The order of alternating stages of the DNA reduplication process. DNA and genes. Appears during sexual reproduction
DNA is a reliable store of genetic information. But it must not only be kept safe, but also passed on to offspring. The survival of the species depends on this. After all, parents must pass on to their children everything that they have achieved in the course of evolution. It records everything: from the number of limbs to the color of the eyes. Of course, microorganisms have much less of this information, but it also needs to be transmitted. To do this, the cell divides. In order for genetic information to go to both daughter cells, it must be doubled, a process called “DNA replication.” It occurs before cell division, no matter which one. It could be a bacterium that has decided to multiply. Or it could be new skin growing at the site of the cut. The process of duplication of deoxyribonucleic acid must be clearly regulated and completed before cell division begins.
Where does doubling occur?
DNA replication occurs directly in the nucleus (in eukaryotes) or in the cytoplasm (in prokaryotes). Nucleic acid consists of nucleotides - adenine, thymine, cytosine and guanine. Both chains of the molecule are built according to the principle of complementarity: adenine in one chain corresponds to thymine, and guanine to cytosine. The doubling of the molecule must take place in such a way that the principle of complementarity is preserved in the daughter helices.
Start of replication - initiation
Deoxyribonucleic acid is a double-stranded helix. DNA replication occurs by adding daughter strands along each parent strand. For this synthesis to become possible, the spirals must be “unraveled” and the chains separated from each other. This role is played by helicase - it unwinds the helix of deoxyribonucleic acid, rotating at high speed. The beginning of DNA duplication cannot begin from any place; such a complex process requires a specific part of the molecule - the replication initiation site. Once the starting point for duplication has been determined and helicase has begun its work of unraveling the helix, the DNA strands move apart to form a replication fork. DNA polymerases sit on them. It is they who will synthesize the daughter chains.
Elongation
In one molecule of deoxyribonucleic acid, from 5 to 50 replication forks can form. The synthesis of daughter chains occurs simultaneously in several parts of the molecule. But it is not easy to complete the construction of complementary nucleotides. The nucleic acid chains are antiparallel to each other. The different directions of the parental chains affect duplication; this determines the complex mechanism of DNA replication. One of the chains is continuously completed by the child and is called the leading one. This is correct, because it is very convenient for polymerase to attach a free nucleotide to the 3’-OH end of the previous one. This synthesis occurs continuously, in contrast to the process on the second chain.
Lagging chain, O'Kazaki fragments
Difficulties arise with the other chain, because there the 5’ end is free, to which it is impossible to attach a free nucleotide. Then DNA polymerase acts from the other side. In order to complete the daughter chain, a primer is created that is complementary to the parent chain. It is formed at the replication fork itself. This is where the synthesis of a small piece begins, but along the “correct” path - the addition of nucleotides occurs at the 3’ end. Thus, the completion of the chain at the second daughter helix occurs discontinuously and has the direction opposite to the movement of the replication fork. These fragments were called O'Kazaki fragments and are about 100 nucleotides long. After the fragment has been built up to the previous finished piece, the primers are cut out by a special enzyme, and the cut site is filled with the missing nucleotides.
Termination
Doubling is completed when both chains have completed their daughter chains, and all O’Kazaki fragments are sewn together. In eukaryotes, DNA replication ends when replication forks meet each other. But in prokaryotes, this molecule is circular, and the process of doubling occurs without first breaking the chain. It turns out that all deoxyribonucleic acid is one large replicon. And duplication ends when the replication forks meet on the opposite side of the ring. After replication is complete, both strands of the parent deoxyribonucleic acid must be linked back together, after which both molecules are twisted to form supercoils. Next, both DNA molecules are methylated at adenine in the -GATC- region. This does not separate the chains or interfere with their complementarity. This is necessary for the folding of molecules into chromosomes, as well as for the regulation of gene reading.
Replication speed and accuracy
The second stage of DNA doubling (elongation) occurs at a speed of about 700 nucleotides per second. If we remember that there are 10 pairs of monomers per turn of nucleic acid, it turns out that during “unwinding” the molecule rotates at a frequency of 70 revolutions per second. For comparison: the rotation speed of the cooler in the computer system unit is approximately 500 revolutions per second. But despite its high speed, DNA polymerase almost never makes mistakes. After all, she simply selects complementary nucleotides. But even if it makes a mistake, DNA polymerase recognizes it, takes a step back, tears off the incorrect monomer and replaces it with the correct one. The mechanism of DNA replication is very complex, but we were able to understand the main points. It is important to understand its significance for both microorganisms and multicellular creatures.
