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 process 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 a "transforming principle."

• Avery, MacLeod, and McCarty (1944): Identified DNA as the transforming substance.

• Hershey-Chase Experiment (1952): Used bacteriophages labeled with radioactive isotopes to show that DNA, not protein, is the genetic material injected into bacteria during infection.

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 of nucleotides, each consisting of a nitrogenous base (A, T, G, or C), a deoxyribose sugar, and a phosphate group.

Double Helix Model

• Watson and Crick Model (1953): DNA is a double helix with two antiparallel strands held together by specific base pairing (A with T, G with C).

• Hydrogen Bonding: A-T pairs form two hydrogen bonds; G-C pairs form three hydrogen bonds.

• X-ray Crystallography: Rosalind Franklin's X-ray diffraction images were critical in elucidating the helical structure.

DNA Replication

Basic Mechanism

DNA replication is the process by which DNA makes a copy of itself during cell division. Each strand serves as a template for the synthesis of a new complementary strand.

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

• Origins of Replication: Replication begins at specific sites, forming replication bubbles that expand in both directions.

Enzymes and Proteins Involved in Replication

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

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

• Topoisomerase: Relieves strain ahead of the replication fork by breaking and rejoining DNA strands.

• Primase: Synthesizes short RNA primers needed to start DNA synthesis.

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

• 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 as Okazaki fragments, which are later joined by DNA ligase.

Proofreading and DNA Repair

• Proofreading: DNA polymerases check and correct errors during replication.

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

• Nucleotide Excision Repair: Damaged DNA segments are removed by nucleases and replaced by DNA polymerase and ligase.

• Mutations: Permanent changes in DNA sequence can be inherited and are a source of genetic variation.

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 forms looped domains and further condenses into chromosomes during cell division.

• Chromosome Territories: During interphase, chromosomes occupy specific regions within the nucleus.

Summary Table: Key Proteins in 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 by breaking, swiveling, and rejoining DNA strands
Primase	Synthesizes an RNA primer at 5' ends of leading strand and Okazaki fragments
DNA polymerase III	Using parental DNA as template, synthesizes new DNA strand by adding nucleotides to an RNA primer or pre-existing DNA strand
DNA polymerase 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