. Thus, tRNAs implement a code correspondence between an amino acid and the corresponding mRNA codon. To perform the adapter function, tRNAs must have a well-defined secondary and tertiary structure.

Each tRNA molecule has a constant secondary structure, has the shape of a two-dimensional cloverleaf and consists of helical regions formed by nucleotides of the same chain, and single-stranded loops located between them. The number of helical regions reaches half of the molecule. Unpaired sequences form characteristic structural elements (branches) that have typical branches:

A) acceptor stem, at the 3/-OH end of which in most cases there is a CCA triplet. The corresponding amino acid is added to the carboxyl group of the terminal adenosine using a specific enzyme;

B) pseudouridine or T C-loop, consists of seven nucleotides with the obligatory sequence 5 / -T CG-3 /, which contains pseudouridine; it is assumed that the T C loop is used to bind tRNA to the ribosome;

B) an additional loop - different in size and composition in different tRNAs;

D) the anticodon loop consists of seven nucleotides and contains a group of three bases (anticodon), which is complementary to the triplet (codon) in the mRNA molecule;

D) dihydrouridyl loop (D-loop), consisting of 8-12 nucleotides and containing from one to four dihydrouridyl residues; the D-loop is believed to be used to bind tRNA to a specific enzyme (aminoacyl-tRNA synthetase).

The tertiary packing of tRNA molecules is very compact and L-shaped. The corner of such a structure is formed by a dihydrouridine residue and a T C-loop, the long leg forms an acceptor stem and a T C-loop, and the short leg forms a D-loop and an anticodon loop.

Polyvalent cations (Mg 2+ , polyamines), as well as hydrogen bonds between the bases and the phosphodiester backbone, participate in the stabilization of the tertiary structure of tRNA.

The complex spatial arrangement of the tRNA molecule is due to multiple highly specific interactions with both proteins and other nucleic acids (rRNA).

Transfer RNA differs from other types of RNA in its high content of minor bases - on average 10-12 bases per molecule, but the total number of them and tRNA increases as organisms move up the evolutionary ladder. In tRNA, various methylated purine (adenine, guanine) and pyrimidine (5-methylcytosine and ribosylthymine) bases, sulfur-containing bases (6-thiouracil) were identified, but the most common (6-thiouracil), but the most common minor component is pseudouridine. The role of unusual nucleotides in tRNA molecules is not yet clear, but it is believed that the lower the level of mitigation of tRNA, the less active and specific it is.

The localization of modified nucleotides is strictly fixed. The presence of minor bases in tRNA makes the molecules resistant to the action of nucleases and, in addition, they are involved in maintaining a certain structure, since such bases are not capable of normal pairing and prevent the formation of a double helix. Thus, the presence of modified bases in tRNA determines not only its structure, but also many special functions of the tRNA molecule.

Most eukaryotic cells contain a set of different tRNAs. For each amino acid there is at least one specific tRNA. tRNAs that bind the same amino acid are called isoacceptor. Each type of cell in the body differs in its ratio of isoacceptor tRNAs.

Matrix (information)

Messenger RNA contains genetic information about the amino acid sequence for essential enzymes and other proteins, i.e. serves as a template for the biosynthesis of polypeptide chains. The share of mRNA in the cell accounts for 5% of the total amount of RNA. Unlike rRNA and tRNA, mRNA is heterogeneous in size, its molecular weight ranges from 25 10 3 to 1 10 6; mRNA is characterized by a wide range of sedimentation constants (6-25S). The presence of mRNA chains of variable length in a cell reflects the diversity of molecular weights of the proteins whose synthesis they provide.

In its nucleotide composition, mRNA corresponds to DNA from the same cell, i.e. is complementary to one of the DNA strands. The nucleotide sequence (primary structure) of mRNA contains information not only about the structure of the protein, but also about the secondary structure of the mRNA molecules themselves. The secondary structure of mRNA is formed due to mutually complementary sequences, the content of which is similar in RNA of different origins and ranges from 40 to 50%. A significant number of paired regions can be formed in the 3/ and 5/ regions of the mRNA.

Analysis of the 5/-ends of the 18s rRNA regions showed that they contain mutually complementary sequences.

The tertiary structure of mRNA is formed mainly due to hydrogen bonds, hydrophobic interactions, geometric and steric restrictions, and electrical forces.

Messenger RNA is a metabolically active and relatively unstable, short-lived form. Thus, the mRNA of microorganisms is characterized by rapid renewal, and its lifespan is several minutes. However, for organisms whose cells contain true membrane-bound nuclei, the lifespan of mRNA can reach many hours and even several days.