A DNA molecule is a structure found on a chromosome. One chromosome contains one such molecule, consisting of two strands. DNA reduplication is the transfer of information after self-reproduction of strands from one molecule to another. It is inherent in both DNA and RNA. This article discusses the process of DNA reduplication.
General information and types of DNA synthesis
It is known that the threads in the molecule are twisted. However, when the process of DNA reduplication begins, they despiral, then move apart, and a new copy is synthesized on each one. Upon completion, two absolutely identical molecules appear, each of which contains a mother and daughter threads. This synthesis is called semi-conservative. The DNA molecules move away, while remaining in a single centromere, and finally separate only when the process of division begins at this centromere.
Another type of synthesis is called reparative. It, unlike the previous one, is not associated with any cellular stage, but begins when DNA damage occurs. If they are too extensive, the cell will eventually die. However, if the damage is local, it can be restored. Depending on the problem, one or two DNA strands can be restored. This, as it is also called, unscheduled synthesis does not take a long time and does not require large energy costs.
But when DNA reduplication occurs, a lot of energy and material are consumed, and its duration lasts for hours.
Reduplication is divided into three periods:
- initiation;
- elongation;
- termination.
Let's take a closer look at this DNA reduplication sequence.
Initiation
Human DNA contains several tens of millions of nucleotide pairs (animals have only one hundred and nine). DNA reduplication begins at many places in the chain for the following reasons. Around the same time, transcription occurs in RNA, but it stops in some specific places during DNA synthesis. Therefore, before such a process, a sufficient amount of substance accumulates in the cytoplasm of the cell in order to support gene expression and so that the vital activity of the cell is not disrupted. Because of this, the process must proceed as quickly as possible. Broadcasting is carried out during this period, but transcription is not carried out. Studies have shown that DNA reduplication occurs at several thousand points at once - small areas with a specific nucleotide sequence. They are joined by special initiation proteins, which in turn are joined by other DNA replication enzymes.
The DNA fragment where synthesis occurs is called a replicon. It starts from the origin and ends when the enzyme completes replication. Replicon is autonomous and also supplies the entire process with its own support.
The process may not begin from all points at once, somewhere it begins earlier, somewhere later; can flow in one or two opposite directions. Events occur in the following order when formed:
- replication fork;
- RNA primer.
Replication fork
This part is the process by which deoxyribonucleic strands are synthesized on detached DNA strands. The forks form the so-called reduplication eye. The process is preceded by a number of actions:
- release from binding to histones in the nucleosome - DNA replication enzymes such as methylation, acetylation and phosphorylation produce chemical reactions as a result of which proteins lose their positive charge, which promotes their release;
- despiralization is unwinding, which is necessary for further release of the threads;
- breaking hydrogen bonds between DNA strands;
- their divergence in different directions of the molecule;
- fixation occurring with the help of SSB proteins.
RNA primer
Synthesis is carried out by an enzyme called DNA polymerase. However, it cannot start it on its own, so this is done by other enzymes - RNA polymerases, which are also called RNA primers. They are synthesized in parallel to deoxyribonucleic strands along Thus, initiation ends with the synthesis of two RNA primers on two DNA strands that are broken and move in different directions.
Elongation
This period begins with the addition of a nucleotide to the 3" end of the RNA seed, which is carried out by the already mentioned DNA polymerase. To the first, it attaches the second, third nucleotide, and so on. The bases of the new strand are connected to the mother strand. It is believed that the synthesis of the strand proceeds in the direction 5" - 3".
Where it occurs towards the replication fork, synthesis proceeds continuously and lengthens at the same time. Therefore, such a thread is called leading or leading. RNA primers are no longer formed on it.
However, on the opposite mother strand, DNA nucleotides continue to attach to the RNA primer, and the deoxyribonucleic strand is synthesized in the direction opposite to the reduplication fork. In this case, it is called lagging or lagging.
On the lagging strand, synthesis occurs in fragments, where at the end of one section, synthesis begins at another nearby section using the same RNA primer. Thus, on the lagging strand there are two fragments that are connected by DNA and RNA. They are called Okazaki fragments.
