DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites

Figure 27.24. Topoisomerase II Mechanism. Topoisomerase II first binds one DNA duplex termed the G (for gate)
segment. The binding of ATP to the two N-terminal domains brings these two domains together. This conformational
change leads to the cleavage of both strands of the G segment and the binding of an additional DNA duplex, the T
segment. This T segment then moves through the break in the G segment and out the bottom of the enzyme. The
hydrolysis of ATP resets the enzyme with the G segment still bound.
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
So far, we have met many of the key players in DNA replication. Here, we ask, Where on the DNA molecule does
replication begin, and how is the double helix manipulated to allow the simultaneous use of the two strands as templates?
In E. coli, DNA replication starts at a unique site within the entire 4.8 × 106 bp genome. This origin of replication, called
the oriC locus, is a 245-bp region that has several unusual features (Figure 27.25). The oriC locus contains four repeats
of a sequence that together act as a binding site for an initiation protein called dnaA. In addition, the locus contains a
tandem array of 13-bp sequences that are rich in A-T base pairs.
The binding of the dnaA protein to the four sites initiates an intricate sequence of steps leading to the unwinding of the
template DNA and the synthesis of a primer. Additional proteins join dnaA in this process. The dnaB protein is a
helicase that utilizes ATP hydrolysis to unwind the duplex. The single-stranded regions are trapped by a single-stranded
binding protein (SSB). The result of this process is the generation of a structure called the prepriming complex, which
makes single-stranded DNA accessible for other enzymes to begin synthesis of the complementary strands.
27.4.1. An RNA Primer Synthesized by Primase Enables DNA Synthesis to Begin
Even with the DNA template exposed, new DNA cannot be synthesized until a primer is constructed. Recall that all
known DNA polymerases require a primer with a free 3 -hydroxyl group for DNA synthesis. How is this primer formed?
An important clue came from the observation that RNA synthesis is essential for the initiation of DNA synthesis. In fact,
RNA primes the synthesis of DNA. A specialized RNA polymerase called primase joins the prepriming complex in a
multisubunit assembly called the primosome. Primase synthesizes a short stretch of RNA (~5 nucleotides) that is
complementary to one of the template DNA strands (Figure 27.26). The primer is RNA rather than DNA because DNA
polymerases cannot start chains de novo. Recall that, to ensure fidelity, DNA polymerase tests the correctness of the
preceding base pair before forming a new phosphodiester bond (Section 27.2.4). RNA polymerases can start chains de
novo because they do not examine the preceding base pair. Consequently, their error rates are orders of magnitude as
high as those of DNA polymerases. The inge-nious solution is to start DNA synthesis with a low-fidelity stretch of
polynucleotide but mark it "temporary" by placing ribonucleotides in it. The RNA primer is removed by hydrolysis by a
5
3 exonuclease; in E. coli, the exonuclease is present as an additional domain of DNA polymerase I, rather than
being present in the Klenow fragment. Thus, the complete polymerase I has three distinct active sites: a 3
5
exonuclease proofreading activity, a polymerase activity, and a 5
3 exonuclease activity.
27.4.2. One Strand of DNA Is Made Continuously, Whereas the Other Strand Is
Synthesized in Fragments
Both strands of parental DNA serve as templates for the synthesis of new DNA. The site of DNA synthesis is called the
replication fork because the complex formed by the newly synthesized daughter strands arising from the parental duplex
resembles a two-pronged fork. Recall that the two strands are antiparallel; that is, they run in opposite directions. As
shown in Figure 27.3, both daughter strands appear to grow in the same direction on cursory examination. However, all
known DNA polymerases synthesize DNA in the 5
3 direction but not in the 3
5 direction. How then does one
of the daughter DNA strands appear to grow in the 3
5 direction?
