Why is the DNA content constant in different cells? The genome: constancy during development

Types of nucleic acids. There are two types of nucleic acids in cells: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These biopolymers are made up of monomers called nucleotides. The nucleotide monomers of DNA and RNA are similar in basic structural features. Each nucleotide consists of three components connected by strong chemical bonds.

Each of the nucleotides that make up RNA contains a five-carbon sugar - ribose; one of four organic compounds which are called nitrogenous bases - adenine, guanine, cytosine, uracil (A, G, C, U); phosphoric acid residue.

The nucleotides that make up DNA contain a five-carbon sugar - deoxyribose, one of four nitrogenous bases: adenine, guanine, cytosine, thymine (A, G, C, T); phosphoric acid residue.

In the composition of nucleotides, a nitrogenous base is attached to the ribose (or deoxyribose) molecule on one side, and a phosphoric acid residue on the other. Nucleotides are connected to each other in long chains. The backbone of such a chain is formed by regularly alternating sugar and phosphoric acid residues, and the side groups of this chain are formed by four types of irregularly alternating nitrogenous bases.

Rice. 7. DNA structure diagram. Hydrogen bonds are indicated by dots

A DNA molecule is a structure consisting of two strands that are connected to each other along their entire length hydrogen bonds(Fig. 7). This structure, characteristic only of DNA molecules, is called a double helix. A feature of the DNA structure is that opposite the nitrogenous base A in one chain lies the nitrogenous base T in the other chain, and opposite the nitrogenous base G is always the nitrogenous base C. Schematically, what has been said can be expressed as follows:

A (adenine) - T (thymine)
T (thymine) - A (adenine)
G (guanine) - C (cytosine)
C (cytosine) - G (guanine)

These pairs of bases are called complementary bases (complementing each other). DNA strands in which the bases are located complementary to each other are called complementary strands. Figure 8 shows two strands of DNA that are connected by complementary regions.

Rice. 8. Section of a double-stranded DNA molecule

The model of the structure of the DNA molecule was proposed by J. Watson and F. Crick in 1953. It was fully confirmed experimentally and played an extremely important role in the development molecular biology and genetics.

The order of arrangement of nucleotides in DNA molecules determines the order of arrangement of amino acids in linear protein molecules, i.e., their primary structure. A set of proteins (enzymes, hormones, etc.) determines the properties of the cell and the organism. DNA molecules store information about these properties and pass them on to generations of descendants, i.e. they are carriers hereditary information. DNA molecules are mainly found in the nuclei of cells and in small quantities in mitochondria and chloroplasts.

Main types of RNA. Hereditary information stored in DNA molecules is realized through protein molecules. Information about the structure of the protein is transmitted to the cytoplasm by special RNA molecules, which are called messenger RNA (mRNA). Messenger RNA is transferred to the cytoplasm, where protein synthesis occurs with the help of special organelles - ribosomes. It is messenger RNA, which is built complementary to one of the DNA strands, that determines the order of amino acids in protein molecules. Another type of RNA also takes part in protein synthesis - transport RNA (tRNA), which brings amino acids to the place of formation of protein molecules - ribosomes, a kind of factories for the production of proteins.

Ribosomes contain a third type of RNA, the so-called ribosomal RNA (rRNA), which determines the structure and functioning of ribosomes.

Each RNA molecule, unlike a DNA molecule, is represented by a single strand; It contains ribose instead of deoxyribose and uracil instead of thymine.

So, nucleic acids perform the most important biological functions in the cell. DNA stores hereditary information about all the properties of the cell and the organism as a whole. Various types RNAs take part in the implementation of hereditary information through protein synthesis.

1. Look at Figure 7 and say what is special about the structure of the DNA molecule. What components make up nucleotides?
2. Why is the consistency of DNA content in different cells of the body considered evidence that DNA is genetic material?
3. Using the table, give comparative characteristics DNA and RNA.

1. A fragment of one DNA strand has the following composition: -A-A-A-T-T-C-C-G-G-. Complete the second chain.
2. Thymines account for 20% of the DNA molecule. total number nitrogenous bases. Determine the amount of nitrogenous bases adenine, guanine and cytosine.
3. What are the similarities and differences between proteins and nucleic acids?

