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 as the hereditary molecule, its structure, the mechanisms of DNA replication, and the organization of DNA within chromosomes.

DNA as the Genetic Material

Historical Experiments Demonstrating DNA's Role

• Griffith's Transformation Experiment (1928): Demonstrated that a substance from dead pathogenic bacteria could genetically transform living nonpathogenic bacteria, suggesting the presence of a 'transforming principle.' This was later identified as DNA.

• Hershey-Chase Experiment (1952): Used bacteriophages labeled with radioactive isotopes to show that DNA, not protein, is the genetic material that enters bacterial cells and directs viral replication.

Chargaff's Rules

• Base Composition: DNA composition varies between species.

• Base Pairing: The amount of adenine (A) equals thymine (T), and the amount of guanine (G) equals cytosine (C).

Structure of DNA

DNA Nucleotide Structure

DNA is a polymer composed of nucleotides, each containing a nitrogenous base (A, T, G, or C), a deoxyribose sugar, and a phosphate group.

Double Helix Model

• Watson and Crick Model: DNA consists of two antiparallel strands forming a right-handed double helix, with complementary base pairing (A-T and G-C) held together by hydrogen bonds.

• X-ray Crystallography: Rosalind Franklin's X-ray diffraction images provided critical evidence for the helical structure of DNA.

Base Pairing and Hydrogen Bonds

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

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

DNA Replication

Models of DNA Replication

• Semiconservative Model: Each new DNA molecule consists of one parental strand and one newly synthesized strand. Supported by the Meselson-Stahl experiment.

• Other Models: Conservative and dispersive models were proposed but not supported by experimental evidence.

Mechanism of DNA Replication

Replication begins at origins of replication, forming replication bubbles that expand in both directions. At each replication fork, several enzymes coordinate the unwinding and synthesis of new DNA strands.

• Helicase: Unwinds the DNA double helix.

• Single-strand binding proteins: Stabilize unwound DNA.

• Topoisomerase: Relieves strain ahead of the replication fork.

• Primase: Synthesizes short RNA primers.

• DNA Polymerase: Adds nucleotides to the 3' end of the primer, synthesizing new DNA in the 5' to 3' direction.

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

Leading and Lagging Strand Synthesis

• Leading Strand: Synthesized continuously toward the replication fork.

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

Key Proteins in DNA Replication

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	Binds to and stabilizes single-stranded DNA
Topoisomerase	Relieves overwinding strain ahead of replication forks by breaking, swiveling, and rejoining DNA strands
Primase	Synthesizes an RNA primer at 5' end of leading strand and at 5' end of each Okazaki fragment of lagging strand
DNA pol III	Using parental DNA as template, synthesizes new DNA strand by covalently adding nucleotides to the 3' end of a pre-existing DNA strand or RNA primer
DNA pol I	Removes RNA nucleotides of primer from 5' end and replaces them with DNA nucleotides
DNA ligase	Joins Okazaki fragments of lagging strand; on leading strand, joins 3' end of DNA that replaces primer to rest of leading strand DNA

Proofreading and Repair

• DNA Polymerase Proofreading: DNA polymerases correct errors during replication by removing and replacing incorrect nucleotides.

• Mismatch Repair: Repair enzymes correct errors missed by DNA polymerase.

• Nucleotide Excision Repair: Damaged DNA segments are removed by nucleases and replaced using the undamaged strand as a template.

Evolutionary Significance of Mutations

• Mutations: Permanent changes in DNA sequence that can be inherited. They are the source of genetic variation and drive evolution by natural selection.

Chromosome Structure and DNA Packaging

Bacterial Chromosomes

• Structure: Double-stranded, circular DNA molecule with associated proteins.

• Supercoiling: DNA is compacted into the nucleoid region by supercoiling.

Eukaryotic Chromosomes and Chromatin

• Chromatin: DNA is combined with histone proteins to form chromatin, which is further organized into higher-order structures.

• Nucleosome: The basic unit of DNA packaging, consisting of DNA wrapped around histone proteins.

• Higher-Order Packing: Nucleosomes coil to form a 30-nm fiber, which is further looped and folded to form metaphase chromosomes.

Chromosome Territories in the Nucleus

• Interphase Chromosomes: Occupy specific regions within the nucleus, preventing entanglement and facilitating regulated gene expression.

Additional info: This guide covers the core concepts of Chapter 16: The Molecular Basis of Inheritance, including the discovery of DNA as genetic material, its structure, the process of replication, and chromosome organization in prokaryotes and eukaryotes. These topics are foundational for understanding gene expression, inheritance, and molecular genetics.