Dna Replication, Repair, And Recombination


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DNA REPLIC ATION, REPAIR, AND RECOMBINATION

5
THE MAINTENANCE OF DNA SEQUENCES DNA REPLICATION MECHANISMS THE INITIATION AND COMPLETION OF DNA REPLICATION IN CHROMOSOMES DNA REPAIR GENERAL RECOMBINATION SITE-SPECIFIC RECOMBINATION

The ability of cells to maintain a high degree of order in a chaotic universe depends upon the accurate duplication of vast quantities of genetic information carried in chemical form as DNA. This process, called DNA replication, must occur before a cell can produce two genetically identical daughter cells. Maintaining order also requires the continued surveillance and repair of this genetic information because DNA inside cells is repeatedly damaged by chemicals and radiation from the environment, as well as by thermal accidents and reactive molecules. In this chapter we describe the protein machines that replicate and repair the cell’s DNA. These machines catalyze some of the most rapid and accurate processes that take place within cells, and their mechanisms clearly demonstrate the elegance and efficiency of cellular chemistry.
While the short-term survival of a cell can depend on preventing changes in its DNA, the long-term survival of a species requires that DNA sequences be changeable over many generations. Despite the great efforts that cells make to protect their DNA, occasional changes in DNA sequences do occur. Over time, these changes provide the genetic variation upon which selection pressures act during the evolution of organisms.
We begin this chapter with a brief discussion of the changes that occur in DNA as it is passed down from generation to generation. Next, we discuss the cellular mechanisms—DNA replication and DNA repair—that are responsible for keeping these changes to a minimum. Finally, we consider some of the most intriguing ways in which DNA sequences are altered by cells, with a focus on DNA recombination and the movement of special DNA sequences in our chromosomes called transposable elements.
THE MAINTENANCE OF DNA SEQUENCES
Although the long-term survival of a species is enhanced by occasional genetic changes, the survival of the individual demands genetic stability. Only rarely do the cell’s DNA-maintenance processes fail, resulting in permanent change in the DNA. Such a change is called a mutation, and it can destroy an organism if it
235

occurs in a vital position in the DNA sequence. Before examining the mechanisms responsible for genetic stability, we briefly discuss the accuracy with which DNA sequences are maintained from one generation to the next.
Mutation Rates Are Extremely Low
The mutation rate, the rate at which observable changes occur in DNA sequences, can be determined directly from experiments carried out with a bacterium such as Escherichia coli—a resident of our intestinal tract and a commonly used laboratory organism. Under laboratory conditions, E. coli divides about once every 30 minutes, and a very large population—several billion—can be obtained from a single cell in less than a day. In such a population, it is possible to detect the small fraction of bacteria that have suffered a damaging mutation in a particular gene, if that gene is not required for the survival of the bacterium. For example, the mutation rate of a gene specifically required for cells to utilize the sugar lactose as an energy source can be determined (using indicator dyes to identify the mutant cells), if the cells are grown in the presence of a different sugar, such as glucose. The fraction of damaged genes is an underestimate of the actual mutation rate because many mutations are silent (for example, those that change a codon but not the amino acid it specifies, or those that change an amino acid without affecting the activity of the protein coded for by the gene). After correcting for these silent mutations, a single gene that encodes an average-sized protein (~103 coding nucleotide pairs) is estimated to suffer a mutation (not necessarily one that would inactivate the protein) once in about 106 bacterial cell generations. Stated in a different way, bacteria display a mutation rate of 1 nucleotide change per 109 nucleotides per cell generation.
The germ-line mutation rate in mammals is more difficult to measure directly, but estimates can be obtained indirectly. One way is to compare the amino acid sequences of the same protein in several species. The fraction of the amino acids that are different between any two species can then be compared with the estimated number of years since that pair of species diverged from a common ancestor, as determined from the fossil record. Using this method, one can calculate the number of years that elapse, on average, before an inherited change in the amino acid sequence of a protein becomes fixed in an organism. Because each such change usually reflects the alteration of a single nucleotide in the DNA sequence of the gene encoding that protein, this value can be used to estimate the average number of years required to produce a single, stable mutation in the gene.
These calculations will nearly always substantially underestimate the actual mutation rate, because many mutations will spoil the function of the protein and vanish from the population because of natural selection—that is, by the preferential death of the organisms that contain them. But there is one family of protein fragments whose sequence does not seem to matter, allowing the genes that encode them to accumulate mutations without being selected against. These are the fibrinopeptides, 20 amino-acid fragments that are discarded from the protein fibrinogen when it is activated to form fibrin during blood clotting. Since the function of fibrinopeptides apparently does not depend on their amino acid sequence, they can tolerate almost any amino acid change. Sequence comparisons of the fibrinopeptides indicate that a typical protein 400 amino acids long would be randomly altered by an amino acid change in the germ line roughly once every 200,000 years.
Another way to estimate mutation rates is to use DNA sequencing to compare corresponding nucleotide sequences from different species in regions of the genome that do not carry critical information. Such comparisons produce estimates of the mutation rate that are in good agreement with those obtained from the fibrinopeptide studies.
E. coli and humans differ greatly in their modes of reproduction and in their generation times. Yet, when the mutation rates of each are normalized to a single round of DNA replication, they are found to be similar: roughly 1 nucleotide change per 109 nucleotides each time that DNA is replicated.
236 Chapter 5 : DNA REPLICATION, REPAIR, AND RECOMBINATION