This dilemma was resolved by Reiji Okazaki, who found that a significant proportion of newly synthesized DNA exists
as small fragments. These units of about a thousand nucleotides (called Okazaki fragments) are present briefly in the
vicinity of the replication fork (Figure 27.27). As replication proceeds, these fragments become covalently joined
through the action of DNA ligase (Section 27.4.3) to form one of the daughter strands. The other new strand is
synthesized continuously. The strand formed from Okazaki fragments is termed the lagging strand, whereas the one
synthesized without interruption is the leading strand. Both the Okazaki fragments and the leading strand are synthesized
in the 5
3 direction. The discontinuous assembly of the lagging strand enables 5
3 polymerization at the
nucleotide level to give rise to overall growth in the 3
5 direction.
27.4.3. DNA Ligase Joins Ends of DNA in Duplex Regions
The joining of Okazaki fragments requires an enzyme that catalyzes the joining of the ends of two DNA chains. The
existence of circular DNA molecules also points to the existence of such an enzyme. In 1967, scientists in several
laboratories simultaneously discovered DNA ligase. This enzyme catalyzes the formation of a phosphodiester bond
between the 3 hydroxyl group at the end of one DNA chain and the 5 -phosphate group at the end of the other (Figure
27.28). An energy source is required to drive this thermodynamically uphill reaction. In eukaryotes and archaea, ATP is
the energy source. In bacteria, NAD+ typically plays this role. We shall examine the mechanistic features that allow
these two molecules to power the joining of two DNA chains.
DNA ligase cannot link two molecules of single-stranded DNA or circularize single-stranded DNA. Rather, ligase seals
breaks in double-stranded DNA molecules. The enzyme from E. coli ordinarily forms a phosphodiester bridge only if
there are at least several base pairs near this link. Ligase encoded by T4 bacteriophage can link two blunt-ended doublehelical fragments, a capability that is exploited in recombinant DNA technology.
Let us look at the mechanism of joining, which was elucidated by I. Robert Lehman (Figure 27.29). ATP donates its
activated AMP unit to DNA ligase to form a covalent enzyme-AMP (enzyme-adenylate) complex in which AMP is linked
to the ε-amino group of a lysine residue of the enzyme through a phosphoamide bond. Pyrophosphate is concomitantly
released. The activated AMP moiety is then transferred from the lysine residue to the phosphate group at the 5 terminus
of a DNA chain, forming a DNA-adenylate complex. The final step is a nucleophilic attack by the 3 hydroxyl group at
the other end of the DNA chain on this activated 5 phosphorus atom.
In bacteria, NAD+ instead of ATP functions as the AMP donor. NMN is released instead of pyrophosphate. Two hightransfer-potential phosphoryl groups are spent in regenerating NAD+ from NMN and ATP when NAD+ is the adenylate
donor. Similarly, two high-transfer-potential phosphoryl groups are spent by the ATP-utilizing enzymes because the
pyrophosphate released is hydrolyzed. The results of structural studies revealed that the ATP- and NAD+-utilizing
enzymes are homologous even though this homology could not be deduced from their amino acid sequences alone.
27.4.4. DNA Replication Requires Highly Processive Polymerases
Enzyme activities must be highly coordinated to replicate entire genomes precisely and rapidly. A prime example is
provided by DNA polymerase III holoenzyme, the enzyme responsible for DNA replication in E. coli. The hallmarks of
this multisubunit assembly are its very high catalytic potency, fidelity, and processivity. Processivity refers to the ability
of an enzyme to catalyze many consecutive reactions without releasing its substrate. The holoenzyme catalyzes the
formation of many thousands of phosphodiester bonds before releasing its template, compared with only 20 for DNA
polymerase I. DNA polymerase III holoenzyme has evolved to grasp its template and not let go until the template has
been completely replicated. A second distinctive feature of the holoenzyme is its catalytic prowess: 1000 nucleotides are
added per second compared with only 10 per second for DNA polymerase I. This acceleration is accomplished with no
loss of accuracy. The greater catalytic prowess of polymerase III is largely due to its processivity; no time is lost in
repeatedly stepping on and off the template.
Processive enzyme
From the Latin procedere, "to go forward."
