DNA division. Replication in biology is an important molecular process of body cells

Nucleic acids play an important role in ensuring the vital activity of cells of living organisms. An important representative of this group of organic compounds is DNA, which carries all the genetic information and is responsible for the manifestation of the necessary characteristics.

What is replication?

As cells divide, they need to increase the amount of nucleic acids in the nucleus to prevent loss of genetic information during the process. In biology, replication is the duplication of DNA by synthesizing new strands.

The main purpose of this process is to transfer genetic information to daughter cells unchanged without any mutations.

Replication enzymes and proteins

The duplication of a DNA molecule can be compared to any metabolic process in a cell that requires the corresponding proteins. Since in biology, replication is an important component of cell division, therefore, many auxiliary peptides are involved here.

• DNA polymerase is the most important reduplication enzyme, which is responsible for the synthesis of the daughter chain. In the cytoplasm of the cell, during the replication process, the presence of nucleic triphosphates is required, which bring all the nucleic bases.

These bases are monomers of nucleic acid, so the entire chain of the molecule is built from them. DNA polymerase is responsible for the assembly process in the correct order, otherwise the appearance of all kinds of mutations is inevitable.

• Primase is a protein that is responsible for the formation of a primer on the template DNA strand. This primer is also called a primer; it has for the enzyme DNA polymerase the presence of initial monomers, from which further synthesis of the entire polynucleotide chain is possible. This function is performed by the primer and its corresponding enzyme.
• Helicase (helicase) forms a replication fork, which is the divergence of template strands by breaking hydrogen bonds. This makes it easier for polymerases to approach the molecule and begin synthesis.
• Topoisomerase. If you imagine a DNA molecule as a twisted rope, as the polymerase moves along the chain, a positive tension will be formed due to the strong twist. This problem is solved by topoisomerase, an enzyme that briefly breaks the chain and unfolds the entire molecule. After which the damaged area is stitched back together, and the DNA does not experience tension.
• Ssb proteins, like clusters, attach to DNA strands at the replication fork to prevent the re-formation of hydrogen bonds before the end of the reduplication process.
• Ligaza. consists of stitching together Okazaki fragments on the lagging strand of a DNA molecule. This occurs by cutting out the primers and inserting native deoxyribonucleic acid monomers in their place.

In biology, replication is a complex multi-step process that is extremely important during cell division. Therefore, the use of various proteins and enzymes is necessary for efficient and correct synthesis.

Reduplication mechanism

There are 3 theories that explain the process of DNA duplication:

1. Conservative states that one daughter nucleic acid molecule is of a template nature, and the second is completely synthesized from scratch.
2. Semi-conservative was proposed by Watson and Crick and confirmed in 1957 in experiments on E. Coli. This theory states that both daughter DNA molecules have one old strand and one newly synthesized one.
3. The dispersive mechanism is based on the theory that daughter molecules have alternating regions along their entire length, consisting of both old and new monomers.

Now a semi-conservative model has been scientifically proven. What is replication at the molecular level? First, helicase breaks the hydrogen bonds of the DNA molecule, thereby opening both strands to the polymerase enzyme. The latter, after the formation of the seeds, begin the synthesis of new chains in the 5’-3’ direction.

The antiparallel property of DNA is the main reason for the formation of leading and lagging strands. On the leading strand, DNA polymerase moves continuously, and on the lagging strand it forms Okazaki fragments, which in the future will be connected using ligase.

Replication Features

How many DNA molecules are in the nucleus after replication? The process itself involves doubling the cell’s genetic makeup, so during the synthetic period of mitosis, the diploid set has twice as many DNA molecules. This entry is usually marked 2n 4c.

In addition to the biological meaning of replication, scientists have found application of the process in various fields of medicine and science. If in biology replication is the doubling of DNA, then in laboratory conditions the reproduction of nucleic acid molecules is used to create several thousand copies.

This method is called polymerase chain reaction (PCR). The mechanism of this process is similar to replication in vivo; therefore, similar enzymes and buffer systems are used for its occurrence.