Cells various types differ from each other mainly because, in addition to the proteins necessary for all cells, without exception, to maintain vital functions, cells of each type synthesize their own set of specialized proteins. For example, keratin is synthesized in epidermal cells, hemoglobin is synthesized in erythrocytes, crystallins are synthesized in lens cells, etc. Since each cell type has specific sets of gene products, one might wonder whether this is simply because the cells have various sets genes? Lens cells, for example, have lost the genes for keratin, hemoglobin, etc., but retained the crystallin genes, or, due to amplification, they selectively increased the number of copies of crystallin genes. However, a number of data show that this is not so: cells of almost all types contain the same complete genome, which was originally present in the fertilized egg. The reason for the differences in cell properties is not the possession of different sets of genes, but their differential expression. In other words, the activity of genes is regulated: they can be turned on and off.

The most convincing evidence of this was obtained in experiments with the transplantation of nuclei into amphibian cells. As a rule, the size of amphibian eggs allows one to inject nuclei obtained from other cells into them using a micropipette. The core of the egg itself is first destroyed by irradiation with ultraviolet light. A prick with a micropipette stimulates the egg to begin development. It turned out that when replacing the egg cell nucleus with a keratinocyte nucleus from the skin of an adult frog or an erythrocyte nucleus, normal swimming tadpoles were obtained. Such experiments have a number of limitations: they are successful when using the nuclei of only some differentiated cells and eggs of certain species. However, the results of other studies allow us to come to the conclusion that the constancy of the genome is maintained during development.

There are several known exceptions to this rule. For example, in some invertebrates, in somatic (non-reproductive) cells, part of the chromosomes present in germline cells (precursors of gametes) is lost already at the early stages of development. In the oocytes of some other animals (including Xenopus laevis), selective replication of ribosomal RNA genes occurs, and in the larvae of some insects, unequal polytenization of chromosomes occurs, resulting in increased amplification of some specific genes. The synthesis of antibodies and antigen-specific receptors by lymphocytes in vertebrates involves the splicing of DNA fragments located in different places in the genome of these specialized cells. Splicing occurs as these cells differentiate. (

Tutorial

Responsible for the release is Finaev V.I.

Editor Belova L.F.

Corrector Protsenko I.A.

LP No. 020565 dated 23.-6.1997 Signed for publication

Offset printing Conditions p.l. – 10.1 Uch.-ed.l. – 9.7

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1. Evidence of the genetic role of DNA

2. Chemical structure nucleic acids

3.1. DNA structure

3.2. Levels of DNA compaction

3.3. DNA replication

3.4. DNA repair

3.5. Functions of DNA

5.1. Basic provisions of the system concept of the gene

5.2. Plasmogens

5.3. Gene properties

5.4. Gene functions

5.5. Gene structure of pro- and eukaryotes

5.6. Regulation of gene function

6. Expression stages genetic information

6.1. Transcription

6.2. Processing

6.3. Broadcast

6.3.1. Properties of the genetic code

6.3.2. Amino acid activation

6.3.3. Broadcast stages

6.4. Protein processing

Brief biographical information

MOLECULAR BASIS OF HERITAGE.

We entered the cage, our cradle, and began

make an inventory of the wealth we have acquired.

Albert Claude (1974)

Evidence of the genetic role of DNA.

Nucleic acids discovered by a Swiss biochemist F. Misher in 1869 in the nuclei of pus cells (leukocytes) and sperm. In 1891, a German biochemist A. Kessel showed that nucleic acids consist of sugar residues, phosphoric acid and four nitrogenous bases, which are derivatives of purine and pyrimidine. He was the first to prove the existence of two types of nucleic acids - DNA And RNA. Then in 1908 - 1909 F. Levene a description of the structure of nucleosides and nucleotides was given, and in 1952 by English researchers led by A. Todd– phosphodiester bond. In the 20s Felgen discovered DNA in chromosomes, and RNAs were found in the nucleus and cytoplasm. In 1950 E. Chargaff with collaborators from Columbia University established differences in the nucleotide composition of DNA in different types.

IN 1953 American biochemist and geneticist J. Watson and the English physicist F. Crick proposed a model of the double helix of DNA. This date is officially considered your birthday new industry biological science – molecular biology.

