Chapter 16: The Molecular Basis of Inheritance – DNA Structure, Replication, and Chromosome Organization

Chapter 16: The Molecular Basis of Inheritance

Introduction

The discovery of DNA as the genetic material and the elucidation of its structure and replication mechanisms are foundational to modern biology. This chapter explores the experiments that identified DNA as the hereditary material, the structure of DNA, the process of DNA replication, and the organization of DNA within chromosomes.

DNA as the Genetic Material

The Search for the Genetic Material

• Genes were shown to be located on chromosomes, which are composed of DNA and protein.

• Early experiments focused on determining whether DNA or protein was the genetic material.

Evidence from Bacterial Transformation

• Frederick Griffith's experiments with Streptococcus pneumoniae demonstrated transformation: harmless bacteria could become pathogenic when mixed with heat-killed pathogenic bacteria, indicating assimilation of foreign DNA.

• Transformation is now defined as a change in genotype and phenotype due to the assimilation of external DNA.

Evidence from Bacteriophages

• Bacteriophages (phages) are viruses that infect bacteria, consisting of DNA (or RNA) enclosed in a protein coat.

• Hershey and Chase (1952) used radioactive labeling to show that DNA, not protein, enters bacterial cells during infection, confirming DNA as the genetic material.

Chargaff’s Rules

• Erwin Chargaff found that DNA composition varies between species and that the amount of adenine (A) equals thymine (T), and guanine (G) equals cytosine (C).

• These findings supported the idea that DNA could carry genetic information.

DNA Structure

Discovery of the Double Helix

• Maurice Wilkins and Rosalind Franklin used X-ray crystallography to study DNA structure, revealing its helical nature.

• James Watson and Francis Crick built a double-helical model of DNA in 1953, based on Franklin’s data and chemical evidence.

Key Features of DNA Structure

• DNA is a double helix with two antiparallel sugar-phosphate backbones and nitrogenous bases paired in the interior.

• Base pairing is specific: A pairs with T, and G pairs with C, via hydrogen bonds.

• Pairing a purine (A or G) with a pyrimidine (T or C) maintains a uniform helix width.

DNA Nucleotide Structure

• Each nucleotide consists of a nitrogenous base, a deoxyribose sugar, and a phosphate group.

• Nucleotides are linked by phosphodiester bonds to form the sugar-phosphate backbone.

DNA Replication

Models of DNA Replication

• Watson and Crick proposed the semiconservative model: each daughter DNA molecule consists of one parental strand and one new strand.

• Alternative models included the conservative and dispersive models, but experimental evidence supports the semiconservative model.

Origins of Replication

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

• Eukaryotic chromosomes have multiple origins; replication proceeds bidirectionally until the entire molecule is copied.

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 overwinding strain ahead of the replication fork.

• Primase: Synthesizes short RNA primers to provide a starting point for DNA synthesis.

• DNA polymerase III: Synthesizes new DNA by adding nucleotides to the 3' end of the primer.

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

• 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
Primase	Synthesizes RNA primer at 5' end of leading strand and each Okazaki fragment
DNA pol III	Synthesizes new DNA strand by adding nucleotides to primer or pre-existing strand
DNA pol I	Removes RNA primer and replaces with DNA nucleotides
DNA ligase	Joins Okazaki fragments and seals nicks in DNA

Mechanism of DNA Synthesis

• DNA polymerases add nucleotides only to the free 3' end, so DNA elongates in the 5' to 3' direction.

• The leading strand is synthesized continuously toward the replication fork.

• The lagging strand is synthesized discontinuously, forming short Okazaki fragments that are later joined by DNA ligase.

Proofreading and DNA Repair

• DNA polymerases proofread newly synthesized DNA, correcting errors.

• Mismatch repair enzymes correct errors missed by DNA polymerase.

• Nucleotide excision repair removes and replaces damaged DNA segments using nucleases, DNA polymerase, and DNA ligase.

Evolutionary Significance of DNA Mutations

• Despite proofreading, some errors persist, resulting in mutations.

• Mutations are the source of genetic variation, driving evolution and the emergence of new species.

Chromosome Structure and DNA Packing

Organization of DNA in Chromosomes

• Bacterial chromosomes are circular and associated with small amounts of protein, forming a supercoiled structure in the nucleoid.

• Eukaryotic chromosomes are linear and associated with large amounts of protein, forming chromatin.

Levels of DNA Packing

• DNA wraps around histone proteins to form nucleosomes ("beads on a string").

• Chromatin fibers are further folded into higher-order structures (10-nm, 30-nm, and 300-nm fibers) to fit into the nucleus.

• During interphase, chromatin is mostly loosely packed (euchromatin), but some regions (centromeres, telomeres) are highly condensed (heterochromatin).

Regulation of Chromatin Structure

• Histone modifications can alter chromatin condensation and affect gene expression.

• The amino-terminal tails of histones are sites for chemical modifications that regulate access to DNA.

Summary Table: Key Proteins in DNA Replication

Protein	Function
Helicase	Unwinds DNA at replication fork
Single-strand binding protein	Stabilizes single-stranded DNA
Topoisomerase	Relieves overwinding strain
Primase	Synthesizes RNA primer
DNA polymerase III	Synthesizes new DNA strand
DNA polymerase I	Replaces RNA primer with DNA
DNA ligase	Joins Okazaki fragments

Key Equations and Concepts

• Base pairing: $A = T$, $G = C$ (Chargaff's rules)

• Direction of synthesis: DNA is synthesized in the 5' → 3' direction.

• Energy for polymerization: Each nucleotide added as a nucleoside triphosphate; hydrolysis of pyrophosphate ($2P_i$) provides energy for bond formation.

Conclusion

The molecular basis of inheritance lies in the structure and replication of DNA. Understanding these processes is essential for comprehending how genetic information is faithfully transmitted and how genetic variation arises, fueling evolution and diversity in living organisms.