Conclusions

Replication has important biological significance for living organisms. Transmission during cell division is not complete without doubling DNA molecules, so the coordinated work of enzymes is important at all stages.

DNA replication- the process of synthesis of a daughter molecule of deoxyribonucleic acid on the matrix of the parent DNA molecule. During subsequent division of the mother cell, each daughter cell receives one copy of a DNA molecule that is identical to the DNA of the original mother cell. This process ensures that genetic information is accurately passed on from generation to generation. DNA replication is carried out by a complex enzyme complex consisting of 15-20 different proteins, called replisome

History of the study

Each DNA molecule consists of one strand of the original parent molecule and one newly synthesized strand. This replication mechanism is called semi-conservative. Currently, this mechanism is considered proven thanks to the experiments of Matthew Meselson and Franklin Stahl (1958). Previously, there were two other models: “conservative” - as a result of replication, one DNA molecule is formed, consisting only of parent chains, and one, consisting only of daughter chains; “dispersive” - all DNA molecules resulting from replication consist of chains, some sections of which are newly synthesized, while others are taken from the parent DNA molecule.

General views

DNA replication is a key event during cell division. It is important that by the time of division the DNA has been replicated completely and only once. This is ensured by certain mechanisms regulating DNA replication. Replication occurs in three stages:

replication initiation

elongation

termination of replication.

Replication regulation occurs mainly at the initiation stage. This is quite easy to implement, because replication can begin not from any DNA section, but from a strictly defined one, called the replication initiation site. There can be either just one or many such sites in the genome. Closely related to the concept of a replication initiation site is the concept replicon . A replicon is a section of DNA that contains an initiation site of replication and is replicated after DNA synthesis begins from this site. Bacterial genomes are typically a single replicon, which means that replication of the entire genome results from just one act of replication initiation. Eukaryotic genomes (as well as their individual chromosomes) consist of a large number of independent replicons, which significantly reduces the total replication time of an individual chromosome. The molecular mechanisms that control the number of replication initiation events at each site during one cell division cycle are called copy number control. In addition to chromosomal DNA, bacterial cells often contain plasmids, which are individual replicons. Plasmids have their own copy number control mechanisms: they can ensure the synthesis of just one copy of the plasmid per cell cycle, or thousands of copies.

Replication begins at the replication initiation site with the unwinding of the DNA double helix, which forms replication fork - site of direct DNA replication. Each site can form one or two replication forks, depending on whether replication is unidirectional or bidirectional. Bidirectional replication is more common. Some time after the start of replication, one can observe in an electron microscope replication eye - a section of a chromosome where DNA has already been replicated, surrounded by longer sections of unreplicated DNA.

At the replication fork, DNA copies a large protein complex (replisome), the key enzyme of which is DNA polymerase. The replication fork moves at a speed of about 100,000 base pairs per minute in prokaryotes and 500-5000 in eukaryotes.

Molecular mechanism of replication

Enzymes (helicase, topoisomerase) and DNA-binding proteins unwind the DNA, keep the template in a diluted state, and rotate the DNA molecule. Correct replication is ensured by the exact matching of complementary base pairs and the activity of DNA polymerase, which is able to recognize and correct the error. Replication in eukaryotes is carried out by several different DNA polymerases. Next, the synthesized molecules are twisted according to the principle of supercoiling and further DNA compaction. Synthesis is energy-consuming.

The strands of the DNA molecule diverge, form a replication fork, and each of them becomes a template on which a new complementary strand is synthesized. As a result, two new double-stranded DNA molecules are formed, identical to the parent molecule.

Characteristics of the replication process

matrix- the sequence of the synthesized DNA chain is uniquely determined by the sequence of the mother chain in accordance with the principle of complementarity;

semi-conservative- one strand of the DNA molecule formed as a result of replication is newly synthesized, and the second is maternal;

goes in the direction from the 5’ end of the new molecule to the 3’ end;

semi-continuous- one of the DNA chains is synthesized continuously, and the second - in the form of a set of individual short fragments (Okazaki fragments);

starts from certain sections of DNA called replication initiation sites

DNA replication is the process of biosynthesis of deoxyribonucleic acid. The material for is adenosine-, guanosine-cytidine- and thymidine triphosphoric acid or ATP, GTP, CTP and TTP.