An enzyme that catalyzes multiple rounds of elongation or digestion
of a polymer while the polymer stays bound. A distributive enzyme,
in contrast, releases its polymeric substrate between successive
catalytic steps.
These striking features of DNA polymerase III do not come cheaply. The holoenzyme consists of 10 kinds of
polypeptide chains and has a mass of ~900 kd, nearly an order of magnitude as large as that of a single-chain DNA
polymerase, such as DNA polymerase I. This replication complex is an asymmetric dimer (Figure 27.30). The
holoenzyme is structured as a dimer to enable it to replicate both strands of parental DNA in the same place at the same
time. It is asymmetric because the leading and lagging strands are synthesized differently. A τ 2 subunit is associated
with one branch of the holoenzyme; γ 2 and (δ δ χ ψ)2 are associated with the other. The core of each branch is the
same, an α ε θ complex. The α subunit is the polymerase, and the ε subunit is the proofreading 3
Each core is catalytically active but not processive. Processivity is conferred by β 2 and τ 2.
5 exonuclease.
The source of the processivity was revealed by the determination of the three-dimensional structure of the β 2 subunit
(Figure 27.31). This unit has the form of a star-shaped ring. A 35-Å-diameter hole in its center can readily accommodate
a duplex DNA molecule, yet leaves enough space between the DNA and the protein to allow rapid sliding and turning
during replication. A catalytic rate of 1000 nucleotides polymerized per second requires the sliding of 100 turns of
duplex DNA (a length of 3400 Å, or 0.34 µm) through the central hole of β 2 per second. Thus, β plays a key role in
2
replication by serving as a sliding DNA clamp.
27.4.5. The Leading and Lagging Strands Are Synthesized in a Coordinated Fashion
The holoenzyme synthesizes the leading and lagging strands simultaneously at the replication fork (Figure 27.32). DNA
polymerase III begins the synthesis of the leading strand by using the RNA primer formed by primase. The duplex DNA
ahead of the polymerase is unwound by an ATP-driven helicase. Single-stranded binding protein again keeps the strands
separated so that both strands can serve as templates. The leading strand is synthesized continuously by polymerase III,
which does not release the template until replication has been completed. Topoisomerases II (DNA gyrase) concurrently
introduces right-handed (negative) supercoils to avert a topological crisis.
The mode of synthesis of the lagging strand is necessarily more complex. As mentioned earlier, the lagging strand is
synthesized in fragments so that 5
3 polymerization leads to overall growth in the 3
5 direction. A looping of
the template for the lagging strand places it in position for 5
3 polymerization (Figure 27.33). The looped laggingstrand template passes through the polymerase site in one subunit of a dimeric polymerase III in the same direction as
that of the leading-strand template in the other subunit. DNA polymerase III lets go of the lagging-strand template after
adding about 1000 nucleotides. A new loop is then formed, and primase again synthesizes a short stretch of RNA primer
to initiate the formation of another Okazaki fragment.
The gaps between fragments of the nascent lagging strand are then filled by DNA polymerase I. This essential enzyme
also uses its 5
3 exonuclease activity to remove the RNA primer lying ahead of the polymerase site. The primer
cannot be erased by DNA polymerase III, because the enzyme lacks 5
3 editing capability. Finally, DNA ligase
connects the fragments.
27.4.6. DNA Synthesis Is More Complex in Eukaryotes Than in Prokaryotes
Replication in eukaryotes is mechanistically similar to replication in prokaryotes but is more challenging for a number of
reasons. One of them is sheer size: E. coli must replicate 4.8 million base pairs, whereas a human diploid cell must
replicate 6 billion base pairs. Second, the genetic information for E. coli is contained on 1 chromosome, whereas, in
human beings, 23 pairs of chromosomes must be replicated. Finally, whereas the E. coli chromosome is circular, human
chromosomes are linear. Unless countermeasures are taken (Section 27.4.7), linear chromosomes are subject to
shortening with each round of replication.
