DNA Replication: Main Goals and Mechanism

Overview of DNA Replication

DNA replication is the process by which a cell copies its entire genome before cell division, ensuring that each daughter cell receives an identical set of genetic information. This process is highly accurate and involves a coordinated effort of multiple enzymes and proteins.

• Goal: To produce two identical DNA molecules from one original DNA molecule.

• Fidelity: Errors during replication are corrected to prevent mutations.

• Key Players: DNA polymerases, helicases, primases, ligases, single-strand binding proteins, and topoisomerases.

The Meselson-Stahl Experiment: Evidence for Semiconservative Replication

The Meselson-Stahl experiment provided definitive evidence that DNA replication is semiconservative, meaning each daughter DNA molecule consists of one parental and one newly synthesized strand.

• Bacteria were grown in heavy (15N) and then light (14N) nitrogen media.

• After one replication, DNA had intermediate density; after two, both intermediate and light DNA were observed.

• This disproved conservative and dispersive models of replication.

Fundamental Rules and Features of DNA Replication

Universal Principles

• Origin of Replication: Replication begins at specific AT-rich sequences called origins.

• Bidirectional: Replication proceeds in both directions from the origin.

• Template Usage: The template strand is read 3' to 5'; new DNA is synthesized 5' to 3'.

• Primer Requirement: DNA polymerases require a primer with a free 3'-OH group (to build in 5' to 3' direction)

• Semi-discontinuous Synthesis: Leading strand is synthesized continuously; lagging strand is synthesized in Okazaki fragments.

Key Properties of DNA Polymerases

Primer Requirement and Catalytic Activity

• Primer: Short RNA strand synthesized by primase; provides a 3'-OH for extension.

• Polymerization: DNA polymerases add nucleotides to the 3' end using dNTPs as substrates.

• Active Site: Requires Mg2+ ions and conserved aspartate residues for catalysis.

Key Reaction:

Structure and Fidelity of DNA Polymerases

• Hand-shaped Structure: Composed of palm, thumb, and fingers domains.

• Two Active Sites: Insertion site (incoming nucleotide) and post-insertion site (new base pair).

• High Fidelity: Only correct base pairs fit; incorrect pairs are excluded.

Proofreading and Exonuclease Activities

• 3'→5' Exonuclease: Removes mismatched nucleotides (proofreading).

• 5'→3' Exonuclease: Removes RNA primers (nick translation, e.g., DNA pol I in E. coli).

DNA Replication in E. coli: Model System

Initiation and Termination

• Replication starts at oriC and ends at ter regions.

• Two replication forks form a theta (θ) structure and proceed bidirectionally.

• Daughter DNA circles are separated and distributed to daughter cells.

Enzymes and Proteins of the E. coli Replisome

DNA Polymerase III: Structure and Function

• Composed of three core units, each with its own catalytic domain.

• Each core interacts with a sliding clamp (β subunit dimer) for high processivity.

• Clamp loader complex (γ complex) links the cores.

Proofreading and Nick Translation

• DNA pol III proofreads via 3'→5' exonuclease activity, ensuring high fidelity (1 error per 109–1010 bp).

• DNA pol I removes RNA primers and replaces them with DNA (nick translation).

• DNA ligase seals nicks between Okazaki fragments.

DNA Replication in Eukaryotes: Special Features

Multiple Origins and Slower Polymerases

• Eukaryotic chromosomes are linear and much larger than prokaryotic DNA.

• Replication occurs at multiple origins (ARS) per chromosome, each with AT-rich sequences.

• DNA synthesis rate is slower (~50 nt/sec), requiring many origins to complete replication in S phase.

Enzymes and the End-Replication Problem

• Eukaryotes have multiple DNA polymerases (e.g., polymerase α, δ, η) with specialized roles.

• Replication of linear chromosome ends (telomeres) presents a unique challenge: after removal of the last RNA primer, a gap remains at the 5' end, leading to progressive shortening of chromosomes.

Telomeres and Telomerase

• Telomeres: Repetitive, noncoding DNA sequences at chromosome ends (e.g., TTAGGG in humans).

• Telomerase: A ribonucleoprotein enzyme with reverse transcriptase activity that extends telomeres using its own RNA template.

• Telomerase activity is present in germline and cancer cells, but not in most somatic cells.

Laboratory Techniques Based on DNA Structure and Replication

Quantification of DNA and RNA

• Nucleic acid bases absorb UV light maximally at 260 nm (A260).

• Concentration can be calculated using the molar extinction coefficient.

• Double-stranded DNA shows lower absorbance than single-stranded DNA or RNA (hypochromic effect).

Thermal Denaturation and Annealing

• Heating DNA separates strands (denaturation); cooling allows reannealing.

• Applications include hybridization assays, gene localization, and transcriptomics.

Polymerase Chain Reaction (PCR)

• Amplifies specific DNA sequences using cycles of denaturation, primer annealing, and extension.

• Requires template DNA, primers, dNTPs, and a thermostable DNA polymerase (e.g., Taq polymerase).

• Applications: gene cloning, DNA sequencing, diagnostics, forensics, and more.

Summary Table: Comparison of E. coli DNA Polymerases

Key Terms and Concepts

• Origin of replication (oriC): Specific DNA sequence where replication begins.

• Okazaki fragments: Short DNA fragments synthesized on the lagging strand.

• Processivity: Number of nucleotides added before the polymerase dissociates from the template.

• Semiconservative replication: Each daughter DNA contains one parental and one new strand.

• Telomerase: Enzyme that extends telomeres to prevent chromosome shortening.