The stability of mRNA can be determined various kinds modifications of its molecule. Thus, it was discovered that the 5/-terminal sequence of mRNA of viruses and eukaryotes is methylated, or “blocked.” The first nucleotide in the 5/-terminal cap structure is 7-methylguanine, which is linked to the next nucleotide by a 5/-5/-pyrophosphate bond. The second nucleotide is methylated at the C-2/-ribose residue, and the third nucleotide may not have a methyl group.

Another ability of mRNA is that at the 3/-ends of many mRNA molecules of eukaryotic cells there are relatively long sequences of adenyl nucleotides, which are attached to the mRNA molecules with the help of special enzymes after completion of synthesis. The reaction takes place in the cell nucleus and cytoplasm.

At the 3/- and 5/- ends of the mRNA, the modified sequences account for about 25% of the total length of the molecule. It is believed that 5/-caps and 3/-poly-A sequences are necessary either to stabilize the mRNA, protecting it from the action of nucleases, or to regulate the translation process.

RNA interference

Several types of RNA have been found in living cells that can reduce the degree of gene expression when complementary to the mRNA or the gene itself. MicroRNAs (21-22 nucleotides in length) are found in eukaryotes and exert their effects through the mechanism of RNA interference. In this case, a complex of microRNA and enzymes can lead to methylation of nucleotides in the DNA of the gene promoter, which serves as a signal to reduce gene activity. When using another type of regulation, the mRNA complementary to the microRNA is degraded. However, there are also miRNAs that increase rather than decrease gene expression. Small interfering RNAs (siRNAs, 20–25 nucleotides) are often produced by the cleavage of viral RNAs, but endogenous cellular siRNAs also exist. Small interfering RNAs also act through RNA interference by mechanisms similar to microRNAs. In animals, so-called Piwi-interacting RNA (piRNA, 29-30 nucleotides) has been found, acting in germ cells against transposition and playing a role in the formation of gametes. In addition, piRNAs can be epigenetically inherited on the maternal line, passing on their ability to inhibit transposon expression to offspring.

Antisense RNAs are widespread in bacteria, many of them suppress gene expression, but some activate expression. Antisense RNAs act by attaching to mRNA, which leads to the formation of double-stranded RNA molecules, which are degraded by enzymes. High molecular weight, mRNA-like RNA molecules have been found in eukaryotes. These molecules also regulate gene expression.

In addition to the role of individual molecules in gene regulation, regulatory elements can be formed in the 5" and 3" untranslated regions of mRNA. These elements can act independently to prevent translation initiation, or they can bind proteins such as ferritin or small molecules such as biotin.

Many RNAs are involved in modifying other RNAs. Introns are excised from pre-mRNA by spliceosomes, which, in addition to proteins, contain several small nuclear RNAs (snRNAs). In addition, introns can catalyze their own excision. The RNA synthesized as a result of transcription can also be chemically modified. In eukaryotes, chemical modifications of RNA nucleotides, for example, their methylation, are performed by small nuclear RNAs (snRNAs, 60-300 nucleotides). This type of RNA is localized in the nucleolus and Cajal bodies. After association of snRNA with enzymes, snRNAs bind to the target RNA by forming base pairs between the two molecules, and the enzymes modify the nucleotides of the target RNA. Ribosomal and transfer RNAs contain many such modifications, the specific position of which is often conserved during evolution. SnRNAs and snRNAs themselves can also be modified. Guide RNAs carry out the process of RNA editing in the kinetoplast, a special region of the mitochondria of kinetoplastid protists (for example, trypanosomes).

Genomes made of RNA

Like DNA, RNA can store information about biological processes. RNA can be used as the genome of viruses and virus-like particles. RNA genomes can be divided into those that do not have a DNA intermediate step and those that are copied into a DNA copy and back into RNA (retroviruses) to reproduce.

Many viruses, such as the influenza virus, contain a genome consisting entirely of RNA at all stages. RNA is contained within a typically protein shell and is replicated using RNA-dependent RNA polymerases encoded within it. Viral genomes consisting of RNA are divided into:

“minus strand RNA”, which serves only as a genome, and a molecule complementary to it is used as mRNA;

double-stranded viruses.

Viroids are another group of pathogens that contain an RNA genome and no protein. They are replicated by RNA polymerases of the host organism.

Retroviruses and retrotransposons

Other viruses have an RNA genome during only one phase of their life cycle. The virions of so-called retroviruses contain RNA molecules, which, when they enter the host cells, serve as a template for the synthesis of a DNA copy. In turn, the DNA template is read by the RNA gene. In addition to viruses, reverse transcription is also used in a class of mobile genome elements - retrotransposons.

The times in which we live are marked by amazing changes, enormous progress, when people receive answers to more and more new questions. Life is rapidly moving forward, and what just recently seemed impossible is beginning to come true. It is quite possible that what today appears to be a plot from the fantasy genre will soon also acquire features of reality.