The first two challenges are met by the use of multiple origins of replication, which are located between 30 and 300 kbp
apart. In human beings, replication requires about 30,000 origins of replication, with each chromosome containing
several hundred. Each origin of replication represents a replication unit, or replicon. The use of multiple origins of
replication requires mechanisms for ensuring that each sequence is replicated once and only once. The events of
eukaryotic DNA replication are linked to the eukaryotic cell cycle (Figure 27.34). In the cell cycle, the processes of DNA
synthesis and cell division (mitosis) are coordinated so that the replication of all DNA sequences is complete before the
cell progresses into the next phase of the cycle. This coordination requires several checkpoints that control the
progression along the cycle.
The origins of replication have not been well characterized in higher eukaryotes but, in yeast, the DNA sequence is
referred to as an autonomously replicating sequence (ARS) and is composed of an AT-rich region made up of discrete
sites. The ARS serves as a docking site for the origin of replication complex (ORC). The ORC is composed of six
proteins with an overall mass of ~400 kd. The ORC recruits other proteins to form the prereplication complex. Several of
the recruited proteins are called licensing factors because they permit the formation of the initiation complex. These
proteins serve to ensure that each replicon is replicated once and only once in a cell cycle. How is this regulation
achieved? After the licensing factors have established the initiation complex, these factors are marked for destruction by
the attachment of ubiquitin and subsequently destroyed by proteasomal digestion (Section 23.2.2).
DNA helicases separate the parental DNA strands, and the single strands are stabilized by the binding of replication
protein A, a single-stranded- DNA-binding protein. Replication begins with the binding of DNA polymerase α , which is
the initiator polymerase. This enzyme has primase activity, used to synthesize RNA primers, as well as DNA polymerase
activity, although it possesses no exonuclease activity. After a stretch of about 20 deoxynucleotides have been added to
the primer, another replication protein, called protein replication factor C (RFC), displaces DNA polymerase α and
attracts proliferating cell nuclear antigen (PCNA). Homologous to the β 2 subunit of E. coli polymerase III, PCNA then
binds to DNA polymerase δ. The association of polymerase δ with PCNA renders the enzyme highly processive and
suitable for long stretches of replication. This process is called polymerase switching because polymerase δ has replaced
polymerase α. Polymerase δ has 3
5 exonuclease activity and can thus edit the replicated DNA. Replication
continues in both directions from the origin of replication until adjacent replicons meet and fuse. RNA primers are
removed and the DNA fragments are ligated by DNA ligase.
27.4.7. Telomeres Are Unique Structures at the Ends of Linear Chromosomes
Whereas the genomes of essentially all prokaryotes are circular, the chromosomes of human beings and other eukaryotes
are linear. The free ends of linear DNA molecules introduce several complications that must be resolved by special
enzymes. In particular, it is difficult to fully replicate DNA ends, because polymerases act only in the 5
3 direction.
The lagging strand would have an incomplete 5 end after the removal of the RNA primer. Each round of replication
would further shorten the chromosome.
The first clue to how this problem is resolved came from sequence analyses of the ends of chromosomes, which are
called telomeres (from the Greek telos, "an end"). Telomeric DNA contains hundreds of tandem repeats of a
hexanucleotide sequence. One of the strands is G rich at the 3 end, and it is slightly longer than the other strand. In
human beings, the repeating G-rich sequence is AGGGTT.
The structure adopted by telomeres has been extensively investigated. Recent evidence suggests that they may form large
duplex loops (Figure 27.35). The single-stranded region at the very end of the structure has been proposed to loop back
to form a DNA duplex with another part of the repeated sequence, displacing a part of the original telomeric duplex. This
looplike structure is formed and stabilized by specific telomere-binding proteins. Such structures would nicely protect
and mask the end of the chromosome.
27.4.8. Telomeres Are Replicated by Telomerase, a Specialized Polymerase That
Carries Its Own RNA Template
How are the repeated sequences generated? An enzyme, termed telomerase, that executes this function has been purified
and characterized. When a primer ending in GGTT is added to the human enzyme in the presence of deoxynucleoside
triphosphates, the sequences GGTTAGGGTT and GGTTAGGGTTAGGGTT, as well as longer products, are generated.