DNA replication mechanism

Biosynthesis is carried out in the presence of a so-called “seed” - a certain amount of single-stranded transformed deoxyribonucleic acid and a catalyst. DNA polymerase acts as a catalyst. This enzyme takes part in the joining of nucleotide residues. In one minute, more than 1000 nucleotide residues are connected. The nucleotide residues in the molecule of a deoxyribonucleic acid fragment are connected to each other by 3', 5'-phosphodiester bonds. DNA polymerase catalyzes the addition of mononucleotide residues to the free 3-hydroxyl end of the transformed deoxyribonucleic acid. First, small parts of the DNA molecule are synthesized. They are susceptible to the action of DNA ligase and form longer fragments of deoxyribonucleic acid. Both fragments are localized in the Transformed deoxyribonucleic acid is used as a growth point for the future DNA molecule and is also a matrix on which an antiparallel chain of deoxyribonucleic acid is formed, which is identical to the transformed DNA in structure and sequence of placement of nucleotide residues. DNA replication occurs during mitotic interphase. Deoxyribonucleic acid is concentrated in chromosomes and chromatin. After the formation of single-stranded deoxyribonucleic acid, its secondary and tertiary structures are formed. Two strands of deoxyribonucleic acid are connected to each other according to the rule of complementarity. DNA replication occurs in the cell nucleus.

The material for the biosynthesis of different groups and types of RNA is high-energy compounds: ATP, GTP, CTP and TTP. can be synthesized in them with the participation of one of the three indicated fragments: DNA-dependent RNA polymerase, polynucleotide-nucleotidyl transferase and RNA-dependent RNA polymerase. The first of them is found in the nuclei of all cells, and is also found in mitochondria. RNA is synthesized on a DNA template in the presence of ribonucleoside triphosphates, Mangan and Magnesium ions. An RNA molecule is formed that is complementary to the DNA template. In order for DNA replication to occur, r-RNA, t-RNA, i-RNA and RNA primers are formed in the nuclei. The first three are transported into the cytoplasm, where they take part in protein biosynthesis.

DNA replication occurs in much the same way as deoxyribonucleic acid translation. The transmission and preservation of hereditary information is carried out in two stages: transcription and translation. What is a gene? A gene is a material unit that is part of a deoxyribonucleic acid molecule (RNA in some viruses). Contained in the chromosomes of cell nuclei. Genetic information is transmitted from DNA through RNA to protein. Transcription is carried out in and consists of the synthesis of mRNA on sections of the deoxyribonucleic acid molecule. It should be said that the nucleotide sequence of deoxyribonucleic acid is “rewritten” into the nucleotide sequence of the mRNA molecule. RNA polymerase attaches to the corresponding section of DNA, “unwinds” its double helix and copies the structure of deoxyribonucleic acid, adding nucleotides according to the principle of complementarity. As the fragment moves, the chain of synthesized RNA moves away from the template, and the DNA double helix behind the enzyme is immediately restored. If RNA polymerase reaches the end of the copied section, the RNA moves away from the matrix into the karyoplasm, after which it moves to the cytoplasm, where it takes part in protein biosynthesis.

During translation, the sequence of nucleotides in the mRNA molecule is translated into the sequence of amino acid residues in the protein molecule. This process occurs in the cytoplasm, where mRNA combines and a polysome is formed.

Replication– transfer of information from DNA to DNA, self-duplication of DNA (DNA biosynthesis).

DNA molecule consisting of two helices doubles during cell division. DNA doubling based on the fact that when unweaving the threads, each thread can be completed complementary copy, thus obtaining two strands of a DNA molecule copying the original one.

