CH 16- The Molecular Basis of Inheritance: Structure and Replication of DNA

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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): Frederick Griffith discovered 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.
Avery, MacLeod, and McCarty (1944): Demonstrated that DNA is the substance that causes bacterial transformation.
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.

Griffith's transformation experiment with mice and bacteria Bacteriophage structure

DNA Structure and Composition

DNA is a polymer composed of nucleotides, each containing a nitrogenous base, a deoxyribose sugar, and a phosphate group. The four nitrogenous bases are adenine (A), thymine (T), guanine (G), and cytosine (C).

Chargaff's Rules: The amount of A equals T, and the amount of G equals C in DNA, but the overall base composition varies between species.
Watson and Crick Model: DNA is a double helix with antiparallel strands, stabilized by specific base pairing (A with T, G with C).

DNA nucleotide structure and base pairing Hydrogen bonding between base pairs Double helix structure and base pairing

Experimental Evidence for DNA Structure

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

$X-ray diffraction image of DNA$

DNA Replication

Models of DNA Replication

Three models were proposed for DNA replication: conservative, semiconservative, and dispersive. The Meselson-Stahl experiment confirmed the semiconservative model, where each new DNA molecule consists of one parental and one newly synthesized strand.

Semiconservative DNA replication model

Mechanism of DNA Replication

Replication begins at origins of replication, forming replication bubbles that expand in both directions. The process involves several key enzymes and proteins:

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 III: Extends the new DNA strand from the primer.
DNA polymerase I: Replaces RNA primers with DNA.
DNA ligase: Joins Okazaki fragments on the lagging strand.

Origins of replication in prokaryotic and eukaryotic cells Replication fork and associated enzymes

Leading and Lagging Strand Synthesis

DNA polymerases can only add nucleotides to the 3' end, so:

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.

Leading and lagging strand synthesis Okazaki fragment synthesis and joining Overview of DNA replication at the fork

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 RNA primer at 5' end of leading strand and at 5' end of each Okazaki fragment of lagging strand
DNA polymerase 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 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

Table of DNA replication proteins and their functions

Proofreading and DNA Repair

DNA polymerases proofread newly synthesized DNA, correcting errors. Additional repair mechanisms include mismatch repair and nucleotide excision repair, which help maintain genetic fidelity.

DNA proofreading and repair mechanisms

Evolutionary Significance of DNA Mutations

Although DNA repair mechanisms are highly accurate, some errors (mutations) persist. These mutations are the source of genetic variation, which is essential for evolution by natural selection.

Chromosome Structure and DNA Packaging

Bacterial Chromosomes

Bacterial chromosomes are typically circular, double-stranded DNA molecules that are supercoiled and located in the nucleoid region of the cell.

Bacterial chromosome supercoiling

Eukaryotic Chromosomes and Chromatin Structure

In eukaryotes, DNA is packaged with proteins (histones) into chromatin. The first level of packing involves the formation of nucleosomes, followed by higher-order folding into fibers and ultimately condensed chromosomes during cell division.

Levels of DNA packaging in eukaryotic chromosomes

Chromosome Territories in the Nucleus

During interphase, chromosomes occupy distinct regions within the nucleus, preventing entanglement and facilitating regulated gene expression.

Chromosome territories in the nucleus

Additional info: This summary integrates foundational experiments, molecular structure, and the enzymatic machinery of DNA replication, as well as the organization of genetic material in cells, providing a comprehensive overview suitable for college-level biology students.