Then everything is repeated. Then another turn of the helix unwinds, the hydrogen bonds are broken, the threads move apart, the leading strand lengthens, the next fragment of the RNA primer is synthesized on the lagging one, after which the Okazaki fragment is synthesized. After this, the RNA primers on the lagging strand are destroyed, and the DNA fragments are combined into one. This happens simultaneously on this circuit:
- formation of new RNA primers;
- synthesis of Okazaki fragments;
- destruction of RNA primers;
- reconnection into one single chain.
Termination
The process continues until two replication forks meet or one of them reaches the end of the molecule. After the forks meet, the daughter DNA strands are joined by an enzyme. If the fork moves to the end of the molecule, DNA reduplication is completed with the help of special enzymes.
Correction
In this process, an important role is played by the control (or correction) of reduplication. All four types of nucleotides arrive at the site of synthesis, and through trial pairing, DNA polymerase selects those that are needed.
The desired nucleotide must be able to form as many hydrogen bonds as a similar nucleotide on the DNA template strand. In addition, there must be a certain constant distance between the sugar-phosphate backbones, corresponding to the three rings in the two bases. If the nucleotide does not meet these requirements, the connection will not occur.
Control is carried out before its inclusion in the chain and before the inclusion of the subsequent nucleotide. After this, a bond is formed in the sugar phosphate backbone.
Mutational variability
The DNA replication mechanism, despite the high percentage of accuracy, always has disturbances in the strands, generally called “gene mutations.” There is one error per thousand nucleotide pairs, which is called convariant reduplication.
It happens for various reasons. For example, with a high or too low concentration of nucleotides, cytosine deamination, the presence of mutagens in the area of synthesis, and more. In some cases, errors can be corrected by reparation processes; in others, correction becomes impossible.
If the damage is in an inactive location, the error will not have severe consequences when the DNA reduplication process occurs. The nucleotide sequence of a particular gene may appear with a pairing error. Then the situation is different, and the negative result can be both the death of this cell and the death of the entire organism. It should also be taken into account that they are based on mutational variability, which makes the gene pool more plastic.
Methylation
At the time of synthesis or immediately after it, methylation of the chains occurs. In humans, this process is believed to be necessary to form chromosomes and regulate gene transcription. In bacteria, this process serves to protect DNA from being cut by enzymes.
DNA replication- This is the process of its doubling before cell division. Sometimes they say "DNA reduplication". Duplication occurs in the S phase of the interphase of the cell cycle.
Obviously, self-copying of genetic material in living nature is necessary. Only in this way can the daughter cells formed during division contain the same amount of DNA as was originally in the original one. Thanks to replication, all genetically programmed structural and metabolic features are transmitted over a number of generations.
During cell division, each DNA molecule from a pair of identical ones goes into its daughter cell. This ensures accurate transmission of hereditary information.
DNA synthesis consumes energy, i.e. it is an energy-consuming process.
DNA replication mechanism
The DNA molecule itself (without duplication) is a double helix. During the process of reduplication, the hydrogen bonds between its two complementary strands are broken. And on each individual chain, which now serves as a template-matrix, a new chain complementary to it is built. In this way, two DNA molecules are formed. Each one gets one strand from its mother’s DNA, the second is newly synthesized. Therefore, the mechanism of DNA replication is semi-conservative(one chain is old, one is new). This replication mechanism was proven in 1958.
In a DNA molecule, the chains are antiparallel. This means that one thread goes in the direction from the 5" end to the 3", and the complementary one goes in the opposite direction. The numbers 5 and 3 indicate the numbers of carbon atoms in deoxyribose, which is part of each nucleotide. Through these atoms, nucleotides are linked to each other by phosphodiester bonds. And where one chain has 3" connections, the other has 5" connections, since it is inverted, that is, it goes in the other direction. For clarity, you can imagine that you put your hand on top of your hand, like a first-grader sitting at a desk.
The main enzyme that carries out the growth of a new strand of DNA can only do this in one direction. Namely: attach a new nucleotide only to the 3" end. Thus, synthesis can only proceed in the direction from 5" to 3".
The chains are antiparallel, which means that synthesis must proceed on them in different directions. If the DNA strands first completely diverged, and then a new complementary one was built on them, then this would not be a problem. In reality, the chains diverge in certain replication origins, and in these places on the matrices synthesis immediately begins.