Conditions required for replication: 1.) Matrix- DNA strands. The splitting of the thread is called replication fork. It can form inside a DNA molecule. They move in different directions, forming replicative eye. There are several such eyes in the eukaryotic DNA molecule, each with two forks. 2.) Substrate. The plastic material is deoxynucleotide triphosphates: dATP, dGTP, dCTP, dTTP. Then they disintegrate until deoxynucleotide monophosphates, two molecules of inorganic phosphate with the release of energy, i.e. they are both the source and energy, And plastic material. 3.) Ions magnesium. 4.) Replicative enzyme complex. A) DNA - unwinding proteins: - DNA-A(causes threads to diverge); - helicases(cleave the DNA strand); - topoisomerase 1 And 2 (unwind beyond the spiral). Are torn apart (3",5")-phosphodiester bonds. Topoisomerase 2 in prokaryotes is called gyrase. b) Proteins that prevent the joining of DNA strands ( SSB proteins). V) DNA polymerase(catalyzes the formation of phosphodiester bonds). DNA polymerase only extends an existing strand, but cannot join two free nucleotides. G) Primaza(catalyzes the formation of a “seed” for synthesis). This is an RNA polymerase in its structure, which connects single nucleotides. d) DNA ligase. 5.) Primers- “seed” for replication. This is a short fragment consisting of ribonucleotide triphosphates(2 - 10). Primer formation is catalyzed primase.

Replication stages: 1.) Initiation(formation of a replication fork); 2.) Elongation(synthesis of new threads); 3.) Primer exclusion; 4.) Termination(completion of the synthesis of two daughter chains).

Initiating replication:- regulate signaling protein molecules – growth factors;- provide enzymes And special proteins.

Required enzymes: DNA topoisomerases- enzymes that unwind DNA superhelixes. DNA helicase– breaks hydrogen bonds in a double-stranded DNA molecule. As a result, replication fork (replicative eye).

Single-stranded DNA-binding proteins bind to single-stranded DNA and prevent them from joining complementarily.

Replication elongation. The synthesis substrates are deoxynucleoside triphosphates, acting as building material and energy sources.

Required enzymes: DNA primase, which catalyzes the synthesis of short RNA primer molecules for DNA polymerase. DNA polymerase ensures the inclusion into the growing “new” chain of nucleotides complementary to the “old” one, that is, the template chain.

The synthesis of new DNA chains can only proceed in the direction from the 5' end towards the 3' end. DNA is synthesized continuously on one strand "leading" chain, and on the other short fragments are formed - “lagging” chain (fragments of Okazaki).

After removing primers DNA ligase sews short fragments of Okazaki together ( termination).

Information is transmitted matrix method. Semi-conservative DNA replication mechanism.

Lagging Strand Synthesis

3’

3’

5’

5’

1. When does replication occur?- In the synthetic phase of interphase, long before cell division. The period between replication and prophase of mitosis is called the postsynthetic phase of interphase, during which the cell continues to grow and checks whether duplication has occurred correctly.

2. If there were 46 chromosomes before doubling, how many will there be after doubling?- The number of chromosomes does not change when DNA is doubled. Before duplication, a person has 46 single chromosomes (consisting of one double strand of DNA), and after duplication, 46 double chromosomes (consisting of two identical double strands of DNA connected to each other at the centromere).

3. Why is replication needed?- So that during mitosis, each daughter cell can receive its own copy of DNA. During mitosis, each of the 46 double chromosomes is divided into two single chromosomes; two sets of 46 single chromosomes are obtained; these two sets diverge into two daughter cells.

Three principles of DNA structure

Semi-conservative- each daughter DNA contains one chain from the maternal DNA and one newly synthesized one.

Complementarity- AT/CG. Opposite to adenine of one DNA strand there is always thymine of another DNA strand, and opposite to cytosine there is always guanine.

Antiparallelism- DNA strands lie opposite ends to each other. These ends are not studied in school, so a little more detail (and then into the wilds).

The monomer of DNA is a nucleotide, the central part of the nucleotide is deoxyribose. It has 5 carbon atoms (in the nearest picture, the lower left deoxyribose has numbered atoms). Let's see: a nitrogenous base is attached to the first carbon atom, the phosphoric acid of a given nucleotide is attached to the fifth, the third atom is ready to attach the phosphoric acid of the next nucleotide. Thus, any DNA chain has two ends:

• 5" end, phosphoric acid is located on it;
• The 3" end contains ribose.