It should be noted that in the years when there was not even a hint of the genetic role of nucleic acids, they were perceived by everyone as a rather strange material, chemically not very complex structure(nitrogen bases, pentoses, phosphoric acid residue). However, their functional significance was deciphered much later, which was due to ignorance of the structural features of nucleic acids. From the point of view of scientists of the late 19th and early 20th centuries, they were inferior in complexity and combinability to proteins whose monomers were 20 types of amino acids. Therefore, it was generally accepted in science that proteins are carriers of hereditary information, because the variety of amino acids made it possible to encode the entire variety of properties and characteristics of living organisms.

Although back in 1914, a Russian researcher Shchepotyev expressed the idea of ​​the possible role of nucleic acids in heredity, but was unable to prove his point of view. However, gradually accumulated scientific facts about the genetic role of nucleic acids.

1928 English microbiologist Frederick Griffith worked with two strains of microorganisms: virulent (had a polysaccharide capsule) and avirulent (did not have a capsule) (Fig. 1). Virulent caused pneumonia in mice and their death. If the virulent strain is heated, it is inactivated and is not dangerous - all mice survive (the postulate of scientists of that time: the gene is of a protein nature; when heated, proteins denature and lose their biological activity). If you mix heated virulent and live avirulent, then some of the mice die. Upon autopsy of mice, virulent capsule forms were found in them. A similar picture was observed if a cell-free extract from virulent forms was added to a living avirulent strain of bacteria. From these experiments, F. Griffith concluded that some factor is transferred from heat-killed virulent forms and cell-free extracts to living non-capsular forms, which converts the avirulent form into a virulent one. This phenomenon is called " transformation"bacteria and remained a mystery for many years."

Rice. 1 F. Griffith's experiments on transformation in bacteria.

1. When mice were infected with avirulent pneumococci, they all survived.

2. When mice were infected with virulent pneumococci, they all died from pneumonia.

3. When mice were infected with heat-killed virulent pneumococci, they all survived.

4. When mice are infected with a mixture of live avirulent and heat-killed

virulent pneumococci, some mice died.

5. When mice were infected with a mixture of live avirulent pneumococci and an extract from heat-killed virulent pneumococci, some of the mice died. (“From Molecules to Man,” 1973, p. 83)

However, F. Griffith could not explain the nature of the transforming factor. American scientists did it O. Avery, J. Mac-Leod, M. Mac-Carty in 1944. They showed that purified pneumococcal DNA extracts could induce bacterial transformation. The purified transforming agent contained small quantity proteins. Proteolytic enzymes did not inactivate it, but deoxyribonuclease did. With their brilliant experiments they showed that DNA is the substance that changes genetic information. These experiments were the first scientific proof genetic role of nucleic acids. This issue was finally resolved in experiments on bacterial viruses - bacteriophages in 1948 – 1952. Bacteriophages have a very simple structure: they consist of a protein shell and a molecule nucleic acid. This makes them an ideal material for studying the question of whether protein or DNA serves as genetic material. In experiments with labeled compounds A. Hershey And M. Chase(1952) it was convincingly shown that DNA is the carrier of genetic information, since the virus injects it into the body bacterial cell, and the protein “shell” remains outside (Fig. 2).

Fig.2. Bacteriophage T 2 With the help of a “tail” it attaches to the bacterium. He introduces his DNA into it, after which it replicates and synthesizes new protein shells. The bacterium then bursts, releasing many new virus particles, each of which can infect a new bacterium (“From Molecules to Man,” 1973, p. 86)

As a result of the experiments described above, it became clear that bacteria and phages serves as genetic material DNA. But is it the carrier of hereditary information in eukaryotic cells? The answer to this question was obtained in transfer experiments whole chromosomes from one cell to another. The recipient cells showed some signs of the donor cell. And then, thanks to the success genetic engineering, were able to add individual genes(DNA containing only one gene) that were lost by the mutant cells. These experiments established that DNA in eukaryotes is the genetic material and the possibility of transfer was proven genes between different types while maintaining their functional properties.

The following facts speak about the genetic function of DNA:

1. Localization of DNA is almost exclusively in chromosomes.

2. The constant number of chromosomes in cells of one species is 2n.

3. The constancy of the amount of DNA in cells of the same species is equal to 2C or 4C, depending on the stage of the cell cycle.