The so-called replication forks. In this case, on one mother chain, synthesis proceeds in the direction of divergence of the fork, and this synthesis occurs continuously, without breaks. On the second template, synthesis proceeds in the opposite direction from the direction of divergence of the original DNA chains. Therefore, such reverse synthesis can only occur in pieces, which are called fragments of Okazaki. Later, such fragments are “stitched” together.
A daughter strand that replicates continuously is called leading, or leading. The one that is synthesized through Okazaki fragments is lagging or lagging, since fragmented replication is slower.
In the diagram, the parent DNA strands gradually diverge in the direction in which the leading daughter strand is synthesized. The synthesis of the lagging chain goes in the opposite direction of the divergence, so it is forced to be carried out in pieces.
Another feature of the main DNA synthesis enzyme (polymerase) is that it cannot begin synthesis itself, only continue. He needs seed or primer. Therefore, a small complementary section of RNA is first synthesized on the parent strand, and then the chain is extended using polymerase. Later the primers are removed and the holes are filled in.
In the diagram, the seeds are shown only on the lagging strand. In fact, they are also on the leading one. However, here you only need one primer per fork.
Since the maternal DNA strands do not always diverge from the ends, but at the points of initialization, it is not so much forks that are actually formed as eyes or bubbles.
Each bubble can have two forks, i.e. the chains will diverge in two directions. However, they can only do one thing. If, nevertheless, the divergence is bidirectional, then from the initialization point on one DNA strand, synthesis will proceed in two directions - forward and backward. In this case, continuous synthesis will be performed in one direction, and Okazaki fragments in the other.
Prokaryotic DNA is not linear, but has a circular structure and only one origin of replication.
The diagram shows the two strands of the parent DNA molecule in red and blue. Newly synthesized strands are shown in dotted lines.
In prokaryotes, DNA self-copying is faster than in eukaryotes. If the reduplication rate in eukaryotes is hundreds of nucleotides per second, then in prokaryotes it reaches a thousand or more.
Replication enzymes
DNA replication is ensured by a whole complex of enzymes called replisome. There are more than 15 enzymes and proteins of replication. The most significant ones are listed below.
The main replication enzyme is the already mentioned DNA polymerase(actually there are several different ones), which directly extends the chain. This is not the only function of the enzyme. The polymerase is able to “check” which nucleotide is trying to attach to the end. If it's not suitable, she deletes it. In other words, partial DNA repair, i.e., its correction of replication errors, occurs already at the stage of synthesis.
Nucleotides found in the nucleoplasm (or cytoplasm in bacteria) exist in the form of triphosphates, i.e. they are not nucleotides, but deoxynucleoside triphosphates (dATP, dTTP, dGTP, dCTP). They are similar to ATP, which has three phosphate residues, two of which are linked by a high-energy bond. When such bonds are broken, a lot of energy is released. Also, deoxynucleoside triphosphates have two high-energy bonds. The polymerase separates the last two phosphates and uses the released energy for the DNA polymerization reaction.
Enzyme helicase separates the template DNA strands by breaking the hydrogen bonds between them.
Since the DNA molecule is a double helix, breaking the bonds provokes even greater twisting. Imagine a rope of two ropes twisted relative to each other, and on one side you pull one end to the right, the other to the left. The woven part will curl even more and become tighter.
To eliminate such tension, it is necessary for the still unbroken double helix to quickly rotate around its axis, “resetting” the resulting superspiralization. However, this is too energy-consuming. Therefore, a different mechanism is implemented in cells. Enzyme topoisomerase breaks one of the threads, passes the second through the gap, and stitches the first again. This is how the resulting supercoils are eliminated.
The template DNA strands that have separated as a result of the action of the helicase try to connect again with their hydrogen bonds. To prevent this from happening, they take action DNA binding proteins. These are not enzymes in the sense that they do not catalyze reactions. Such proteins attach to the DNA strand along its entire length and prevent the complementary strands of the template DNA from closing.
Primers are synthesized RNA primase. And they are deleted exonuclease. After the primer is removed, the hole is filled in by another type of polymerase. However, in this case, individual sections of DNA are not stitched together.
Individual parts of the synthesized chain are cross-linked by a replication enzyme such as DNA ligase.
