The Molecular Basis of Inheritance: Structure and Replication of DNA

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 experimental evidence for DNA as the hereditary molecule, its structure, and the mechanisms by which it is replicated and packaged within cells.

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 non-pathogenic bacteria, suggesting a "transforming principle."

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

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

Chargaff's Rules

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

• Species Variation: The relative amounts of A, T, G, and C vary between species, supporting DNA's role in heredity.

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 (1953): Proposed the double helix structure of DNA, with two antiparallel strands held together by specific base pairing (A-T and G-C).

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

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

DNA Replication

Models of Replication

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

• Other Models: Conservative and dispersive models were proposed but disproven.

Origins of Replication

Replication begins at specific sites called origins of replication, forming replication bubbles that expand in both directions.

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 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 growing DNA strand in the 5' to 3' direction.

• DNA Polymerase I: Removes RNA primers and replaces them with DNA.

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

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 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

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.

Proofreading and DNA Repair

• Proofreading: DNA polymerases correct errors during replication by removing mismatched nucleotides.

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

• Nucleotide Excision Repair: Nucleases remove damaged DNA, which is then replaced by DNA polymerase and sealed by DNA ligase.

DNA Packaging in Chromosomes

Bacterial Chromosomes

• Consist of double-stranded, circular DNA molecules associated with a small amount of protein.

• DNA is supercoiled and located in the nucleoid region.

Eukaryotic Chromosomes and Chromatin Structure

• DNA is combined with proteins (histones) to form chromatin.

• Chromatin undergoes multiple levels of packing: DNA double helix → nucleosomes → 30-nm fiber → looped domains → condensed chromosome.

• During interphase, chromosomes occupy specific regions in the nucleus and do not become entangled.

Summary Table: Key Concepts in DNA Structure and Replication

Concept	Key Points
DNA as Genetic Material	Griffith, Avery, Hershey-Chase experiments; DNA, not protein, is hereditary material
DNA Structure	Double helix, antiparallel strands, complementary base pairing (A-T, G-C)
Replication	Semiconservative, origins of replication, leading/lagging strands, Okazaki fragments
Enzymes	Helicase, primase, DNA polymerases, ligase, topoisomerase, single-strand binding proteins
Repair	Proofreading, mismatch repair, nucleotide excision repair
Chromosome Structure	Supercoiling in prokaryotes, chromatin packing in eukaryotes