Figure 5–1 Different proteins evolve at very different rates. A comparison of the rates of amino acid change found in hemoglobin, histone H4, cytochrome c, and the fibrinopeptides.The first three proteins have changed much more slowly during evolution than the fibrinopeptides, the number in parentheses indicating how many million years it has taken, on average, for one acceptable amino acid change to appear for every 100 amino acids that the protein contains. In determining rates of change per year, it is important to realize that two species that diverged from a common ancestor 100 million years ago are separated from each other by 200 million years of evolutionary time.
Many Mutations in Proteins Are Deleterious and Are Eliminated by Natural Selection
When the number of amino acid differences in a particular protein is plotted for several pairs of species against the time that has elapsed since the pair of species diverged from a common ancestor, the result is a reasonably straight line: the longer the period since divergence, the larger the number of differences. For convenience, the slope of this line can be expressed in terms of a “unit evolutionary time” for that protein, which is the average time required for 1 amino acid change to appear in a sequence of 100 amino acid residues. When various proteins are compared, each shows a different but characteristic rate of evolution (Figure 5–1).
Since most DNA nucleotides are thought to be subject to roughly the same rate of random mutation, the different rates observed for different proteins must reflect differences in the probability that an amino acid change will be harmful for each protein. For example, from the data in Figure 5–1, we can estimate that about six of every seven random amino acid changes are harmful in cytochrome c, and that virtually all amino acid changes are harmful in histone H4. The individual animals that carried such harmful mutations were presumably eliminated from the population by natural selection.
Low Mutation Rates Are Necessary for Life as We Know It
Since so many mutations are deleterious, no species can afford to allow them to accumulate at a high rate in its germ cells. Although the observed mutation frequency is low, it is nevertheless thought to limit the number of essential proteins that any organism can encode to perhaps 60,000. By an extension of the same argument, a mutation frequency tenfold higher would limit an organism to about 6000 essential genes. In this case, evolution would probably have stopped at an organism less complex than a fruit fly.
Whereas germ cells must be protected against high rates of mutation to maintain the species, the somatic cells of multicellular organisms must be protected from genetic change to safeguard each individual. Nucleotide changes in somatic cells can give rise to variant cells, some of which, through natural selection, proliferate rapidly at the expense of the rest of the organism. In an extreme case, the result is an uncontrolled cell proliferation known as cancer, a disease that causes about 30% of the deaths each year in Europe and North America. These deaths are due largely to an accumulation of changes in the DNA sequences of somatic cells (discussed in Chapter 23). A significant increase in the mutation frequency would presumably cause a disastrous increase in the incidence of cancer by accelerating the rate at which somatic cell variants arise. Thus, both for the perpetuation of a species with a large number of genes (germ cell stability) and for the prevention of cancer resulting from mutations in somatic cells (somatic cell stability), multicellular organisms like ourselves depend on the remarkably high fidelity with which their DNA sequences are maintained.
As we see in subsequent sections, successful DNA maintenance depends both on the accuracy with which DNA sequences are duplicated and distributed to daughter cells, and on a set of enzymes that repair most of the changes in the DNA caused by radiation, chemicals, or other accidents.
THE MAINTENANCE OF DNA SEQUENCES

