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DNA Replication and Repair: Structure, Mechanisms, and Fidelity

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DNA Replication and Repair

Introduction

DNA replication is a fundamental process in all living organisms, ensuring the accurate transmission of genetic information from one generation to the next. This process involves the coordinated action of multiple enzymes and proteins to duplicate the DNA molecule, followed by mechanisms that repair errors and damage to maintain genome integrity.

Modern Definition of a Gene and Chromosome Structure

Genes and Chromosomes

  • Gene: A gene is a sequence of DNA that contains the information necessary to produce a functional product, typically a protein or functional RNA.

  • Chromosome: Chromosomes are long DNA molecules associated with proteins (mainly histones in eukaryotes) that help package and organize the DNA within the cell nucleus.

  • Biomolecules: Genes are composed of nucleic acids (DNA or RNA in some viruses), while chromosomes are made up of DNA and proteins.

Experimental Evidence: The Hershey-Chase Experiment

Purpose and Design

  • The Hershey-Chase experiment (1952) demonstrated that DNA, not protein, is the genetic material in viruses.

  • Bacteriophages (viruses that infect bacteria) were labeled with radioactive isotopes: phosphorus-32 (labels DNA) and sulfur-35 (labels protein).

  • After infection of bacteria, only the radioactive DNA entered the cells, indicating that DNA carries genetic information.

Labeling Reasoning

  • Radioactive phosphorus (32P): Incorporated into DNA (due to phosphate backbone), not proteins.

  • Radioactive sulfur (35S): Incorporated into proteins (due to cysteine and methionine), not DNA.

Summary Table: Labeling Biomolecules

Radioactive Isotope

Labels

Does Not Label

Phosphorus-32

DNA

Protein

Sulfur-35

Protein

DNA

DNA Structure

Primary and Secondary Structure

  • Primary structure: The sequence of nucleotides in a single DNA strand.

  • Secondary structure: The double helix formed by two antiparallel strands held together by complementary base pairing.

  • Polarity: DNA strands have directionality, with a 5' (phosphate) end and a 3' (hydroxyl) end.

Base Pairing

  • Adenine (A) pairs with Thymine (T) via two hydrogen bonds.

  • Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

Chargaff's Rules

  • In double-stranded DNA, the amount of A equals T, and the amount of G equals C: and .

DNA Replication: Overview

When and Where Replication Occurs

  • DNA replication occurs during the S phase of interphase in the cell cycle.

Basic Steps of Replication

  1. Strand separation: The two DNA strands are separated by helicase.

  2. Base pairing: Each parental strand serves as a template for the synthesis of a new complementary strand.

  3. Polymerization: DNA polymerase adds nucleotides to the growing DNA strand in the 5' to 3' direction.

Mechanism of DNA Replication

Enzymes and Proteins Involved

Protein/Enzyme

Function

Helicase

Unwinds the DNA double helix

Single-strand binding proteins (SSBPs)

Stabilize single-stranded DNA

Topoisomerase

Relieves tension ahead of the replication fork

Primase

Synthesizes short RNA primers

DNA polymerase III

Main enzyme for DNA synthesis (adds nucleotides to 3' end)

Sliding clamp

Holds DNA polymerase in place

DNA polymerase I

Removes RNA primers and replaces them with DNA

DNA ligase

Joins Okazaki fragments on the lagging strand

Leading and Lagging Strand Synthesis

  • Leading strand: Synthesized continuously in the 5' to 3' direction, toward the replication fork.

  • Lagging strand: Synthesized discontinuously in short fragments (Okazaki fragments) in the 5' to 3' direction, away from the replication fork.

  • Okazaki fragments are later joined by DNA ligase.

Directionality of Synthesis

  • Both leading and lagging strands are synthesized in the 5' to 3' direction.

Summary Table: Leading vs. Lagging Strand

Feature

Leading Strand

Lagging Strand

Synthesis Direction

Toward replication fork

Away from replication fork

Continuity

Continuous

Discontinuous (Okazaki fragments)

Primer Requirement

One primer

Multiple primers

DNA Replication: Detailed Steps

Initiation

  • Replication begins at origins of replication.

  • Helicase unwinds DNA; SSBPs stabilize single strands; topoisomerase relieves supercoiling.

Elongation

  • Primase synthesizes RNA primers.

  • DNA polymerase III extends the primers, synthesizing new DNA in the 5' to 3' direction.

  • On the lagging strand, primase and DNA polymerase III repeatedly synthesize short Okazaki fragments.

  • DNA polymerase I removes RNA primers and replaces them with DNA.

  • DNA ligase seals the nicks between Okazaki fragments.

Termination

  • Replication ends when the entire DNA molecule has been copied.

Fidelity and Repair of DNA Replication

Accuracy of DNA Replication

  • DNA replication is highly accurate, with an error rate of less than one mistake per billion nucleotides.

  • For the human genome (~3 billion base pairs), this means only a few errors per cell division.

Mechanisms Ensuring Fidelity

  1. Correct base pairing: DNA polymerase selects the correct nucleotide based on template strand.

  2. Proofreading: DNA polymerase has 3' to 5' exonuclease activity to remove incorrectly paired nucleotides.

  3. Mismatch repair: Additional enzymes recognize and repair mismatches missed during replication.

DNA Damage and Repair Mechanisms

  • Nucleotide excision repair: Removes bulky lesions (e.g., thymine dimers) and replaces damaged DNA.

  • Mismatch repair: Corrects errors that escape proofreading, distinguishing old and new strands (in E. coli, methylation marks the old strand).

  • Defects in DNA repair pathways can increase the risk of cancer due to accumulation of mutations.

Summary Table: DNA Repair Mechanisms

Repair Mechanism

When It Occurs

What It Fixes

Proofreading

During replication

Incorrectly paired bases

Mismatch repair

After replication

Mismatched bases

Nucleotide excision repair

Anytime

Bulky lesions, thymine dimers

Key Terms and Concepts

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

  • Replication fork: The Y-shaped region where DNA is split into two separate strands for copying.

  • Replisome: The complex of enzymes and proteins that carry out DNA replication.

  • Phosphodiester bond: The covalent bond linking nucleotides in the DNA backbone.

  • Hydrogen bonds: Weak bonds between complementary bases that hold the two DNA strands together.

Example: Synthesis Directionality

  • Both leading and lagging strands are synthesized in the 5' to 3' direction, but the lagging strand is made in fragments away from the replication fork.

Example: DNA Damage

  • Thymine dimers: Covalent bonds between adjacent thymine bases caused by UV light, repaired by nucleotide excision repair.

  • Benzopyrene adducts: Bulky lesions caused by chemical mutagens, also repaired by nucleotide excision repair.

Summary

  • DNA replication is a highly regulated, accurate process involving multiple enzymes and proteins.

  • Fidelity is maintained by proofreading and repair mechanisms.

  • Defects in replication or repair can lead to mutations and disease.

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