One of most important discoveries in the second half of the twentieth century, nucleic acids RNA and DNA became available, thanks to which man came closer to unraveling the secrets of nature.

Nucleic acids

Nucleic acids are organic compounds with high molecular weight properties. They contain hydrogen, carbon, nitrogen and phosphorus.

They were discovered in 1869 by F. Miescher, who examined pus. However, then their discovery was not given much importance. Only later, when these acids were discovered in all animal and plant cells, did their enormous role become understood.

There are two types of nucleic acids: RNA and DNA (ribonucleic and deoxyribonucleic acids). This article is devoted to ribonucleic acid, but for a general understanding we will also consider what DNA is.

What's happened

DNA is made up of two strands that are connected according to the law of complementarity by hydrogen bonds of nitrogenous bases. The long chains are twisted into a spiral; one turn contains almost ten nucleotides. The diameter of the double helix is ​​two millimeters, the distance between nucleotides is about half a nanometer. The length of one molecule sometimes reaches several centimeters. The length of the DNA in the nucleus of a human cell is almost two meters.

The structure of DNA contains all DNA has replication, which means the process during which two completely identical daughter molecules are formed from one molecule.

As already noted, the chain is made up of nucleotides, which in turn consist of nitrogenous bases (adenine, guanine, thymine and cytosine) and a phosphorus acid residue. All nucleotides differ in their nitrogenous bases. Hydrogen bonding does not occur between all bases; adenine, for example, can only bond with thymine or guanine. Thus, there are as many adenyl nucleotides in the body as thymidyl nucleotides, and the number of guanyl nucleotides is equal to cytidyl nucleotides (Chargaff’s rule). It turns out that the sequence of one chain predetermines the sequence of another, and the chains seem to mirror each other. This pattern, where the nucleotides of two chains are arranged in an orderly manner and are also combined selectively, is called the principle of complementarity. In addition to hydrogen bonds, the double helix also interacts hydrophobically.

The two chains are multidirectional, that is, they are located in opposite directions. Therefore, opposite the three" end of one is the five" end of the other chain.

Outwardly, it resembles a spiral staircase, the railing of which is a sugar-phosphate frame, and the steps are complementary nitrogen bases.

What is ribonucleic acid?

RNA is a nucleic acid with monomers called ribonucleotides.

By chemical properties it is very similar to DNA in that both are polymers of nucleotides that are a phospholated N-glycoside that is built on a pentose (five-carbon sugar) residue, with a phosphate group at the fifth carbon and a nitrogen base at the first carbon.

It is a single polynucleotide chain (except for viruses), which is much shorter than DNA.

One RNA monomer is the remains of the following substances:

• nitrogen bases;
• five-carbon monosaccharide;
• phosphorus acids.

RNA has pyrimidine (uracil and cytosine) and purine (adenine, guanine) bases. Ribose is a monosaccharide nucleotide of RNA.

Differences between RNA and DNA

Nucleic acids differ from each other in the following properties:

• its quantity in a cell depends on the physiological state, age and organ affiliation;
• DNA contains the carbohydrate deoxyribose, and RNA contains ribose;
• the nitrogenous base in DNA is thymine, and in RNA it is uracil;
• classes perform different functions, but are synthesized on a DNA template;
• DNA consists of a double helix, while RNA consists of a single strand;
• it is not typical for it to act on DNA;
• RNA has more minor bases;
• the chains vary significantly in length.

History of the study

Cell RNA was first discovered by German biochemist R. Altmann while studying yeast cells. In the mid-twentieth century, the role of DNA in genetics was proven. Only then were the types of RNA, functions, and so on described. Up to 80-90% of the mass in the cell is r-RNA, which together with proteins forms a ribosome and participates in protein biosynthesis.

In the sixties of the last century, it was first suggested that there must be a certain species that carries the genetic information for protein synthesis. After this, it was scientifically established that there are such information ribonucleic acids that represent complementary copies of genes. They are also called messenger RNAs.

So-called transport acids are involved in decoding the information recorded in them.

Later, methods began to be developed to identify the nucleotide sequence and establish the structure of RNA in the acid space. Thus, it was discovered that some of them, called ribozymes, can cleave polyribonucleotide chains. As a result, it began to be assumed that at the time when life arose on the planet, RNA acted without DNA and proteins. Moreover, all transformations were carried out with her participation.

The structure of the ribonucleic acid molecule

Almost all RNA is a single chain of polynucleotides, which, in turn, consist of monoribonucleotides - purine and pyrimidine bases.

Nucleotides are designated by the initial letters of the bases:

• adenine (A), A;
• guanine (G), G;
• cytosine (C), C;
• uracil (U), U.