amino acid changes per 100 amino acids

mamma

bird ls from m

ma sm from

amma

repti mals frreptiles ls carples fro om

200

fibrinopeptides (0.7)

160

(5)

bin

120

glo

o

hem

80

40 cytochrome c (21) histone H4 (500)
200 400 600 800 1000
millions of years since divergence of species

237

from m fis reptiles vertebra lamprhes
te e

s from y

insects

Summary
In all cells, DNA sequences are maintained and replicated with high fidelity. The mutation rate, approximately 1 nucleotide change per 109 nucleotides each time the DNA is replicated, is roughly the same for organisms as different as bacteria and humans. Because of this remarkable accuracy, the sequence of the human genome (approximately 3 ¥ 109 nucleotide pairs) is changed by only about 3 nucleotides each time a cell divides. This allows most humans to pass accurate genetic instructions from one generation to the next, and also to avoid the changes in somatic cells that lead to cancer.

DNA REPLICATION MECHANISMS
All organisms must duplicate their DNA with extraordinary accuracy before each cell division. In this section, we explore how an elaborate “replication machine” achieves this accuracy, while duplicating DNA at rates as high as 1000 nucleotides per second.
Base-Pairing Underlies DNA Replication and DNA Repair
As discussed briefly in Chapter 1, DNA templating is the process in which the nucleotide sequence of a DNA strand (or selected portions of a DNA strand) is copied by complementary base-pairing (A with T, and G with C) into a complementary DNA sequence (Figure 5–2). This process entails the recognition of each nucleotide in the DNA template strand by a free (unpolymerized) complementary nucleotide, and it requires that the two strands of the DNA helix be separated. This separation allows the hydrogen-bond donor and acceptor groups on each DNA base to become exposed for base-pairing with the appropriate incoming free nucleotide, aligning it for its enzyme-catalyzed polymerization into a new DNA chain.
The first nucleotide polymerizing enzyme, DNA polymerase, was discovered in 1957. The free nucleotides that serve as substrates for this enzyme were found to be deoxyribonucleoside triphosphates, and their polymerization into DNA required a single-stranded DNA template. The stepwise mechanism of this reaction is illustrated in Figures 5–3 and 5–4.
The DNA Replication Fork Is Asymmetrical
During DNA replication inside a cell, each of the two old DNA strands serves as a template for the formation of an entire new strand. Because each of the two daughters of a dividing cell inherits a new DNA double helix containing one old and one new strand (Figure 5–5), the DNA double helix is said to be replicated “semiconservatively” by DNA polymerase. How is this feat accomplished?
Analyses carried out in the early 1960s on whole replicating chromosomes revealed a localized region of replication that moves progressively along the parental DNA double helix. Because of its Y-shaped structure, this active region

template S strand 5¢

S strand 5¢
3¢ S¢ strand parent DNA double helix





new S¢ strand



new S strand



3¢ template S¢ strand

238 Chapter 5 : DNA REPLICATION, REPAIR, AND RECOMBINATION

Figure 5–2 The DNA double helix 3¢ acts as a template for its own
duplication. Because the nucleotide A 5¢ will successfully pair only with T, and G
only with C, each strand of DNA can serve as a template to specify the sequence of nucleotides in its complementary strand by DNA base3¢ pairing. In this way, a double-helical DNA molecule can be copied precisely.