4. Half the amount of DNA in the nuclei of germ cells

5. The influence of mutagens on the chemical structure of DNA.

6. The phenomenon of genetic recombination in bacteria during their conjugation.

7. The phenomenon of transduction - transfer genetic material from one strain of bacteria to another using phage DNA.

8. Infectious function of isolated viral nucleic acid.

Geneticists were able to figure out why, even though the DNA in all cells of the body is the same, the cells themselves develop differently. They found a code that blocks information sections of the genetic code. Moreover, the code turned out to be universal for different types.

IN genetic code In addition to the information that defines all the proteins that a cell can produce, another coding mechanism has been found. The code lays down the procedure for blocking information. It is inaccessible for reading in those parts of the DNA molecule where the chain is wound around histones - a kind of protein coils, and the code indicates the places of twisting.

The nucleotide sequences that determine the location of blocked pieces of DNA were described by Eran Segal from the Israeli Weizmann Institute and Jonathan Widom from Northwestern University in Illionois in the latest issue of the journal Nature.

Biologists have suspected for years that special factors favor the regions of DNA that wrap around nucleosomes most easily. But what these factors were was unclear. Scientists analyzed more than two hundred sections of yeast DNA folded into nucleosomes.

And they discovered hidden marks - a special sequence of nucleotide pairs in some parts of the chain that determine the availability of the genetic material that follows them. They are located in the previously considered “junk” part of the DNA.

By knowing these key sites, the researchers were able to correctly predict the location of 50% of nucleosomes in cells of similar tissues in other species (each cell contains about 30 million nucleosomes).

In fact, the discovery means the establishment of a mechanism for blocking genetic information that is universal for all living organisms.

Dr. Segal, he said, was very surprised by such a good result. According to his assumption, nucleosomes often move, opening new sections of DNA for reading. The location of the unraveled half of the coiled DNA is determined by competition between nucleosomes and other locking mechanisms.

On free sections of DNA, if it is necessary to transcribe a gene (to create a new protein), a similar natural mechanism of marks is implemented. Scientists have known about this code for a long time: in front of the gene that determines the substance, there are 6–8 nucleotide pairs that “explain” it.

The nucleosome coils themselves are composed of histone proteins. In the process of evolution, histones have proven themselves to be the most resistant to changes. They also practically do not differ between different types of living organisms. Thus, the histones of peas and cows differ in only two of the 102 amino acid compounds. And since any information about a protein is contained in the form of a sequence of nucleotide pairs in the DNA code, scientists have long assumed that there is a mechanism for blocking information in the DNA code, similar to many organisms. Written as a sequence of nucleotide pairs, it may be just the nucleosome code.

And the combination of the reading code and the blocking code determines what a given cell will turn into during the development of the organism from the embryo.

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Chromosomes consist of chromatin - a combination of DNA and proteins (histones). This complex has a complex spatial configuration.

The nature of the connection (packaging) in the chromosome of one very long DNA molecule (its length reaches hundreds and even thousands of micrometers) and numerous, relatively compact protein molecules has not yet been fully elucidated.

It is assumed that a chain of many protein molecules is in the middle, and DNA is twisted around in the form of a spiral. In addition to these two main compounds, small amounts of RNA, lipids and some salts were found in chromatin.

Constancy of the amount of DNA in the nucleus

Each species of plant and animal contains a strictly defined and constant amount of DNA in the cell nucleus. Different species of organisms have significantly different DNA content. For example, in one nucleus of a haploid cell (in a sperm) sea ​​urchin contains 0.9 10 -9 mg of DNA, carp - 1.64 10 -9, rooster - 1.26 10 -9, bull - 3.42 10 -9, human - 3.25 10 - 9 mg. For some plants these numbers are significantly higher. In a lily, for example, a haploid cell contains 58.0·10 -9 mg of DNA.

In the nuclei of all somatic (diploid) cells of each type of organism, the DNA content is also constant and twice the amount of DNA in the haploid cells of this species.

Even more important is the specificity of the nucleotide composition of DNA. Soviet scientist academician A.N. Belozersky established that DNA isolated from different tissues of the same organism has the same nucleotide composition. It does not depend on the age of the organism or the influence external environment. At the same time, DNA isolated from cells of different species contains nitrogenous bases in different proportions.