They are linked together by tri- and pentaphosphodiester bonds.

A very different number of nucleotides (from several tens to tens of thousands) are included in the structure of RNA. They can form a secondary structure consisting mainly of short double-stranded strands formed by complementary bases.

Structure of the ribnucleic acid molecule

As already mentioned, the molecule has a single-stranded structure. RNA receives its secondary structure and shape as a result of the interaction of nucleotides with each other. It is a polymer whose monomer is a nucleotide consisting of a sugar, a phosphorus acid residue and a nitrogen base. Externally, the molecule is similar to one of the DNA chains. The nucleotides adenine and guanine, which are part of RNA, are classified as purines. Cytosine and uracil are pyrimidine bases.

Synthesis process

For an RNA molecule to be synthesized, the template is a DNA molecule. However, the reverse process also happens, when new molecules of deoxyribonucleic acid are formed on the ribonucleic acid matrix. This occurs during the replication of some types of viruses.

Other ribonucleic acid molecules can also serve as the basis for biosynthesis. Many enzymes are involved in its transcription, which occurs in the cell nucleus, but the most important of them is RNA polymerase.

Species

Depending on the type of RNA, its functions also differ. There are several types:

• messenger RNA;
• ribosomal rRNA;
• transport tRNA;
• minor;
• ribozymes;
• viral.

Information ribonucleic acid

Such molecules are also called matrix molecules. They make up approximately two percent of the total number in the cell. In eukaryotic cells they are synthesized in the nuclei on DNA templates, then passing into the cytoplasm and binding to ribosomes. Next, they become templates for protein synthesis: transfer RNAs that carry amino acids are attached to them. This is how the process of converting information occurs, which is implemented in the unique structure of the protein. In some viral RNAs it is also a chromosome.

Jacob and Mano are the discoverers of this species. Without a rigid structure, its chain forms curved loops. When not working, mRNA gathers into folds and curls up into a ball, but unrolls when working.

mRNA carries information about the sequence of amino acids in the protein that is being synthesized. Each amino acid is encoded in certain place using genetic codes, which are characterized by:

• triplet - from four mononucleotides it is possible to build sixty-four codons (genetic code);
• non-crossing - information moves in one direction;
• continuity - the principle of operation is that one mRNA - one protein;
• universality - one or another type of amino acid is encoded in the same way in all living organisms;
• degeneracy - there are twenty known amino acids, and sixty-one codons, that is, they are encoded by several genetic codes.

Ribosomal ribonucleic acid

Such molecules make up the vast majority of cellular RNA, eighty to ninety percent of the total. They combine with proteins and form ribosomes - these are organelles that perform protein synthesis.

Ribosomes are composed of sixty-five percent rRNA and thirty-five percent protein. This polynucleotide chain easily bends along with the protein.

The ribosome consists of amino acid and peptide sections. They are located on contacting surfaces.

Ribosomes move freely in the right places. They are not very specific and can not only read information from mRNA, but also form a matrix with them.

Transport ribonucleic acid

tRNAs are the most studied. They make up ten percent of the cell's ribonucleic acid. These types of RNA bind to amino acids thanks to a special enzyme and are delivered to the ribosomes. In this case, amino acids are transported by transport molecules. However, it happens that different codons encode an amino acid. Then several transport RNAs will carry them.

It curls up into a ball when inactive, and when functioning it has the appearance of a clover leaf.

It distinguishes the following sections:

• an acceptor stem having the nucleotide sequence ACC;
• a site that serves to attach to a ribosome;
• an anticodon that codes for the amino acid that is attached to this tRNA.

Minor type of ribonucleic acid

Recently, RNA species have been added to a new class, the so-called small RNAs. They are most likely universal regulators that turn genes on or off in embryonic development, and also control processes within cells.

Ribozymes have also recently been identified; they actively participate when RNA acid is fermented, acting as a catalyst.

Viral types of acids

The virus is capable of containing either ribonucleic acid or deoxyribonucleic acid. Therefore, with the corresponding molecules, they are called RNA-containing. When such a virus enters a cell, reverse transcription occurs - new DNA appears on the basis of ribonucleic acid, which is integrated into the cells, ensuring the existence and reproduction of the virus. In another case, complementary RNA is formed on the incoming RNA. Viruses are proteins; life activity and reproduction occur without DNA, but only on the basis of the information contained in the RNA of the virus.

Replication

To improve our overall understanding, it is necessary to consider the process of replication that produces two identical nucleic acid molecules. This is how cell division begins.

It involves DNA polymerases, DNA-dependent, RNA polymerases and DNA ligases.