O _
O O
_O O O O
_O O O O
_O O O O
_O O O O

Figure 5–3 The chemistry of DNA synthesis. The addition of a deoxyribonucleotide to the 3¢ end of a polynucleotide chain (the primer strand) is the fundamental reaction by which DNA is synthesized. As shown, base-pairing between an incoming deoxyribonucleoside triphosphate and an existing strand of DNA (the template strand) guides the formation of the new strand of DNA and causes it to have a complementary nucleotide sequence.

5¢ end of strand
O _ OPO
O H2C O

primer strand

O _ OPO
O H2C O

C A

OH

C 3¢ end of strand

OOO

_

O P O P O P O CH2 O

_

_

_

OOO

pyrophosphate OH

incoming deoxyribonucleoside triphosphate

3¢ end of strand O

G T

CH2

P O
CH2

template strand

P O

CH2
G
P O

CH2
A
P O

Figure 5–4 DNA synthesis catalyzed by DNA polymerase. (A) As indicated, DNA

T

polymerase catalyzes the stepwise addition of a deoxyribonucleotide to the 3¢-OH end of a

polynucleotide chain, the primer strand, that is paired to a second template strand.The newly

synthesized DNA strand therefore polymerizes in the 5¢-to-3¢ direction as shown in the previous

figure. Because each incoming deoxyribonucleoside triphosphate must pair with the template

strand to be recognized by the DNA polymerase, this strand determines which of the four

possible deoxyribonucleotides (A, C, G, or T) will be added.The reaction is driven by a large,

favorable free-energy change, caused by the release of pyrophosphate and its subsequent

hydrolysis to two molecules of inorganic phosphate. (B) The structure of an E. coli DNA

polymerase molecule, as determined by x-ray crystallography. Roughly speaking, it resembles a

right hand in which the palm, fingers, and thumb grasp the DNA.This drawing illustrates a DNA

polymerase that functions during DNA repair, but the enzymes that replicate DNA have similar

features. (B, adapted from L.S. Beese,V. Derbyshire, and T.A. Steitz, Science 260:352–355, 1993.)

3¢ HO

5¢ triphosphate

incoming deoxyribonucleoside triphosphate 5¢

CH2
OP O
5¢ end of strand

incoming deoxyribonucleoside
triphosphate
5¢-to-3¢ direction of chain growth

3¢ HO


5¢ primer strand
template strand 3¢

3¢ HO

+ pyrophosphate


gap in helix
“fingers”

“thumb”



template

strand





primer strand

5¢ (A)

3¢ (B)

“palm”

DNA REPLICATION MECHANISMS

239

is called a replication fork (Figure 5–6). At a replication fork, the DNA of both new daughter strands is synthesized by a multienzyme complex that contains the DNA polymerase.
Initially, the simplest mechanism of DNA replication seemed to be the continuous growth of both new strands, nucleotide by nucleotide, at the replication fork as it moves from one end of a DNA molecule to the other. But because of the antiparallel orientation of the two DNA strands in the DNA double helix (see Figure 5–2), this mechanism would require one daughter strand to polymerize in the 5¢-to-3¢ direction and the other in the 3¢-to-5¢ direction. Such a replication fork would require two different DNA polymerase enzymes. One would polymerize in the 5¢-to-3¢ direction, where each incoming deoxyribonucleoside triphosphate carried the triphosphate activation needed for its own addition. The other would move in the 3¢-to-5¢ direction and work by so-called “head growth,” in which the end of the growing DNA chain carried the triphosphate activation required for the addition of each subsequent nucleotide (Figure 5–7). Although head-growth polymerization occurs elsewhere in biochemistry (see pp. 89–90), it does not occur in DNA synthesis; no 3¢-to-5¢ DNA polymerase has ever been found.
How, then, is overall 3¢-to-5¢ DNA chain growth achieved? The answer was first suggested by the results of experiments in the late 1960s. Researchers added highly radioactive 3H-thymidine to dividing bacteria for a few seconds, so that only the most recently replicated DNA—that just behind the replication fork— became radiolabeled. This experiment revealed the transient existence of pieces of DNA that were 1000–2000 nucleotides long, now commonly known as Okazaki fragments, at the growing replication fork. (Similar replication intermediates were later found in eucaryotes, where they are only 100–200 nucleotides long.) The Okazaki fragments were shown to be polymerized only in the 5¢-to-3¢chain direction and to be joined together after their synthesis to create long DNA chains.
A replication fork therefore has an asymmetric structure (Figure 5–8). The DNA daughter strand that is synthesized continuously is known as the leading strand. Its synthesis slightly precedes the synthesis of the daughter strand that is synthesized discontinuously, known as the lagging strand. For the lagging strand, the direction of nucleotide polymerization is opposite to the overall direction of DNA chain growth. Lagging-strand DNA synthesis is delayed because it must wait for the leading strand to expose the template strand on which each Okazaki fragment is synthesized. The synthesis of the lagging strand