The antiparallel rule is that at one end of a double strand of DNA (for example, at the top end of the nearest picture), one strand has a 5" end and the other has a 3" end. It is important for the replication process that DNA polymerase can only extend the 3" end. A DNA chain can only grow at its 3" end.

In this picture, the process of DNA doubling occurs from bottom to top. It can be seen that the left chain grows in the same direction, and the right one grows in the opposite direction.

In the following picture top new chain("leading strand") elongates in the same direction in which duplication occurs. Bottom new chain("lagging strand") cannot extend in the same direction, because there it has a 5" end, which, as we remember, does not grow. Therefore, the lower strand grows with the help of short (100-200 nucleotides) Okazaki fragments, each of which grows in the 3" direction. Each Okazaki fragment grows from the 3" end of the primer ("RNA primers", the primers are red in the figure).

Replication enzymes

Overall direction of replication- the direction in which DNA duplication occurs.
Parental DNA- old (maternal) DNA.
Green cloud next to "Parental DNA"- a helicase enzyme that breaks hydrogen bonds between the nitrogenous bases of the old (mother) DNA chain.
Gray ovals on DNA strands that have just been separated from each other- destabilizing proteins that prevent DNA strands from connecting.
DNA pol III- DNA polymerase, which adds new nucleotides to the 3" end of the upper (leading, continuously synthesized) DNA strand (Leading strand).
Primase- primase enzyme, which makes primer (red Lego piece). Now we count the primers from left to right:

• the first primer is still unfinished, primaza is making it right now;
• from the second primer, DNA polymerase builds DNA - in the direction opposite to the direction of DNA doubling, but in the direction of the 3" end;
• from the third primer the DNA chain has already been built (Lagging strand), she came close to the fourth primer;
• the fourth primer is the shortest because DNA polymerase (DNA pol I) removes it (aka RNA, it has nothing to do with DNA, we only needed the right end from it) and replaces it with DNA;
• The fifth primer is no longer in the picture, it has been completely cut out, leaving a gap in its place. DNA ligase (DNA ligase) stitches this break so that the lower (lagging) DNA strand is intact.

The enzyme topoisomerase is not indicated in the super picture, but it will appear later in the tests, so let’s say a few words about it. Here is a rope consisting of three large strands. If three comrades take hold of these three strands and begin to pull them in three different directions, then very soon the rope will stop unraveling and will curl into tight loops. The same thing could happen with DNA, which is a two-stranded rope, if not for topoisomerase.

Topoisomerosis cuts one of the two DNA strands, after which (second picture, red arrow) the DNA rotates around one of its strands, so that tight loops are not formed (topological stress is reduced).

Terminal underreplication

From the super picture with replication enzymes, it is clear that in the place left after the removal of the primer, DNA polymerase completes the next Okazaki fragment. (Is it really clear? If anything, the Okazaki fragments in the super-painting are indicated by numbers in circles.) When the replication in the super-painting reaches its logical (left) end, then the last (leftmost) Okazaki fragment will not have a “next”, so there will be no one to complete the DNA on the empty space left after removing the primer.

Here's another drawing for you. The black DNA strand is old, maternal. DNA duplication, unlike the superpattern, occurs from left to right. Since the new (green) DNA on the right has a 5" end, it is lagging and is extended by individual fragments (Okazaki). Each Okazaki fragment grows from the 3" end of its primer (blue rectangle). Primers, as we remember, are removed by DNA polymerase, which at this point completes the next Okazaki fragment (this process is indicated by a red ellipsis). At the end of the chromosome there is no one to fill this section, since there is no next Okazaki fragment, there is already an empty space there (Gap). Thus, after each replication, both 5" ends of the daughter chromosomes are shortened (terminal underreplication).

Stem cells (in the skin, red bone marrow, testes) must divide much more than 60 times. Therefore, the enzyme telomerase functions in them, which lengthens telomeres after each replication. Telomerase extends the overhanging 3" end of the DNA so that it grows to the size of the Okazaki fragment. After this, primase synthesizes a primer on it, and DNA polymerase extends the under-replicated 5" end of the DNA.