The replication process consists of the following steps:

• despiralization - there is a sequential unwinding of the maternal DNA, capturing the entire molecule;
• breaking of hydrogen bonds, in which the chains diverge and a replication fork appears;
• adjustment of dNTPs to the released bases of the mother chains;
• the cleavage of pyrophosphates from dNTP molecules and the formation of phosphodiester bonds due to the released energy;
• respiralization.

After the formation of a daughter molecule, the nucleus, cytoplasm and the rest are divided. Thus, two daughter cells are formed that have fully received all the genetic information.

In addition, the primary structure of proteins that are synthesized in the cell is encoded. DNA takes an indirect part in this process, and not a direct one, which consists in the fact that it is on DNA that the synthesis of RNA and proteins involved in the formation takes place. This process is called transcription.

Transcription

The synthesis of all molecules occurs during transcription, that is, the rewriting of genetic information from a specific DNA operon. The process is similar to replication in some ways and quite different in others.

The similarities are the following parts:

• the beginning comes from the despiralization of DNA;
• hydrogen bonds between the bases of the chains are broken;
• NTFs are complementarily adjusted to them;
• hydrogen bonds are formed.

Differences from replication:

• during transcription, only the DNA section corresponding to the transcripton is unraveled, while during replication, the entire molecule is untwisted;
• during transcription, the adaptable NTPs contain ribose and uracil instead of thymine;
• information is written off only from a certain area;
• Once the molecule is formed, the hydrogen bonds and the synthesized chain are broken, and the chain slips off the DNA.

For normal functioning, the primary structure of RNA must consist only of DNA sections copied from exons.

Newly formed RNAs begin the maturation process. Silent sections are cut out, and informative sections are stitched together, forming a polynucleotide chain. Further, each species has transformations unique to it.

In mRNA, attachment occurs at the initial end. The polyadenylate is added to the final section.

In tRNA, bases are modified to form minor species.

In rRNA, individual bases are also methylated.

Protects proteins from destruction and improves transport into the cytoplasm. RNA in a mature state combines with them.

The meaning of deoxyribonucleic and ribonucleic acids

Nucleic acids are of great importance in the life of organisms. They are stored, transported into the cytoplasm and passed on to inheritance. daughter cells information about proteins synthesized in each cell. They are present in all living organisms; the stability of these acids plays a critical role for the normal functioning of both cells and the entire organism. Any changes in their structure will lead to cellular changes.

Structure of nucleic acids

Nucleic acids – phosphorus-containing biopolymers of living organisms, ensuring the preservation and transmission of hereditary information.

Macromolecules of nucleic acids were discovered in 1869 by the Swiss chemist F. Miescher in the nuclei of leukocytes found in manure. Later, nucleic acids were identified in all cells of plants and animals, fungi, bacteria and viruses.

Note 1

There are two types of nucleic acids – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

As the names indicate, the DNA molecule contains the pentose sugar deoxyribose, and the RNA molecule contains ribose.

A large number of varieties of DNA and RNA are now known, which differ from each other in structure and significance in metabolism.

Example 1

The bacterial cell of Escherichia coli contains about 1000 varieties of nucleic acids, and animals and plants have even more.

Each type of organism has its own set of these acids. DNA is localized primarily in the chromosomes of the cell nucleus (% of the total DNA of the cell), as well as in chloroplasts and mitochondria. RNA is found in the cytoplasm, nucleoli, ribosomes, mitochondria, and plastids.

A DNA molecule consists of two polynucleotide chains, helically twisted relative to each other. The chains are arranged antiparallel, that is, the 3-end and the 5-end.

The structural components (monomers) of each such chain are nucleotides. In nucleic acid molecules, the number of nucleotides varies - from 80 in transfer RNA molecules to several tens of thousands in DNA.

Any DNA nucleotide contains one of four nitrogenous bases ( adenine, thymine, cytosine and guanine), deoxyribose And phosphoric acid residue.

Note 2

Nucleotides differ only in their nitrogenous bases, between which there are related relationships. Thymine, cytosine and uracil are pyrimidine bases, while adenine and guanine are purine bases.

Adjacent nucleotides in a polynucleotide chain are linked by covalent bonds formed between the deoxyribose of a DNA molecule (or ribose of RNA) of one nucleotide and the phosphoric acid residue of another.

Note 3

Although there are only four types of nucleotides in a DNA molecule, due to changes in the sequence of their location in a long chain, DNA molecules achieve enormous diversity.

Two polynucleotide chains are combined into a single DNA molecule using hydrogen bonds, which are formed between the nitrogenous bases of nucleotides of different chains.

In this case, adenine (A) can only combine with thymine (T), and guanine (G) can only combine with cytosine (C). As a result, in various organisms the number of adenyl nucleotides is equal to the number of thymidyl nucleotides, and the number of guanyl nucleotides is equal to the number of cytidyl nucleotides. This pattern is called "Chargaff's rule". In this way, the sequence of nucleotides in one chain is determined according to their sequence in the other.