REPLICATION
REPLICATION
REPLICATION
Figure 5–5 The semiconservative nature of DNA replication. In a round of replication, each of the two strands of DNA is used as a template for the formation of a complementary DNA strand.The original strands therefore remain intact through many cell generations.

replication forks
1 mm
240 Chapter 5 : DNA REPLICATION, REPAIR, AND RECOMBINATION

Figure 5–6 Two replication forks moving in opposite directions on a circular chromosome. An active zone of DNA replication moves progressively along a replicating DNA molecule, creating a Y-shaped DNA structure known as a replication fork: the two arms of each Y are the two daughter DNA molecules, and the stem of the Y is the parental DNA helix. In this diagram, parental strands are orange; newly synthesized strands are red. (Micrograph courtesy of Jerome Vinograd.)

3¢ 5¢
OH 3¢ OH
HO 5¢
3¢ 5¢



sugar OH 3¢ base
5¢ triphosphate

Figure 5–7 An incorrect model for DNA replication. Although it might seem to be the simplest possible model for DNA replication, the mechanism illustrated here is not the one that cells use. In this scheme, both daughter DNA strands would grow continuously, using the energy of hydrolysis of the two terminal phosphates (yellow circles highlighted by red rays) to add the next nucleotide on each strand.This would require chain growth in both the 5¢-to-3¢ direction (top) and the 3¢-to-5¢ direction (bottom). No enzyme that catalyzes 3¢-to-5¢ nucleotide polymerization has ever been found.

by a discontinuous “backstitching” mechanism means that only the 5¢-to-3¢ type of DNA polymerase is needed for DNA replication.

The High Fidelity of DNA Replication Requires Several Proofreading Mechanisms
As discussed at the beginning of this chapter, the fidelity of copying DNA during replication is such that only about 1 mistake is made for every 109 nucleotides copied. This fidelity is much higher than one would expect, on the basis of the accuracy of complementary base-pairing. The standard complementary base pairs (see Figure 4–4) are not the only ones possible. For example, with small changes in helix geometry, two hydrogen bonds can form between G and T in DNA. In addition, rare tautomeric forms of the four DNA bases occur transiently in ratios of 1 part to 104 or 105. These forms mispair without a change in helix geometry: the rare tautomeric form of C pairs with A instead of G, for example.
If the DNA polymerase did nothing special when a mispairing occurred between an incoming deoxyribonucleoside triphosphate and the DNA template, the wrong nucleotide would often be incorporated into the new DNA chain, producing frequent mutations. The high fidelity of DNA replication, however, depends not only on complementary base-pairing but also on several “proofreading” mechanisms that act sequentially to correct any initial mispairing that might have occurred.
The first proofreading step is carried out by the DNA polymerase, and it occurs just before a new nucleotide is added to the growing chain. Our knowledge of this mechanism comes from studies of several different DNA polymerases, including one produced by a bacterial virus, T7, that replicates inside E. coli. The correct nucleotide has a higher affinity for the moving polymerase than does the incorrect nucleotide, because only the correct nucleotide can correctly base-pair with the template. Moreover, after nucleotide binding, but before the nucleotide is covalently added to the growing chain, the enzyme must undergo a conformational change. An incorrectly bound nucleotide is more likely to dissociate during this step than the correct one. This step therefore allows the polymerase to “double-check” the exact base-pair geometry before it catalyzes the addition of the nucleotide.









