DNA Replication and Recombination: Mechanisms and Enzymes

DNA Replication: Models and Experimental Evidence

Watson and Crick's Semiconservative Model

Watson and Crick proposed that DNA replicates in a semiconservative manner, meaning each daughter DNA molecule consists of one parental and one newly synthesized strand.

• Conservative replication: The parental molecule remains intact, and an entirely new molecule is synthesized.

• Dispersive replication: Parental and new DNA are interspersed in both strands after replication.

• Semiconservative replication: Each daughter molecule contains one old (parental) and one new strand.

Example: The semiconservative model is supported by experimental evidence and is the accepted mechanism for DNA replication in all organisms.

Meselson-Stahl Experiment

The Meselson-Stahl experiment (1958) provided definitive evidence for semiconservative replication in Escherichia coli using isotopic labeling and density gradient centrifugation.

• Bacteria were grown in a medium containing heavy nitrogen ($^{15}N$), then transferred to a medium with light nitrogen ($^{14}N$).

• After one round of replication, DNA had intermediate density, consistent only with the semiconservative model.

• Subsequent generations showed both intermediate and light DNA, further confirming the model.

Key Point: The experiment ruled out conservative and dispersive models.

Taylor, Woods, and Hughes Experiment

Taylor, Woods, and Hughes confirmed semiconservative replication in eukaryotes by labeling chromosomes and tracking the distribution of labeled DNA during cell division.

• Radioactive labeling of DNA in root tip cells of Vicia faba (broad bean) showed that each chromosome after replication contained one labeled and one unlabeled chromatid.

Key Questions in DNA Replication

Replication Initiation and Directionality

Critical questions in DNA replication include:

• Where does replication start? (Origin of replication)

• In what direction does it proceed? (Unidirectional or bidirectional)

In most organisms, replication is bidirectional from a defined origin.

Origins of Replication

Prokaryotic Origins

In E. coli, replication begins at a single origin called oriC.

• Initiator proteins bind to oriC, unwinding the DNA and recruiting other replication machinery.

• This results in the formation of two replication forks that proceed in opposite directions (bidirectional replication).

DNA Polymerases

Discovery and Functions in Prokaryotes

DNA polymerase I was the first discovered; it can synthesize DNA and has both 3'→5' and 5'→3' exonuclease activity.

• Mutants lacking DNA polymerase I can still replicate DNA, but are deficient in DNA repair, indicating the presence of other DNA polymerases.

Types of DNA Polymerases in E. coli

• Five DNA polymerases: I, II, III, IV, and V.

• DNA polymerase III: Main enzyme for DNA synthesis during replication.

• DNA polymerase I: Removes RNA primers and fills in gaps.

• Both have 3'→5' exonuclease activity for proofreading.

Mechanism of DNA Replication

Replication Fork and Replisome

The replication fork is the Y-shaped region where DNA is unwound and new strands are synthesized.

• Replisome: A large protein complex at the replication fork, including DNA polymerase III holoenzyme (10 subunits), clamp loader, helicase, primase, and single-stranded binding proteins (SSBPs).

Steps in DNA Replication

• Unwinding of DNA by helicase.

• Relief of supercoiling by topoisomerases (e.g., DNA gyrase).

• Synthesis of RNA primers by primase.

• Continuous synthesis on the leading strand; discontinuous synthesis (Okazaki fragments) on the lagging strand.

• Removal of RNA primers and gap filling by DNA polymerase I.

• Sealing of nicks by DNA ligase.

• Proofreading by 3'→5' exonuclease activity.

DNA Replication in Eukaryotes

Complexity and Multiple Origins

Eukaryotic DNA replication is fundamentally similar to prokaryotic replication but is more complex due to:

• Much larger genome size and multiple linear chromosomes.

• DNA packaged with histones and other proteins (chromatin).

• Multiple origins of replication per chromosome (up to 25,000 replicons per genome).

Chromatin Structure and Replicons

Each replicon is a unit of DNA replicated from a single origin. Chromatin structure influences the accessibility of origins and replication timing.

Eukaryotic DNA Polymerases

Eukaryotes have multiple DNA polymerases, each with specialized functions. The table below summarizes key polymerases and their proposed functions:

Polymerase	Human Name	Yeast Name	Proposed Function
Pol α	POLA1	POL1	Initiates DNA synthesis; synthesizes RNA primer and short DNA segment
Pol δ	POLD1	POL3	Lagging strand synthesis; DNA repair
Pol ε	POLE	POL2	Leading strand synthesis
Pol β	POLB	—	Base excision repair
Pol γ	POLG	MIP1	Mitochondrial DNA replication
Pol η, ι, κ, λ, μ, ν, θ, ζ, Rev1	Various	Various	Translesion synthesis, DNA repair

Additional info: Most eukaryotic DNA polymerases are multisubunit enzymes.

Polymerase Switching

• Pol α/primase complex initiates synthesis with an RNA primer and short DNA segment.

• Pol δ or Pol ε then takes over for elongation (polymerase switching).

Replicating Linear DNA and Telomeres

End-Replication Problem

Linear chromosomes pose a challenge for complete replication of the lagging strand ends, leading to progressive shortening.

• Telomerase is an enzyme that extends telomeres using an RNA template, preventing loss of genetic information.

Homologous Recombination

Mechanism and Importance

Homologous recombination is a process by which genetic material is exchanged between similar or identical DNA molecules, crucial for genetic diversity and DNA repair.

• Involves strand invasion, formation of a Holliday junction, and resolution to produce recombinant DNA molecules.

Example: Homologous recombination occurs during meiosis and in the repair of double-strand breaks.