This ability of nucleotides to selectively combine is called complementarity, and this property ensures the formation of new DNA molecules based on the original molecule (replication).

Note 4

The double helix is ​​stabilized by numerous hydrogen bonds (two are formed between A and T, three between G and C) and hydrophobic interactions.

The DNA diameter is 2 nm, the helix pitch is 3.4 nm, and each turn contains 10 nucleotide pairs.

The length of a nucleic acid molecule reaches hundreds of thousands of nanometers. This significantly exceeds the largest protein macromolecule, the length of which, when unfolded, is no more than 100–200 nm.

Self duplication of a DNA molecule

Each cell division, provided that the nucleotide sequence is strictly observed, is preceded by the replication of a DNA molecule.

It begins with the DNA double helix temporarily unwinding. This occurs under the action of the enzymes DNA topoisomerase and DNA helicase. DNA polymerase and DNA primase catalyze the polymerization of nucleoside triphosphates and the formation of a new chain.

The accuracy of replication is ensured by the complementary (A - T, G - C) interaction of the nitrogenous bases of the template chain that is being built.

Note 5

Each polynucleotide chain is a template for a new complementary chain. As a result, two DNA molecules are formed, one half of each of which comes from the mother molecule, and the other is newly synthesized.

Moreover, new chains are synthesized first in the form of short fragments, and then these fragments are “stitched” into long chains by a special enzyme.

The two new DNA molecules formed are an exact copy the original molecule due to replication.

This process is the basis for the transfer of hereditary information, which takes place at the cellular and organismal levels.

Note 6

Key Feature DNA replication - its high accuracy, which is ensured by a special complex of proteins - the “replication machine”.

Functions of the “replication machine”:

• produces carbohydrates that form a complementary pair with the nucleotides of the mother matrix chain;
• acts as a catalyst in the formation of a covalent bond between the end of the growing chain and each new nucleotide;
• corrects the chain by removing nucleotides that are incorrectly incorporated.

The number of errors in the “replication machine” is very small, less than one error per 1 billion nucleotides.

However, there are cases when the “replication machine” can skip or insert several extra bases, include a C instead of a T or an A instead of a G. Each such replacement of a nucleotide sequence in a DNA molecule is a genetic error and is called mutation. In all subsequent generations of cells, such errors will be reproduced again, which can lead to noticeable negative consequences.

Types of RNA and their functions

RNA is a single polynucleotide chain (some viruses have two chains).

Monomers are ribonucleotides.

Nitrogen bases in nucleotides:

• adenine (A);*
• guanine (G);
• cytosine (C);
• uracil (U).*

Monosaccharide – ribose.

In the cell it is localized in the nucleus (nucleolus), mitochondria, chloroplasts, ribosomes, and cytoplasm.

It is synthesized by template synthesis according to the principle of complementarity on one of the DNA strands, is not capable of replication (self-duplication), and is labile.

There are various types RNA, which differ in molecular size, structure, location in the cell and functions.

Low molecular weight transfer RNAs (tRNAs) constitute about 10% of the total amount of cellular RNA.

In the process of transmitting genetic information, each tRNA can attach and transfer only a certain amino acid (for example, lysine) to ribosomes, the site of protein synthesis. But for each amino acid there is more than one tRNA. Therefore, there are many more than 20 different tRNAs, which differ in their primary structure (have a different nucleotide sequence).

Ribosomal RNAs (rRNAs) make up up to 85% of all RNA cells. Being part of ribosomes, they thereby perform a structural function. rRNA also takes part in the formation of the active center of the ribosome, where peptide bonds are formed between amino acid molecules during the process of protein biosynthesis.

Featuring messenger or messenger RNA (mRNA) the synthesis of proteins in the cell is programmed. Although their content in the cell is relatively low - about 5% - of total mass Of all RNA cells, mRNA comes first in importance, since they directly transfer the DNA code for protein synthesis. In this case, each cell protein is encoded by a specific mRNA. This is explained by the fact that RNA, during its synthesis, receives information from DNA about the structure of the protein in the form of a copied nucleotide sequence and transfers it to the ribosome for processing and implementation.

Note 7

The significance of all types of RNA is that they are a functionally unified system aimed at carrying out the synthesis of cell-specific proteins in the cell.

Chemical structure and role of ATP in energy metabolism

Adenosine triphosphoric acid (ATP ) is contained in every cell - in the hyaloplasm (the soluble fraction of the cytoplasm), mitochondria, chloroplasts and the nucleus.

It provides energy for most of the reactions occurring in the cell. With the help of ATP, the cell is able to move, synthesize new molecules of proteins, fats and carbohydrates, get rid of breakdown products, carry out active transport, etc.