leading strand

5¢ 3¢ 3¢

most recently



synthesized

5¢ 3¢





DNA

5¢ 3¢



5¢ 3¢

lagging strand with



Okazaki fragments

5¢ 5¢

Figure 5–8 The structure of a DNA replication fork. Because both daughter DNA strands are polymerized in the 5¢-to-3¢ direction, the DNA synthesized on the lagging strand must be made initially as a series of short DNA molecules, called Okazaki fragments.

DNA REPLICATION MECHANISMS

241

The next error-correcting reaction, known as exonucleolytic proofreading, takes place immediately after those rare instances in which an incorrect nucleotide is covalently added to the growing chain. DNA polymerase enzymes cannot begin a new polynucleotide chain by linking two nucleoside triphosphates together. Instead, they absolutely require a base-paired 3¢-OH end of a primer strand on which to add further nucleotides (see Figure 5–4). Those DNA molecules with a mismatched (improperly base-paired) nucleotide at the 3¢-OH end of the primer strand are not effective as templates because the polymerase cannot extend such a strand. DNA polymerase molecules deal with such a mismatched primer strand by means of a separate catalytic site (either in a separate subunit or in a separate domain of the polymerase molecule, depending on the polymerase). This 3¢-to-5¢ proofreading exonuclease clips off any unpaired residues at the primer terminus, continuing until enough nucleotides have been removed to regenerate a base-paired 3¢-OH terminus that can prime DNA synthesis. In this way, DNA polymerase functions as a “self-correcting” enzyme that removes its own polymerization errors as it moves along the DNA (Figures 5–9 and 5–10).
The requirement for a perfectly base-paired primer terminus is essential to the self-correcting properties of the DNA polymerase. It is apparently not possible for such an enzyme to start synthesis in the complete absence of a primer without losing any of its discrimination between base-paired and unpaired growing 3¢-OH termini. By contrast, the RNA polymerase enzymes involved in gene transcription do not need efficient exonucleolytic proofreading: errors in making RNA are not passed on to the next generation, and the occasional defective RNA molecule that is produced has no long-term significance. RNA polymerases are thus able to start new polynucleotide chains without a primer.
An error frequency of about 1 in 104 is found both in RNA synthesis and in the separate process of translating mRNA sequences into protein sequences. This level of mistakes is 100,000 times greater than that in DNA replication, where a series of proofreading processes makes the process remarkably accurate (Table 5–1).

Only DNA Replication in the 5¢-to-3¢ Direction Allows Efficient Error Correction
The need for accuracy probably explains why DNA replication occurs only in the 5¢-to-3¢ direction. If there were a DNA polymerase that added deoxyribonucleoside triphosphates in the 3¢-to-5¢ direction, the growing 5¢-chain end, rather than the incoming mononucleotide, would carry the activating triphosphate. In this





3¢ P
E

3¢ 5¢

P
E 3¢

primer strand
3¢ 5¢
template strand

primer strand

OH C*



OH

TTTT

C OH

AAAAAAAAA 3¢

template strand

rare tautomeric form of C (C*) happens to base-pair with A and is thereby incorporated by DNA polymerase into the primer strand

OH T T T T C*
AAAAAAAAA

rapid tautomeric shift of C* to normal cytosine (C) destroys its base-pairing with A
OH
TTTT C AAAAAAAAA

unpaired 3¢-OH end of primer blocks further elongation of primer strand by DNA polymerase

TTTT

OH C

T OH

AAAAAAAAA

OH C

3¢-to-5¢ exonuclease activity attached to DNA polymerase chews back to create a basepaired 3¢-OH end on the primer strand

OH TTTT
AAAAAAAAA

DNA polymerase continues the process of adding nucleotides to the base-paired 3¢-OH end of the primer strand
OH T OH TTTTT AAAAAAAAA

T OH

POLYMERIZING

EDITING

Figure 5–10 Editing by DNA polymerase. Outline of the structures of DNA polymerase complexed with the DNA template in the polymerizing mode (left) and the editing mode (right). The catalytic site for the exonucleolytic (E) and the polymerization (P) reactions are indicated.To determine these structures by x-ray crystallography, researchers “froze” the polymerases in these two states, by using either a mutant polymerase defective in the exonucleolytic domain (right), or by withholding the Mg2+ required for polymerization (left).