Elizabeth Blackburn and Carol Greider discovered that the enzyme contains an RNA molecule that serves as the
template for elongation of the G-rich strand (Figure 27.36). Thus, the enzyme carries the information necessary to
generate the telomere sequences. The exact number of repeated sequences is not crucial.
Subsequently, a protein component of telomerases also was identified. From its amino acid sequence, this component is
clearly related to reverse transcriptases, enzymes first discovered in retroviruses that copy RNA into DNA. Thus,
telomerase is a specialized reverse transcriptase that carries its own template. Telomeres may play important roles in
cancer-cell biology and in cell aging.
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
Figure 27.25. Origin of Replication in E. coli. OriC has a length of 245 bp. It contains a tandem array of three nearly
identical 13-nucleotide sequences (green) and four binding sites (yellow) for the dnaA protein. The relative orientations
of the four dnaA sites are denoted by arrows.
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
Figure 27.26. Priming. DNA replication is primed by a short stretch of DNA that is synthesized by primase, an RNA
polymerase. The RNA primer is removed at a later stage of replication.
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
Figure 27.27. Okazaki Fragments. At a replication fork, both strands are synthesized in a 5
3 direction. The
leading strand is synthesized continuously, whereas the lagging strand is synthesized in short pieces termed Okazaki
fragments.
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
Figure 27.28. DNA Ligase Reaction. DNA ligase catalyzes the joining of one DNA strand with a free 3 -hydroxyl
group to another with a free 5 -phosphate group. In eukaryotes and archaea, ATP is cleaved to AMP and PPi to drive this
reaction. In bacteria, NAD+ is cleaved to AMP and nicotinamide mononucleotide (NMN).
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
Figure 27.29. DNA Ligase Mechanism. DNA ligation proceeds by the transfer of an AMP unit first to a lysine side
chain on DNA ligase and then to the 5 -phosphate group of the substrate. The AMP unit is released on formation of the
phosphodiester linkage in DNA.
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
Figure 27.30. Proposed Architecture of DNA Polymerase III Holoenzyme. [After A. Kornberg and T. Baker, DNA
Replication, 2d ed. (W. H. Freeman and Company, 1992).]
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
Figure 27.31. Structure of the Sliding Clamp. The dimeric β2 subunit of DNA polymerase III forms a ring that
surrounds the DNA duplex. It allows the polymerase enzyme to move without falling off the DNA substrate.
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
Figure 27.32. Replication Fork. Schematic representation of the enzymatic events at a replication fork in E. coli.
Enzymes shaded in yellow catalyze chain initiation, elongation, and ligation. The wavy lines on the lagging strand
denote RNA primers. [After A. Kornberg and T. Baker, DNA Replication, 2d ed. (W. H. Freeman and Company, 1992).]
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
Figure 27.33. Coordination between the Leading and the Lagging Strands. The looping of the template for the
lagging strand enables a dimeric DNA polymerase III holoenzyme to synthesize both daughter strands. The leading
strand is shown in red, the lagging strand in blue, and the RNA primers in green. [Courtesy of Dr. Arthur Kornberg.]
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
Figure 27.34. Eukaryotic Cell Cycle. DNA replication and cell division must take place in a highly coordinated fashion
in eukaryotes. Mitosis (M) takes place only after DNA synthesis (S). Two gaps (G1 and G2) in time separate the two
processes.
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
Figure 27.35. Proposed Model for Telomeres. A single-stranded segment of the G-rich strand extends from the end of
the telomere. In one model for telomeres, this single-stranded region invades the duplex to form a large duplex loop.
III. Synthesizing the Molecules of Life
27. DNA Replication, Recombination, and Repair
27.4. DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites
Figure 27.36. Telomere Formation. Mechanism of synthesis of the G-rich strand of telomeric DNA. The RNA
template of telomerase is shown in blue and the nucleotides added to the G-rich strand of the primer are shown in red.
[After E. H. Blackburn. Nature 350(1991):569.]