The ATP molecule is formed by a nitrogenous base, the five-carbon sugar ribose and three phosphoric acid residues. The phosphate groups in the ATP molecule are connected to each other by high-energy (macroergic) bonds.

As a result of hydrolytic elimination of the final phosphate group, adenosine diphosphoric acid (ADP) and energy is released.

After the elimination of the second phosphate group, adenosine monophosphoric acid (AMP) and another portion of energy is released.

ATP is formed from ADP and inorganic phosphate due to the energy that is released during oxidation organic matter and during the process of photosynthesis. This process is called phosphorylation. In this case, at least 40 kJ/mol of ATP accumulated in its high-energy bonds must be used.

This means that the main significance of the processes of respiration and photosynthesis is that they supply energy for the synthesis of ATP, with the participation of which a significant number of different processes occur in the cell.

ATP is restored extremely quickly. Example In humans, each ATP molecule is broken down and renewed again 2400 times a day, therefore its average lifespan is less than 1 minute.

ATP synthesis occurs mainly in mitochondria and chloroplasts. ATP, which is formed, enters through the channels of the endoplasmic reticullum to those parts of the cell where energy is needed.

Any type of cellular activity occurs due to the energy that is released during ATP hydrolysis. The remaining energy (about 50%) that is released during the breakdown of molecules of proteins, fats, carbohydrates and others organic compounds, dissipates in the form of heat, dissipates and has no practically significant significance for the life of the cell.

If previously the prevailing opinion was that RNA played a minor role, it is now clear that it is a necessary and essential element of cell life. Mechanisms of many...

From Masterweb

09.04.2018 14:00

Various types of DNA and RNA - nucleic acids - are one of the objects of study of molecular biology. One of the most promising and rapidly developing areas in this science is recent years was RNA research.

Briefly about the structure of RNA

So, RNA, ribonucleic acid, is a biopolymer, the molecule of which is a chain formed by four types of nucleotides. Each nucleotide, in turn, consists of a nitrogenous base (adenine A, guanine G, uracil U or cytosine C) combined with the sugar ribose and a phosphoric acid residue. Phosphate residues, combining with ribose of neighboring nucleotides, “crosslink” the constituent blocks of RNA into a macromolecule - a polynucleotide. This is how the primary structure of RNA is formed.

The secondary structure - the formation of a double chain - is formed in some parts of the molecule in accordance with the principle of complementarity of nitrogenous bases: adenine forms a pair with uracil through a double, and guanine with cytosine - a triple hydrogen bond.

In its working form, the RNA molecule also forms a tertiary structure - a special spatial structure, conformation.

RNA synthesis

All types of RNA are synthesized using the enzyme RNA polymerase. It can be DNA- and RNA-dependent, that is, it can catalyze synthesis on both DNA and RNA templates.

The synthesis is based on base complementarity and antiparallel direction of reading the genetic code and proceeds in several stages.

First, RNA polymerase is recognized and binds to a special sequence of nucleotides on DNA - the promoter, after which the double helix of DNA unwinds in a small area and the assembly of an RNA molecule begins over one of the chains, called the template (the other DNA chain is called coding - it is its copy that is synthesized RNA). The asymmetry of the promoter determines which DNA strand will serve as a template, and thereby allows RNA polymerase to initiate synthesis in the correct direction.

The next stage is called elongation. The transcription complex, including RNA polymerase and the untwisted region with the DNA-RNA hybrid, begins to move. As this movement proceeds, the growing RNA chain gradually separates, and the DNA double helix unwinds in front of the complex and is restored behind it.

The final stage of synthesis occurs when RNA polymerase reaches a special region of the template called the terminator. Termination (completion) of the process can be achieved in various ways.

Main types of RNA and their functions in cells

They are as follows:

• Matrix or information (mRNA). Through it, transcription is carried out - the transfer of genetic information from DNA.
• Ribosomal (rRNA), which ensures the process of translation - protein synthesis on an mRNA matrix.
• Transport (tRNA). Recognizes and transports amino acids to the ribosome, where protein synthesis occurs, and also takes part in translation.
• Small RNAs are a large class of small molecules that perform various functions during the processes of transcription, RNA maturation, and translation.
• RNA genomes are coding sequences that contain genetic information in some viruses and viroids.

In the 1980s, the catalytic activity of RNA was discovered. Molecules with this property are called ribozymes. Not many natural ribozymes are known yet; their catalytic ability is lower than that of proteins, but in the cell they perform exclusively important functions. Currently underway successful work on the synthesis of ribozymes, which also have practical significance.

Let's take a closer look at the different types of RNA molecules.

Messenger (messenger) RNA

This molecule is synthesized over an untwisted section of DNA, thus copying the gene encoding a particular protein.