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DNA replication is the process of synthesis of a daughter molecule of deoxyribonucleic acid, which occurs during cell division on the matrix of the parent DNA molecule.
At the same time, the genetic material encrypted in DNA doubles and is divided between daughter cells.
DNA replication is carried out by an enzyme DNA polymerase.
The replication mechanism is based on enzymatic synthesis deoxyribonucleic acid (DNA)
The strict specificity of pairing of nitrogenous bases in the DNA molecule determines complementarity base sequences in two strands and provides high accuracy
According to Watson and Crick, the process of DNA replication involves:
1) gap hydrogen bonds and unraveling threads double helix;
2) synthesis on single strands of complementary chains.
As a result, two similar molecules arise from one double-stranded DNA, and in each of the daughter molecules one polynucleotide chain is the parent one, and the other is newly synthesized ( semi-conservative mechanism Replication).
Reduplication process:
Unwinding of a molecule's helix - separation of one chain from another into parts of a DNA molecule
The effect of the enzyme DNA polymerase on a molecule
Attaching complementary nucleotides to each DNA strand
Formation of two DNA molecules from one.
Functional unit of replication – replicon (beginning – initiation, end – completion). Once started, replication continues until all replicon will not be duplicated (doubled).
The growth of a polynucleotide chain occurs only from its 3" end, i.e. in the 5": 3 direction. The enzyme that catalyzes this reaction is DNA polymerase.
Replication fork asymmetrical. Of the two synthesized daughter DNA strands, one is built continuously, and the other intermittently. The first is called leading, or leading, chain, and the second - lagging behind.
Short RNA segments complementary to the template DNA strand serve as primers for the synthesis of fragments of the lagging strand. These RNA primers, consisting of approximately 10 nucleotides, are synthesized at certain intervals on a lagging strand template of ribonucleoside triphosphates in the 5" : 3" direction using an enzyme RNA primases.
RNA primers then they are extended with deoxynucleotides from the 3" end by DNA polymerase, which continues to grow until the chain being built reaches the RNA primer attached to the 5" end of the previous fragment. The fragments formed in this way (the so-called. fragments of Okazaki) the lagging chain has 1000-2000 deoxyribonucleotide residues in bacteria; in animal cells their length does not exceed 200 nucleotides.
To ensure the formation of a continuous DNA chain from many such fragments, a special DNA repair system comes into play, removing the RNA primer and replacing it with DNA. An enzyme completes the entire process DNA ligase, catalyzing the formation of a phosphodiester bond between the 3"-OH group of the new DNA fragment and the 5"-phosphate group of the previous fragment.
Unwinding the double helix and spaces. chain separation is carried out using several special proteins. Helicases unwind short sections of DNA located immediately before the replication fork.
Several molecules are attached to each of the separated chains DNA binding proteins which prevent the formation of complementary pairs and reverse reunification of chains.
When ring replicon(for example, in a plasmid) the described process is called. q-replication. Circular DNA molecules are twisted on themselves (supercoiled); when the double helix unwinds during replication, they must continuously rotate around their own axis. In this case, torsional stress occurs, which is eliminated by breaking one of the circuits. The two ends are then immediately reconnected to each other. This function is performed by an enzyme DNA topoisomerase .
DNA polymerase
DNA polymerase is an enzyme involved in DNA replication. Enzymes of this class catalyze the polymerization of deoxyribonucleotides along a chain of DNA nucleotides, which the enzyme “reads” and uses as a template. The type of a new nucleotide is determined by the principle of complementarity with the template from which it is read. The assembled molecule is complementary to the template monohelix and identical to the second component of the double helix.
There are DNA-dependent DNA polymerase, which uses one of the DNA strands as a template, and RNA-dependent DNA polymerase, which is also capable of reading information from RNA (reverse transcription).
DNA polymerase begins DNA replication by binding to a stretch of nucleotide chain. The average number of nucleotides added by DNA polymerase enzymes during one act of binding/dissociation with the template is called processivity.
DNA helicases
DNA helicases are enzymes that unwind a double-stranded DNA helix using energy from the hydrolysis of NTP triphosphates. The single-stranded DNA produced is involved in various processes such as replication, recombination, and repair. DNA helicases are essential for replication, repair, recombination, and transcription. Helicases are present in all organisms.
What carbohydrate is included in RNA nucleotides?
