The Molecular Basis of Inheritance: DNA Structure, Replication, and Repair

Chapter 16: The Molecular Basis of Inheritance

Introduction

This chapter explores the molecular mechanisms underlying the inheritance of genetic information, focusing on the structure of DNA, the process of DNA replication, and the systems that ensure the fidelity of genetic transmission. Understanding these processes is fundamental to modern biology and genetics.

DNA Structure

The Double Helix Model

The structure of deoxyribonucleic acid (DNA) was elucidated in 1953 by James Watson and Francis Crick. DNA consists of two antiparallel strands forming a double helix, with complementary base pairing between adenine (A) and thymine (T), and between guanine (G) and cytosine (C).

• Nucleotide: The basic unit of DNA, composed of a phosphate group, a deoxyribose sugar, and a nitrogenous base.

• Base Pairing: Hydrogen bonds form between specific pairs: A with T (2 hydrogen bonds), G with C (3 hydrogen bonds).

• Antiparallel Strands: The two DNA strands run in opposite directions (5′ → 3′ and 3′ → 5′).

Example: The sequence 5′-ATGC-3′ on one strand pairs with 3′-TACG-5′ on the other.

DNA Replication

Overview and Significance

DNA replication is the process by which a cell copies its DNA before cell division, ensuring genetic continuity. Accurate replication is essential for inheritance and cellular function.

• Semiconservative Model: Each new DNA molecule consists of one parental (old) strand and one newly synthesized strand.

• Replication Fork: The Y-shaped region where the DNA is unwound and new strands are synthesized.

• Origins of Replication: Specific sequences where replication begins; eukaryotic chromosomes have multiple origins.

Key Enzymes and Proteins in DNA Replication

• Helicase: Unwinds the DNA double helix at the replication fork.

• Single-Strand Binding Proteins: Stabilize unwound DNA strands.

• Topoisomerase: Relieves strain caused by unwinding by breaking, swiveling, and rejoining DNA strands.

• Primase: Synthesizes a short RNA primer to provide a starting point for DNA synthesis.

• DNA Polymerase: Adds nucleotides to the 3′ end of the primer, synthesizing the new DNA strand.

• DNA Ligase: Joins Okazaki fragments on the lagging strand.

Mechanism of DNA Synthesis

• Directionality: DNA polymerases can only add nucleotides to the free 3′ end, so synthesis occurs in the 5′ → 3′ direction.

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized discontinuously away from the fork as Okazaki fragments, later joined by DNA ligase.

• Nucleoside Triphosphates: Each incoming nucleotide is a nucleoside triphosphate (e.g., dATP), which loses two phosphates as pyrophosphate during incorporation.

Equation:

$\text{(DNA)n} + \text{dNTP} \rightarrow \text{(DNA)}_{n+1} + \text{PP}_i$

Table: Major Proteins in Bacterial DNA Replication

Protein	Function
Helicase	Unwinds parental double helix at replication forks
Single-strand binding protein	Binds to and stabilizes single-stranded DNA
Topoisomerase	Relieves overwinding strain ahead of replication forks
Primase	Synthesizes RNA primer
DNA polymerase III	Synthesizes new DNA strand by adding nucleotides to primer
DNA polymerase I	Removes RNA nucleotides of primer, replaces with DNA
DNA ligase	Joins Okazaki fragments

Proofreading and Repair of DNA

Ensuring Fidelity

DNA polymerases proofread each nucleotide as it is added. If an incorrect nucleotide is incorporated, it is removed and replaced. Additional repair mechanisms correct errors that escape proofreading.

• Mismatch Repair: Enzymes remove and replace incorrectly paired nucleotides.

• Nucleotide Excision Repair: A nuclease cuts out damaged DNA, which is then replaced by DNA polymerase and sealed by DNA ligase.

Example: UV-induced thymine dimers are repaired by nucleotide excision repair.

Evolutionary Significance of DNA Mutations

Source of Genetic Variation

Despite high fidelity, some errors persist, resulting in mutations. These mutations are the raw material for evolution, providing genetic variation upon which natural selection acts.

• Mutation: A permanent change in the DNA sequence.

• Significance: Mutations can be beneficial, neutral, or harmful, and are essential for evolution and the emergence of new species.

Replicating the Ends of DNA Molecules

Telomeres and Telomerase

Linear DNA molecules in eukaryotes pose a unique challenge: the replication machinery cannot fully replicate the 5′ ends, leading to progressive shortening of chromosomes with each cell division.

• Telomeres: Repetitive nucleotide sequences at chromosome ends that protect genes from erosion.

• Telomerase: An enzyme that extends telomeres in germ cells, preventing loss of essential genetic information.

• Connection to Aging: Telomere shortening is associated with cellular aging; telomerase activity is high in germ cells and some cancer cells.

Equation:

$\text{Telomerase} + \text{Telomere DNA} \rightarrow \text{Extended Telomere DNA}$

Summary

• DNA's double-helical structure enables accurate replication and transmission of genetic information.

• Multiple enzymes and proteins coordinate the complex process of DNA replication.

• Proofreading and repair mechanisms maintain genetic fidelity, while mutations provide evolutionary potential.

• Special mechanisms, such as telomeres and telomerase, address the challenges of replicating linear chromosomes in eukaryotes.