The RNA of eukaryotic cells, before becoming, in turn, a matrix for protein synthesis, must mature, that is, go through a complex of various modifications - processing.

First of all, even at the transcription stage, the molecule is capped: a special structure of one or more modified nucleotides – a cap – is attached to its end. It plays an important role in many downstream processes and increases mRNA stability. The so-called poly(A) tail, a sequence of adenine nucleotides, is attached to the other end of the primary transcript.

The pre-mRNA then undergoes splicing. This is the removal from the molecule of non-coding regions - introns, of which there are many in eukaryotic DNA. Next, the mRNA editing procedure occurs, during which its composition is chemically modified, as well as methylation, after which the mature mRNA leaves the cell nucleus.

Ribosomal RNA

The basis of the ribosome, a complex that ensures protein synthesis, is made up of two long rRNAs, which form ribosomal subparticles. They are synthesized together in the form of one pre-rRNA, which is then separated during processing. The large subparticle also includes low molecular weight rRNA, synthesized from a separate gene. Ribosomal RNAs have a tightly packed tertiary structure that serves as a scaffold for proteins present in the ribosome that perform auxiliary functions.

In the idle phase, the ribosomal subunits are separated; When the translation process is initiated, the rRNA of the small subparticle combines with the messenger RNA, after which the elements of the ribosome are completely combined. When the RNA of a small subunit interacts with mRNA, the latter is pulled through the ribosome (which is equivalent to the movement of the ribosome along the mRNA). The ribosomal RNA of the large subunit is a ribozyme, that is, it has enzymatic properties. It catalyzes the formation of peptide bonds between amino acids during protein synthesis.

It should be noted that the largest part Ribosomal accounts for 70-80% of all RNA in a cell. DNA has a large number genes encoding rRNA, which ensures its very intense transcription.

Transfer RNA

This molecule is recognized by a specific amino acid with the help of a special enzyme and, combining with it, transports the amino acid to the ribosome, where it serves as an intermediary in the process of translation - protein synthesis. Transfer occurs by diffusion in the cytoplasm of the cell.

Newly synthesized tRNA molecules, like other types of RNA, undergo processing. Mature tRNA in its active form has a cloverleaf-like conformation. On the “petiole” of the leaf - the acceptor site - there is a CCA sequence with a hydroxyl group that binds to the amino acid. At the opposite end of the “leaf” is an anticodon loop that binds to the complementary codon on the mRNA. The D-loop serves to bind transfer RNA to the enzyme when interacting with an amino acid, and the T-loop serves to bind to the large subunit of the ribosome.

Small RNAs

These types of RNA play an important role in cellular processes and are now being actively studied.

For example, small nuclear RNAs in eukaryotic cells are involved in mRNA splicing and possibly have catalytic properties along with spliceosomal proteins. Small nucleolar RNAs are involved in the processing of ribosomal and transfer RNA.

Small interfering and microRNAs are the most important elements of the gene expression regulation system necessary for the cell to control its own structure and vital functions. This system is an important part of the cell's immune antiviral response.

There is also a class of small RNAs that function in complex with Piwi proteins. These complexes play a huge role in the development of germline cells, spermatogenesis and the suppression of mobile genetic elements.

RNA genome

The RNA molecule can be used as a genome by most viruses. Viral genomes are different - single- and double-stranded, circular or linear. Also, RNA virus genomes are often segmented and generally shorter than DNA genomes.

There is a family of viruses genetic information which, encoded in RNA, after infection of the cell by reverse transcription is rewritten into DNA, which is then introduced into the genome of the victim cell. These are so-called retroviruses. These include, in particular, the human immunodeficiency virus.

The importance of RNA research in modern science

If previously the prevailing opinion was about the secondary role of RNA, it is now clear that it is a necessary and important element of intracellular life. Many processes of primary importance cannot occur without the active participation of RNA. Mechanisms of such processes for a long time remained unknown, but thanks to the study of different types of RNA and their functions, many details are gradually becoming clearer.

It is possible that RNA played a decisive role in the emergence and development of life at the dawn of Earth's history. The results of recent studies support this hypothesis, indicating the extraordinary antiquity of many cell functioning mechanisms involving certain types of RNA. For example, the recently discovered riboswitches in mRNA (a system of protein-free regulation of gene activity at the transcription stage), according to many researchers, are echoes of the era when primitive life was built on the basis of RNA, without the participation of DNA and proteins. MicroRNAs are also considered to be a very ancient component of the regulatory system. The structural features of catalytically active rRNA indicate its gradual evolution through the addition of new fragments to the ancient protoribosome.

A thorough study of which types of RNA and how they are involved in certain processes is also extremely important for theoretical and applied fields of medicine.

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