Figure 5–9 Exonucleolytic proofreading by DNA polymerase during DNA replication. In this example, the mismatch is due to the incorporation of a rare, transient tautomeric form of C, indicated by an asterisk. But the same proofreading mechanism applies to any misincorporation at the growing 3¢-OH end.

242 Chapter 5 : DNA REPLICATION, REPAIR, AND RECOMBINATION

TABLE 5–1 The Three Steps That Give RiseTo High-fidelity DNA Synthesis

REPLICATION STEP

ERRORS PER NUCLEOTIDE POLYMERIZED

5¢Æ3¢ polymerization

1 ¥ 105

3¢Æ5¢ exonucleolytic proofreading

1 ¥ 102

Strand-directed mismatch repair

1 ¥ 102

Total

1 ¥ 109

The third step, strand-directed mismatch repair, is described later in this chapter.

case, the mistakes in polymerization could not be simply hydrolyzed away, because the bare 5¢-chain end thus created would immediately terminate DNA synthesis (Figure 5–11). It is therefore much easier to correct a mismatched base that has just been added to the 3¢ end than one that has just been added to the 5¢ end of a DNA chain. Although the mechanism for DNA replication (see Figure 5–8) seems at first sight much more complex than the incorrect mechanism depicted earlier in Figure 5–7, it is much more accurate because all DNA synthesis occurs in the 5¢-to-3¢ direction.
Despite these safeguards against DNA replication errors, DNA polymerases occasionally make mistakes. However, as we shall see later, cells have yet another chance to correct these errors by a process called strand-directed mismatch repair. Before discussing this mechanism, however, we describe the other types of proteins that function at the replication fork.



primer strand



PPP P P P P P P

HYPOTHETICAL 3¢-TO-5¢
STRAND GROWTH

PPP PP

PPP PP

ACTUAL 5¢-TO-3¢ STRAND GROWTH





PPP P P P P P P P P

Figure 5–11 An explanation for the 5¢-to-3¢ direction of DNA chain growth. Growth in the 5¢-to-3¢ direction, shown on the right, allows the chain to continue to be elongated when a mistake in polymerization has been removed by exonucleolytic proofreading (see Figure 5–9). In contrast, exonucleolytic proofreading in the hypothetical 3¢-to-5¢ polymerization scheme, shown on the left, would block further chain elongation. For convenience, only the primer strand of the DNA double helix is shown.

