DNA Replication and Recombination

Lecture 3 - Bio206 Notes

Date: Feb. 4th, 2026

DNA Replication: Overview

What is DNA replication?

  • DNA replication is the process by which DNA makes a copy of itself.
  • It is essential for cell division, ensuring that each daughter cell receives a complete set of genetic material.

Importance of DNA Replication

  • DNA is the genetic material of living organisms.
  • It needs to be accurately replicated and passed on to daughter cells during cell division.

Models of DNA Replication

Three Possible Models

  1. Semiconservative Model:
    • As proposed by Watson and Crick.
    • Each new double helix consists of one old and one new strand.
  2. Conservative Model:
    • The parental double helix remains intact, and both daughter helices are newly synthesized.
  3. Dispersive Model:
    • Both strands of each daughter helix contain segments of both original and newly synthesized DNA.

Watson-Crick Model of Replication

Mechanism of DNA Replication

  1. Unwinding of Double Helix:
    • The double helix unwinds to expose the bases on each strand.
  2. Template Function:
    • Each strand can serve as a template for the synthesis of new strands.
  3. Base Pairing:
    • New strands form through the insertion of complementary base pairs.
  4. Outcome:
    • One double helix becomes two identical daughter double helices.

Experimental Evidence for DNA Replication Models

Meselson-Stahl Experiment (1958)

  • Objective: To separate preexisting "parental" DNA from newly synthesized daughter DNA.
  • Method:
    • E. coli was grown in a medium containing heavy isotope 15N and then switched to one containing light isotope 14N.
    • A cesium chloride (CsCl) gradient was formed via high-speed centrifugation to separate the DNA based on density.
  • Importance of Nitrogen Composition:
    • The nitrogen composition indicates where the nitrogen ends up in DNA.

Confirmation of Semiconservative Replication

  • Following replication in 14N, the models were tested to see which was ruled out.
  • The results ultimately confirmed that the semiconservative model is correct.

Mechanism of DNA Replication

Key Enzyme: DNA Polymerase

  • Catalyzes: Formation of new phosphodiester bonds during DNA synthesis.
  • Energy Source:
    • Derived from high-energy phosphate bonds associated with deoxynucleotide triphosphates (dNTPs).
  • Process Overview:
    • The DNA synthesis comprises two stages:
    1. Initiation: Proteins open the double helix and prepare for complementary base pairing.
    2. Elongation: Proteins connect the correct sequence of nucleotides on newly formed DNA strands.

Requirements for DNA Polymerase Action

  1. Four dNTPs – Required for incorporation into the chain and as energy sources.
  2. Single-stranded template – May be unwound by other proteins.
  3. Primer with exposed 3' hydroxyl – Needed to initiate synthesis.

Initiation of DNA Replication

  1. Initiator Protein: Binds to the origin of replication.
  2. Helicase: Unwinds the double helix.
  3. Single-strand Binding Proteins: Keep the DNA helix open during replication.
  4. Primase Function: Synthesizes the RNA primer that is complementary and antiparallel to the template strand.

Elongation of DNA Replication

  1. Polymerization: DNA polymerase III catalyzes the formation of phosphodiester bonds between adjacent nucleotides.
  2. Strands:
    • Leading Strand: Continuously synthesized.
    • Lagging Strand: Synthesized discontinuously in short segments known as Okazaki fragments.
  3. Completion:
    • DNA polymerase I replaces RNA primers with DNA.
    • DNA ligase covalently joins Okazaki fragments.

Bidirectional Replication of a Circular Bacterial Chromosome

  • Replication proceeds in two directions from a single origin (Ori).
  • Supercoiling: Unwinding creates supercoiled DNA ahead of the replication fork.
  • Role of DNA Topoisomerase: Relaxes supercoils by cutting and resealing the sugar-phosphate backbone.

Maintaining Accuracy of Genetic Information

Mechanisms Ensuring Fidelity

  1. Redundancy: Each strand of the double helix can specify the sequence of the other strand.
  2. Precision of Replication Machinery:
    • DNA polymerases I and III possess proofreading abilities.
  3. DNA Repair Enzymes: Responsible for eliminating mismatches post-replication.

Genetic Diversity Through Recombination

Types of Meiotic Events

  1. Independent Assortment:
    • Homologous chromosomes segregate freely during meiosis, creating new allele combinations.
  2. Crossing Over:
    • Exchange of DNA segments between homologous chromosomes, ensuring proper segregation and creating new combinations of genetic material.

DNA Recombination Mechanism

Overview of the Process

  • Initiated by the Spo11 protein, which breaks the phosphodiester bonds in one chromatid.
  • Includes several steps:
    1. Formation of double-strand breaks.
    2. Resection to create single-stranded tails.
    3. Strand invasion and formation of heteroduplexes.
    4. Formation of double Holliday junctions and their migration.
    5. Resolution of the Holliday junctions by resolvase and ligase.

Mismatches and Gene Conversion

  • Heteroduplex regions may contain mismatches that can be repaired by DNA repair enzymes.
  • Gene Conversion: Deviations in segregation ratios indicating the conversion of alleles, which can happen through the recombination process.

Summary of Key Concepts

  • DNA replication ensures genetic continuity.
  • The semiconservative model is a widely accepted replication mechanism.
  • Fidelity of replication is maintained through redundancy and proofreading.
  • Recombination contributes to genetic diversity during meiosis through independent assortment and crossing over.