Tests

1. Replication is a process in which:
A) transfer RNA synthesis occurs;
B) DNA synthesis (copying) occurs;
C) ribosomes recognize anticodons;
D) peptide bonds are formed.

2. Match the functions of enzymes involved in the replication of prokaryotes with their names.

3. During replication in eukaryotic cells, removal of primers
A) is carried out by an enzyme with only DNAase activity
B) forms Okazaki fragments
B) occurs only in lagging strands
D) occurs only in the nucleus

4. If you extract the DNA of bacteriophage fX174, you will find that it contains 25% A, 33% T, 24% G, and 18% C. How could you explain these results?
A) The results of the experiment are incorrect; there was an error somewhere.
B) One could assume that the percentage of A is approximately equal to that of T, which is also true for C and G. Therefore, Chargaff's rule is not violated, DNA is double-stranded and replicates semi-conservatively.
B) Since the percentages of A and T and, accordingly, C and G are different, DNA is a single strand; it is replicated by a special enzyme that follows a special replication mechanism with a single strand as a template.
D) Since neither A is equal to T, nor G is equal to C, then DNA must be single-stranded; it is replicated by synthesizing the complementary strand and using this double-stranded form as a template.

5. The diagram refers to double-stranded DNA replication. For each of squares I, II, III, select one enzyme that functions in this area.

A) Telomerase
B) DNA topoisomerase
B) DNA polymerase
D) DNA helicase
D) DNA ligase

6. A bacterial culture from a medium with a light nitrogen isotope (N-14) was transferred to a medium containing a heavy isotope (N-15) for a time corresponding to one division, and then returned to a medium with a light nitrogen isotope. Analysis of the DNA composition of bacteria after a period corresponding to two replications showed:

Options answer	DNA
	light	average	heavy
A	3/4	1/4	-
B	1/4	3/4	-
IN	-	1/2	1/2
G	1/2	1/2	-

7. One rare genetic disease is characterized by immunodeficiency, mental and physical retardation, and microcephaly. Suppose that in a DNA extract from a patient with this syndrome you found almost equal amounts of long and very short stretches of DNA. Which enzyme is most likely missing/defective in this patient?
A) DNA ligase
B) Topoisomerase
B) DNA polymerase
D) Helicase

8. The DNA molecule is a double helix containing four different types of nitrogenous bases. Which of the following statements regarding both replication and the chemical structure of DNA is correct?
A) The base sequences of the two strands are the same.
B) In a double strand of DNA, the content of purines is equal to the content of pyrimidines.
C) Both chains are synthesized in the 5’→3’ direction continuously.
D) The addition of the first base of the newly synthesized nucleic acid is catalyzed by DNA polymerase.
E) The error correction activity of DNA polymerase occurs in the 5’→3’ direction.

9. Most DNA polymerases also have the activity:
A) ligase;
B) endonuclease;
B) 5"-exonuclease;
D) 3"-exonuclease.

10. DNA helicase is a key DNA replication enzyme that unwinds double-stranded DNA into single-stranded DNA. An experiment to determine the properties of this enzyme is described below.

Which of the following statements regarding this experiment is correct?
A) The band appearing at the top of the gel is ssDNA only, 6.3 kb in size.
B) The band appearing at the bottom of the gel is 300bp labeled DNA.
B) If the hybridized DNA is treated with DNA helicase only and the reaction is carried out to completion, the arrangement of the bands looks like that shown in lane 3 in b.
D) If the hybridized DNA is treated with boiling only without helicase treatment, the band arrangement appears as shown in lane 2 in b.
E) If the hybridized DNA is treated with boiled helicase only, the band arrangement looks like that shown in lane 1 in b.

District Olympiad 2001
- All-Russian Olympiad 2001
- International Olympiad 2001
- International Olympiad 1991
- International Olympiad 2008
- District Olympiad 2008
- International Olympiad 2010
The full texts of these Olympiads can be found.