PPP

PROOFREADING P

5¢ end produced if one nucleotide
is removed by proofreading





3¢ end produced

PPPPPPP

when one nucleotide is removed by

proofreading

PPP

PPP

incoming correct deoxyribonucleotide
triphosphate

P P incoming correct deoxyribonucleotide triphosphate

PPP





PPPPPPPP

REACTION DOES NOT PROCEED, AS NO HIGH-ENERGY BOND WOULD BE CLEAVED

HIGH-ENERGY BOND IS CLEAVED, PROVIDING THE ENERGY FOR POLYMERIZATION

DNA REPLICATION MECHANISMS

243

A Special Nucleotide-Polymerizing Enzyme Synthesizes Short RNA Primer Molecules on the Lagging Strand
For the leading strand, a special primer is needed only at the start of replication: once a replication fork is established, the DNA polymerase is continuously presented with a base-paired chain end on which to add new nucleotides. On the lagging side of the fork, however, every time the DNA polymerase completes a short DNA Okazaki fragment (which takes a few seconds), it must start synthesizing a completely new fragment at a site further along the template strand (see Figure 5–8). A special mechanism is used to produce the base-paired primer strand required by this DNA polymerase molecule. The mechanism involves an enzyme called DNA primase, which uses ribonucleoside triphosphates to synthesize short RNA primers on the lagging strand (Figure 5–12). In eucaryotes, these primers are about 10 nucleotides long and are made at intervals of 100–200 nucleotides on the lagging strand.
The chemical structure of RNA was introduced in Chapter 1 and described in detail in Chapter 6. Here, we note only that RNA is very similar in structure to DNA. A strand of RNA can form base pairs with a strand of DNA, generating a DNA/RNA hybrid double helix if the two nucleotide sequences are complementary. The synthesis of RNA primers is thus guided by the same templating principle used for DNA synthesis (see Figures 1–5 and 5–2).
Because an RNA primer contains a properly base-paired nucleotide with a 3¢-OH group at one end, it can be elongated by the DNA polymerase at this end to begin an Okazaki fragment. The synthesis of each Okazaki fragment ends when this DNA polymerase runs into the RNA primer attached to the 5¢ end of the previous fragment. To produce a continuous DNA chain from the many DNA fragments made on the lagging strand, a special DNA repair system acts quickly to erase the old RNA primer and replace it with DNA. An enzyme called DNA ligase then joins the 3¢ end of the new DNA fragment to the 5¢ end of the previous one to complete the process (Figures 5–13 and 5–14).
Why might an erasable RNA primer be preferred to a DNA primer that would not need to be erased? The argument that a self-correcting polymerase cannot start chains de novo also implies its converse: an enzyme that starts chains anew cannot be efficient at self-correction. Thus, any enzyme that primes the synthesis of Okazaki fragments will of necessity make a relatively inaccurate copy (at least 1 error in 105). Even if the copies retained in the final product constituted as little as 5% of the total genome (for example, 10 nucleotides per 200nucleotide DNA fragment), the resulting increase in the overall mutation rate would be enormous. It therefore seems likely that the evolution of RNA rather than DNA for priming brought a powerful advantage to the cell: the ribonucleotides in the primer automatically mark these sequences as “suspect copy” to be efficiently removed and replaced.
Special Proteins Help to Open Up the DNA Double Helix in Front of the Replication Fork
For DNA synthesis to proceed, the DNA double helix must be opened up ahead of the replication fork so that the incoming deoxyribonucleoside triphosphates can form base pairs with the template strand. However, the DNA double helix is very stable under normal conditions; the base pairs are locked in place so strongly that temperatures approaching that of boiling water are required to separate the two strands in a test tube. For this reason, DNA polymerases and





3¢HO





RNA primer 3¢HO


DNA primase 5¢


Figure 5–12 RNA primer synthesis. A schematic view of the reaction catalyzed by DNA primase, the enzyme that synthesizes the short RNA primers made on the lagging strand using DNA as a template. Unlike DNA polymerase, this enzyme can start a new polynucleotide chain by joining two nucleoside triphosphates together.The primase synthesizes a short polynucleotide in the 5¢-to-3¢ direction and then stops, making the 3¢ end of this primer available for the DNA polymerase.

3¢ 5¢
laggingstrand template

RNA primer


new RNA primer synthesis by DNA primase







DNA polymerase adds to new RNA primer to start new Okazaki fragment



5¢ 3¢







DNA polymerase finishes DNA fragment









old RNA primer erased and replaced by DNA



Figure 5–13 The synthesis of one of the many DNA fragments on



the lagging strand. In eucaryotes, RNA primers are made at intervals

spaced by about 200 nucleotides on the lagging strand, and each RNA

primer is approximately 10 nucleotides long.This primer is erased by a

special DNA repair enzyme (an RNAse H) that recognizes an RNA strand in

an RNA/DNA helix and fragments it; this leaves gaps that are filled in by



DNA polymerase and DNA ligase.



5¢ 3¢
nick sealing by DNA ligase joins new Okazaki fragment to the growing chain
5¢ 3¢

244 Chapter 5 : DNA REPLICATION, REPAIR, AND RECOMBINATION

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Dna Replication, Repair, And Recombination