1) ribose2) glucose3) uracil4) deoxyribose
2) Polymers include:
1) starch, protein, cellulose 3) cellulose, sucrose, starch
2) protein, glycogen, fat 4) glucose, amino acid, nucleotide.
3) The scientist who discovered the cell:
1) R. Hooke; 3) T. Schwann
2); R. Brown 4) M. Schleiden
4. Find the correct continuation of the expression “photolysis of water occurs inside...”:
1) mitochondria on the walls of the cristae; 3) plastid, in the stroma;
2) plastids, in thylakoids; 4) EPS membranes.
5. During the light phase of photosynthesis, the plant uses light energy to produce:
1) ATP from ADP and F; 3) NADP + + H 2 -> NADP H;
2) Glucose and carbon dioxide; 4) O 2 from CO 2.
6.Dark reactions of photosynthesis occur in:
a) chloroplast stroma; c) thylakoid membranes;
b) ribosomes of chloroplasts; d) grains.
What do photosynthesis and the process of glucose oxidation have in common?
1) both processes occur in mitochondria;
2) both processes occur in chloroplasts;
3) as a result of these processes, glucose is formed;
4) as a result of these processes, ATP is formed.
8. As a result of what process are organic substances formed from inorganic ones?
1) protein biosynthesis; 3) ATP synthesis;
2) photosynthesis; 4) glycolysis.
9. The energetically valuable product of anaerobic glycolysis is two molecules:
1) lactic acid; 3) ATP;
2) pyruvic acid; 4) ethanol.
10. Which nucleotide is not part of DNA:
1) thymine; 2) uracil; 3) adenine; 4) cytosine
Appears during sexual reproduction
1) less variety of genotypes and phenotypes than with asexual
2) greater diversity of genotypes and phenotypes than with asexual
3) less viable offspring
4) offspring less adapted to the environment
Each new cell comes from the same one through its
1) division 3) mutation
2) adaptations 4) modifications
The formation of organs in the embryonic development of mammals occurs at the stage
1) blastula 3) crushing
2) neurula 4) gastrula
From what embryonic structures are the nervous system and epidermis of animal skin formed?
1) mesoderm 3) endoderm
2) ectoderm 4) blastometers
Nuclear division during reproduction occurs in
1) amoeba vulgaris 3) staphylococcus
2) cholera vibrio 4) anthrax bacillus
Parents' genetic information is combined into offspring during reproduction
1) budding 3) seeds
2) vegetative 4) spores
17. The number of chromosomes during sexual reproduction in each generation would double if the process had not formed during evolution:
18. The first anaphase of meiosis ends:
1) divergence to the poles of homologous chromosomes;
2) chromatid divergence;
3) formation of gametes;
4) crossing over.
19. Cell DNA carries information about the structure:
1) proteins, fats and carbohydrates; 3) amino acids;
2) proteins and fats; 4) enzymes.
20. The gene encodes information about the structure:
1) several proteins;
2) one of the complementary DNA strands;
3) amino acid sequence in one protein molecule;
4) one amino acid.
21. When one DNA molecule replicates, new chains are synthesized. Their number in two new molecules is equal to:
1) four; 2) two; 3) alone; 4) three.
22. If 20% of a DNA molecule consists of cytosine nucleotides, then the percentage of thymine nucleotides is equal to:
1) 40%; 2) 30%; 3) 10%; 4) 60%.
23.Broadcasting is the process:
1) formation of mRNA; 3) formation of a protein chain on the ribosome;
2) DNA doubling; 4) connections of t-RNA with amino acids.
24. What law will manifest itself in the inheritance of traits during crossing?
organisms with genotypes: Aa x Aa?
1) uniformity 3) linked inheritance
2) splitting 4) independent inheritance
25. Indicate the features of modification variability.
1) occurs suddenly
2) manifests itself in individual individuals of the species
3) changes are due to the reaction norm
4) manifests itself similarly in all individuals of the species
5) is adaptive in nature
6) passed on to offspring
Match the substances and structures involved in protein synthesis with their functions by placing the necessary letters next to the numbers.
Determine the sequence in which the DNA reduplication process occurs
A) unwinding of the helix of the molecule
B) the effect of enzymes on the molecule
C) separation of one chain from another into parts of a DNA molecule
D) attachment of complementary nucleotides to each DNA strand
D) the formation of two DNA molecules from one