12.1 DNA replication mechanisms and enzymes

DNA replication is the process by which a cell copies its entire genome before division, ensuring each daughter cell receives a complete set of genetic information. Understanding the enzymatic machinery behind replication is foundational for grasping how mutations arise, how cells maintain genomic integrity, and why replication errors can lead to disease.

This section covers the key enzymes involved in replication and walks through the mechanics of how leading and lagging strands are synthesized.

DNA Replication Enzymes

Essential Enzymes for DNA Replication

Each enzyme in the replication machinery has a specific job. Think of the replication fork as an assembly line where every protein must act in the right order.

• DNA polymerase catalyzes the synthesis of new DNA strands by adding nucleotides to the growing strand in the 5' to 3' direction. This directionality is absolute and drives much of the complexity at the replication fork.
  • In prokaryotes, DNA polymerase III is the primary replicative polymerase, while DNA polymerase I removes RNA primers and fills the resulting gaps.
  • In eukaryotes, DNA polymerase δ handles lagging strand synthesis and DNA polymerase ε handles the leading strand.
• Helicase unwinds the double-stranded DNA by breaking the hydrogen bonds between complementary base pairs, separating the two strands so they can each serve as templates. It uses energy from ATP hydrolysis to travel along the DNA and pry the strands apart.
• Primase synthesizes short RNA primers (roughly 8–12 nucleotides long) that provide a free 3'-OH group for DNA polymerase to extend. This is necessary because DNA polymerase cannot initiate synthesis de novo. It can only add nucleotides onto a pre-existing primer.
• Single-strand binding proteins (SSBs) stabilize the separated single strands after helicase unwinds them, preventing the strands from re-annealing or forming secondary structures before polymerase arrives.

Enzymes for Completing DNA Replication

Once the bulk of synthesis is done, additional enzymes clean up and finalize the new strands.

• DNA ligase joins the discontinuous Okazaki fragments on the lagging strand into a continuous strand. It does this by catalyzing the formation of a phosphodiester bond between the 3'-OH end of one fragment and the 5'-phosphate end of the next.
• Topoisomerase relieves the torsional strain and supercoiling that builds up ahead of the replication fork as helicase unwinds the DNA. Without topoisomerase, the DNA ahead of the fork would become overwound and eventually halt replication.
  • Topoisomerase I introduces a transient single-strand break, allows the DNA to swivel and relax, then reseals the nick. Found in both prokaryotes and eukaryotes.
  • Topoisomerase II (including bacterial gyrase) introduces a transient double-strand break, passes a segment of DNA through the gap, and reseals it. This is especially important in prokaryotes for removing positive supercoils ahead of the fork.
• RNase H (in eukaryotes) or DNA polymerase I (in prokaryotes) removes the RNA primers after Okazaki fragment synthesis, and the resulting gaps are filled with DNA before ligase seals the final nicks.

DNA Replication Process

Semiconservative Replication and Initiation

Semiconservative replication means that each daughter DNA molecule contains one original (parental) strand and one newly synthesized strand. This was demonstrated by the Meselson-Stahl experiment (1958), which used heavy $^{15}N$ nitrogen isotopes and CsCl density gradient centrifugation to distinguish parental from newly made DNA. After one round of replication in light $^{14}N$ medium, all DNA showed an intermediate density, ruling out both conservative and dispersive models.

Replication begins at a specific sequence called the origin of replication (ori).

• Prokaryotes typically have a single origin (e.g., oriC in E. coli), so replication proceeds bidirectionally from one point.
• Eukaryotes have multiple origins across each chromosome, allowing their much larger genomes to be replicated within a reasonable timeframe. Multiple replication forks converge and merge as synthesis proceeds.

At each origin, initiator proteins recognize and bind the ori sequence, locally melting (denaturing) the AT-rich region of the DNA. Helicase is then loaded, and the replication fork forms: a Y-shaped structure where the two parental strands are being unwound and copied simultaneously.

Leading and Lagging Strand Synthesis

Because DNA polymerase can only synthesize in the 5' to 3' direction, the two template strands at the fork are handled differently.

• Leading strand: The template here runs 3' to 5' toward the fork, so polymerase can follow the fork continuously. After a single RNA primer is laid down, DNA polymerase extends it smoothly in the same direction the fork is moving. This strand requires only one priming event.

• Lagging strand: The template runs 5' to 3' toward the fork, which means polymerase must work away from the fork. Synthesis happens in short bursts:

  1. Primase synthesizes an RNA primer on the lagging strand template.
  2. DNA polymerase extends the primer, synthesizing a stretch of DNA called an Okazaki fragment.
  3. When the polymerase reaches the RNA primer of the previous fragment, it stops.
  4. The RNA primer is removed (by RNase H/Pol I in prokaryotes, or RNase H/FEN1 in eukaryotes) and replaced with DNA.
  5. DNA ligase seals the nick between adjacent fragments, creating a continuous strand.

• Okazaki fragments are roughly 1,000–2,000 nucleotides in prokaryotes and 100–200 nucleotides in eukaryotes. The size difference reflects the different spacing of priming events and the overall speed of replication in each system.