Chapter 16: The Molecular Basis of Inheritance

Introduction

This chapter explores the molecular foundation of genetic inheritance, focusing on the discovery, structure, replication, and packaging of DNA. Understanding these concepts is essential for grasping how genetic information is transmitted from one generation to the next.

DNA as the Genetic Material

Discovery of DNA's Role

The identification of DNA as the genetic material was a pivotal moment in biology. Early experiments demonstrated that DNA carries hereditary information.

• Frederick Griffith's Experiment (1928): Griffith worked with two strains of Streptococcus pneumoniae: one pathogenic (S strain) and one harmless (R strain).

• When heat-killed S cells were mixed with living R cells, some R cells became pathogenic, indicating a transfer of genetic material.

• This process was termed transformation: a change in genotype and phenotype due to assimilation of foreign DNA.

Griffith's Experiment Table

Chargaff's Rules

Erwin Chargaff discovered that DNA composition varies between species, but within a species:

• The amount of adenine (A) equals thymine (T).

• The amount of guanine (G) equals cytosine (C).

This is known as Chargaff's rules and is fundamental to base pairing in DNA.

Structure of DNA

Double Helix Model

The structure of DNA was elucidated through X-ray crystallography by Rosalind Franklin and model building by James Watson and Francis Crick.

• DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a deoxyribose sugar, and a phosphate group.

• The molecule consists of two strands forming a double helix, with sugar-phosphate backbones on the outside and nitrogenous bases paired in the interior.

• The strands are antiparallel (run in opposite directions).

• Base pairing: A pairs with T via two hydrogen bonds; G pairs with C via three hydrogen bonds.

Example: The uniform width of the helix is maintained by pairing a purine (A or G) with a pyrimidine (C or T).

DNA Replication

Semiconservative Model

Watson and Crick proposed that DNA replication is semiconservative: each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

• Alternative models (conservative and dispersive) were disproven by experimental evidence.

Mechanism of Replication

DNA replication involves multiple enzymes and occurs at specific sites called origins of replication.

• Replication begins at origins, forming replication bubbles that expand in both directions.

• At the end of each bubble is a replication fork where DNA is unwound.

• Helicase unwinds the DNA helix.

• Single-strand binding proteins stabilize unwound DNA.

• Topoisomerase relieves strain ahead of the fork.

• Primase synthesizes a short RNA primer to initiate synthesis.

• DNA polymerase adds nucleotides to the 3' end of the primer, synthesizing new DNA in the 5' to 3' direction.

Leading vs. Lagging Strand

• Leading strand: Synthesized continuously toward the replication fork.

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

DNA Replication Table

Proofreading and Repair

DNA polymerases proofread newly synthesized DNA and correct errors. Additional repair mechanisms include:

• Mismatch repair: Corrects errors missed by DNA polymerase.

• Nucleotide excision repair: Removes and replaces damaged DNA segments.

Mutations that escape repair can be inherited and contribute to genetic variation.

Telomeres and Telomerase

Linear eukaryotic chromosomes have telomeres at their ends, which protect genes from erosion during replication.

• Telomeres do not prevent shortening but delay it.

• Telomerase is an enzyme that extends telomeres in germ cells, maintaining chromosome integrity.

Significance: Telomere shortening is associated with aging and cell senescence.

DNA Packaging in Chromosomes

Chromatin Structure

DNA is packaged with proteins to form chromatin, which condenses to form chromosomes.

• Histones are proteins responsible for the first level of DNA packing.

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

• Chromatin can be loosely packed (euchromatin) or tightly packed (heterochromatin).

• Heterochromatin is generally transcriptionally inactive due to tight packing.

Example: Centromeres and telomeres are regions of heterochromatin.

Chromatin Packing Table

Summary of Key Terms

• Transformation: Uptake of foreign DNA by a cell, changing its genotype and phenotype.

• Chargaff's rules: A = T, G = C in DNA.

• Double helix: Two antiparallel strands of DNA wound around each other.

• Semiconservative replication: Each new DNA molecule contains one old and one new strand.

• Okazaki fragments: Short DNA segments synthesized on the lagging strand.

• Telomeres: Repetitive DNA sequences at chromosome ends.

• Chromatin: DNA-protein complex forming chromosomes.

Key Equations

• Base pairing:

• Direction of DNA synthesis:

Additional info: Some explanations and tables were expanded for clarity and completeness based on standard biology curriculum.