Watson and Crick immediately saw the relationship of the double helix to genetic replication. They proposed that each strand of the chromosome serves as a template to specify a new, complementary DNA strand. A template is a pattern for making something; DNA acts as a template because each strand specifies the new daughter strand by base‐pairing. This template feature makes DNA replication semiconservative: after replication, each daughter chromosome has one strand of newly synthesized DNA and one strand of DNA from the parental chromosome. See Figure 1 .
These enzymes copy DNA sequences by using one strand as a template. The reaction catalyzed by DNA polymerases is the addition of deoxyribonucleotides to a DNA chain by using dNTPs as substrates, as shown in Figure 2.
Figure 2
All DNA polymerases require a template strand, which is copied. DNA polymerases also require a primer, which is complementary to the template. The reaction of DNA polymerases is thus better understood as the addition of nucleotides to a primer to make a sequence complementary to a template. The requirement for template and primer are exactly what would be expected of a replication enzyme. Because DNA is the information store of the cell, any ability of DNA polymerases to make DNA sequences from nothing would lead to the degradation of the cell's information copy.
More than one DNA polymerase exists in each cell. The key distinction among the enzyme forms is their processitivity—how long a chain they synthesize before falling off the template. A DNA polymerase used in replication is more processive than a repair enzyme. The replication enzyme needs to make a long enough chain to replicate the entire chromosome. The repair enzyme needs only to make a long enough strand to replace the damaged sequences in the chromosome. The best‐studied bacterium, E. coli, has three DNA polymerase types.
DNA polymerase I (Pol I) is primarily a repair enzyme, although it also has a function in replication. About 400 Pol I molecules exist in a single bacterium. DNA polymerase I only makes an average of 20 phosphodiester bonds before dissociating from the template. These properties make good sense for an enzyme that is going to replace damaged DNA. Damage occurs at separate locations so the large number of Pol I molecules means that a repair enzyme is always close at hand.
DNA polymerase I has nucleolytic (depolymerizing) activities, which are an intimate part of their function. The 5′ to 3′ exonuclease activity removes base‐paired sequences ahead of the polymerizing activity. During replication, this can remove primers ahead of the polymerizing function of the polymerase. See Figure 3 .
Figure 3
Another intimate function of DNA polymerase I (and of the other forms of DNA polymerase found in E. coli) is the 3′ to 5′ exonuclease activity. This activity can de‐polymerize DNA starting from the newly synthesized end. Imagining why DNA polymerase would have an activity that opposes the action of the enzyme is a little difficult. The 3′ to 5′ exonuclease activity serves an editing function to ensure the fidelity of replication. Suppose DNA polymerase were to make a mistake and add a T opposite a G in the template strand. When the enzyme begins the next step of polymerization, the T is not properly paired with the template. The 3′ to 5′ exonucleolytic activity of DNA polymerase then removes the unpaired nucleotide, releasing TMP, until a properly paired stretch is detected. Then polymerization can resume. This cycle costs two high‐energy phosphate bonds because TTP is converted to TMP. While this may seem wasteful of energy, the editing process does keep the information store of the cell intact, as shown in Figure 4 .
DNA polymerase II is a specialized repair enzyme. Like Pol I, a large number of Pol II molecules reside in the cell (about 100). The enzyme is more processive than Pol I. Pol II has the same editing (3′ to 5′) activity as Pol I, but not the 5′ to 3′ exonuclease activity.
The actual replication enzyme in E. coli is DNA polymerase III. Its properties contrast with Pol I and Pol II in several respects. Pol III is much more processive than the other enzymes, making about 500,000 phosphodiester bonds on the average. In other words, it is about 5,000 times more processive than Pol I and 50 times more processive than Pol II. Pol III is a multisubunit enzyme. It lacks a 5′ to 3′ exonucleolytic activity, although a subunit of the enzyme carries out the editing (3′ to 5′) function during replication. Finally, only about 10 molecules of Pol III reside in each cell. This remains consistent with the function of Pol III in replication, because the chromosome only needs to be copied once per generation. Therefore, the cell only requires a few molecules of the enzyme. Pol III synthesizes DNA at least a hundred times more rapidly than the other polymerases. It can synthesize half of the bacterial chromosome in a little more than 20 minutes, which is the fastest that the bacterium can replicate.
Figure 4
Chromosomal replication
The process of chromosomal replication in bacteria is complex. Bacterial chromosomes are double‐stranded DNA and almost always circular. DNA replication starts at a specific sequence, the origin, on the chromosome and proceeds in two directions towards another specific region, the terminus, as shown in Figure 5 .
Figure 5
Initiation
At an origin, the replication process first involves DNA strand opening so that each strand of the DNA molecule is available as a template. Initiation is the rate‐limiting step for replication of the chromosome. Like other metabolic pathways, the control of replication is exerted at the first committed step.
Initiation sequences contain a set of repeated sequences, which bind the essential initiator protein, DnaA. The DnaA protein opens the helix to make a short region of separated strands. Then a specialized single‐strand binding protein binds to the DNA strands to keep them apart. This process makes a template, but replication can't happen because no primer yet exists. See Figure 6.
Figure 6
Chain initiation occurs when a specialized RNA polymerase enzyme called primase makes a short RNA primer. DNA polymerase III extends this RNA primer on both strands. Because DNA polymerase synthesizes DNA only in one direction (5′ to 3′), only one strand is copied in each direction (left and rightward in the next figure). At the end of the initiation process, two replication forks exist, going in opposite directions from the “bubble” at the origin of replication, as shown in Figure 7.
Figure 7
Elongation
Because only one strand can serve as a template for synthesis in the 5′ to 3′ direction (the template goes in the 3′ to 5′ direction, because the double helix is antiparallel), only one strand, the leading strand, can be elongated continuously. Ahead of the replication fork, DNA gyrase (topoisomerase II) helps unwind the DNA double helix and keep the double strands from tangling during replication.
Synthesis of the second (lagging) strand is more complicated because it is going in the wrong direction to serve as a template. No DNA polymerase exists to synthesize DNA in the 3′ to 5′ direction, so copying of the lagging strand is discontinuous—that is, short strands of DNA are made and subsequently matured by joining them together. An RNA primer, which is made by primase, initiates each of these small pieces of DNA. Then DNA polymerase III elongates the primer until it butts up against the 5′ end of the next primer molecule.
DNA polymerase I then uses its polymerizing and 5′ to 3′ exonuclease activities to remove the RNA primer and fill in this sequence with DNA. Because Pol I is not very processive, it falls off the lagging strand after a relatively short‐length synthesis. DNA polymerases can't seal up the nicks that result from the replacement of RNA primers with DNA. Instead, another enzyme, DNA ligase, seals off the nicks by using high energy phosphodiester bonds in ATP or NAD to join a free 3′ hydroxyl with an adjacent 5′ phosphate.
A multienzyme complex simultaneously carries out both leading and lagging strand replication. You can see the best model of the process in the next figure; the lagging strand may curl around so it presents the correct face to the enzyme. The two replication forks proceed around the chromosome, until they meet at the terminus. Termination is poorly defined biochemically, but it is known to require some form of DNA gyrase activity. See Figure 8.
Figure 8