The Molecular Basis of Inheritance: DNA Structure, Replication, and Chromosome Organization

The Molecular Basis of Inheritance

Introduction

The molecular basis of inheritance centers on the role of DNA as the genetic material. This chapter explores the evidence supporting DNA's function, its structural features, the mechanisms of DNA replication, and the organization of DNA within chromosomes.

DNA as the Genetic Material

Historical Evidence for DNA's Role

Early experiments demonstrated that DNA, not protein, is the hereditary material. Key studies include Griffith's transformation experiment and the Hershey-Chase experiment with bacteriophages.

• Griffith's Experiment: Showed that a substance from heat-killed pathogenic bacteria could transform harmless bacteria into pathogenic forms, suggesting a "transforming principle" (later identified as DNA).

• Hershey-Chase Experiment: Used radioactive labeling to show that DNA, not protein, enters bacterial cells during viral infection and directs viral replication.

Structure of DNA

Nucleotide Composition and Base Pairing

DNA is a polymer composed of nucleotides, each containing a phosphate group, a deoxyribose sugar, and a nitrogenous base (adenine, thymine, guanine, or cytosine). Chargaff's rules state that the amount of adenine equals thymine, and the amount of guanine equals cytosine.

• Nucleotide Structure: Each nucleotide consists of a phosphate, a sugar (deoxyribose), and a nitrogenous base.

• Base Pairing: Adenine pairs with thymine via two hydrogen bonds; guanine pairs with cytosine via three hydrogen bonds.

Double Helix Model

Watson and Crick proposed the double helix model, where two antiparallel strands are held together by complementary base pairing. X-ray crystallography by Franklin provided critical evidence for the helical structure.

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

• Helical Structure: The double helix has a uniform diameter and completes one turn every 10 base pairs.

DNA Replication

Mechanisms and Models

DNA replication is the process by which genetic information is copied for cell division. Three models were proposed: conservative, semiconservative, and dispersive. The semiconservative model, supported by Meselson and Stahl, states that each new DNA molecule consists of one old and one new strand.

• Semiconservative Replication: Each daughter DNA molecule contains one parental strand and one newly synthesized strand.

• Replication Origin: Replication begins at specific sites called origins, forming replication bubbles.

Enzymes and Proteins Involved

Multiple proteins coordinate DNA replication, including helicase, primase, DNA polymerases, single-strand binding proteins, and topoisomerase.

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

• Primase: Synthesizes a short RNA primer to initiate DNA synthesis.

• DNA Polymerase: Adds nucleotides to the growing DNA strand in the 5' to 3' direction.

• Single-Strand Binding Proteins: Stabilize unwound DNA.

• Topoisomerase: Relieves strain ahead of the replication fork.

Leading and Lagging Strand Synthesis

DNA polymerase synthesizes the leading strand continuously toward the replication fork, while the lagging strand is synthesized discontinuously as Okazaki fragments, which are joined by DNA ligase.

• Leading Strand: Synthesized continuously in the 5' to 3' direction.

• Lagging Strand: Synthesized in short fragments (Okazaki fragments) away from the replication fork.

• DNA Ligase: Joins Okazaki fragments to form a continuous strand.

Key Replication Proteins

The following table summarizes the main proteins involved in bacterial DNA replication and their functions:

Protein	Function
Helicase	Unwinds parental double helix at replication forks
Single-strand binding protein	Stabilizes unwound DNA
Topoisomerase	Relieves strain ahead of replication fork
Primase	Synthesizes RNA primer
DNA pol III	Uses parental DNA as template, synthesizes new DNA strand
DNA pol I	Removes RNA primer, replaces with DNA
DNA ligase	Joins Okazaki fragments

Proofreading and DNA Repair

Mechanisms of DNA Repair

DNA polymerases proofread newly synthesized DNA, correcting errors. Additional repair mechanisms include mismatch repair and nucleotide excision repair, which remove and replace damaged DNA segments.

• Proofreading: DNA polymerase checks and corrects mismatched bases during replication.

• Mismatch Repair: Enzymes correct errors in base pairing.

• Nucleotide Excision Repair: Nuclease removes damaged DNA, which is replaced by DNA polymerase and sealed by DNA ligase.

Chromosome Structure and DNA Packing

Bacterial vs. Eukaryotic Chromosomes

Bacterial chromosomes are circular and supercoiled, while eukaryotic chromosomes are linear and highly organized with proteins called histones forming chromatin.

• Bacterial Chromosome: Double-stranded, circular DNA found in the nucleoid.

• Eukaryotic Chromosome: Linear DNA packed with histones into chromatin, forming nucleosomes and higher-order structures.

Chromatin Organization

During interphase, chromosomes occupy specific regions in the nucleus, preventing entanglement and facilitating gene regulation.

Summary

DNA is the hereditary material, structured as a double helix of nucleotides. Its replication is semiconservative, involving a coordinated set of enzymes and proteins. DNA repair mechanisms ensure fidelity, and DNA is packed into chromosomes with varying complexity in prokaryotes and eukaryotes.

Key Equations

• Chargaff's Rule: $\text{[A]} = \text{[T]}, \text{[G]} = \text{[C]}$

• Semiconservative Replication: $\text{New DNA} = \text{1 parental strand} + \text{1 new strand}$

Additional info: The notes expand on brief textbook points to provide a comprehensive overview suitable for exam preparation, including definitions, examples, and visual aids.