Chapter 16: The Molecular Basis of Inheritance

Introduction

This chapter explores the molecular mechanisms underlying genetic inheritance, focusing on the structure and replication of DNA, the experiments that established DNA as the genetic material, and the organization of DNA within chromosomes.

DNA as the Genetic Material

Discovery of DNA's Role

The identification of DNA as the hereditary material was a pivotal moment in biology, established through a series of experiments in the early 20th century.

• Frederick Griffith's Experiment (1928): Griffith worked with two strains of Streptococcus pneumoniae: a pathogenic (S) strain and a 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, defined as a change in genotype and phenotype due to assimilation of foreign DNA.

Example: In Griffith's experiment, only the combination of heat-killed S cells and living R cells caused disease in mice, demonstrating transformation.

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 provided key evidence for the base-pairing structure of DNA.

Structure of DNA

Components of DNA

• Nucleotide: The basic unit of DNA, consisting of a nitrogenous base (A, T, G, or C), a deoxyribose sugar, and a phosphate group.

• Double Helix: DNA consists of two antiparallel strands forming a right-handed double helix.

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

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

Key Discoveries

• Rosalind Franklin: Used X-ray crystallography to produce images of DNA, revealing its helical structure.

• Watson and Crick: Built a model of DNA as a double helix with antiparallel sugar-phosphate backbones and complementary base pairing.

DNA Replication

Overview of Replication

DNA replication ensures genetic information is accurately passed from parent to daughter cells during cell division.

• Replication is semiconservative: each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

Steps in DNA Replication

• Origin of Replication: Replication begins at specific sites where the DNA strands are separated, forming a replication bubble.

• Replication Fork: The Y-shaped region where new DNA strands are synthesized.

• Enzymes Involved:

  • Helicase: Unwinds the DNA double helix.

  • Single-Strand Binding Proteins: Stabilize unwound DNA.

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

  • Primase: Synthesizes a short RNA primer to provide a starting point for DNA polymerase.

  • DNA Polymerase: Adds nucleotides to the 3' end of the primer, synthesizing new DNA in the 5' to 3' direction.

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

Leading vs. Lagging Strand

• Leading Strand: Synthesized continuously toward the replication fork.

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

Proofreading and Repair

• DNA Polymerase: Proofreads newly synthesized DNA, correcting errors.

• Mismatch Repair: Other enzymes remove and replace incorrectly paired nucleotides.

• Nucleotide Excision Repair: Damaged DNA segments are cut out by nucleases and replaced.

Telomeres and Telomerase

• Telomeres: Repetitive nucleotide sequences at the ends of eukaryotic chromosomes that protect genes from erosion during replication.

• Telomerase: An enzyme that extends telomeres in germ cells, preventing their shortening.

Significance: Telomere shortening is associated with aging and limits the number of times a cell can divide.

Chromosome Structure and DNA Packaging

Organization of DNA in Cells

• Prokaryotes: DNA is a double-stranded, circular molecule located in the nucleoid region and is supercoiled.

• Eukaryotes: DNA is linear and associated with proteins to form chromatin, which is highly organized within the nucleus.

Levels of DNA Packing

• Histones: Proteins responsible for the first level of DNA packing; DNA wraps around histone octamers to form nucleosomes.

• Nucleosome: The basic unit of chromatin, consisting of DNA wound twice around a core of eight histone proteins.

• Chromatin: Can be loosely packed (euchromatin) or tightly packed (heterochromatin). Heterochromatin is generally transcriptionally inactive.

Table: Comparison of Euchromatin and Heterochromatin

Summary

• DNA is the hereditary material, as demonstrated by transformation experiments and chemical analysis.

• The double helix structure of DNA allows for accurate replication and transmission of genetic information.

• Multiple enzymes coordinate the process of DNA replication, ensuring fidelity and repair.

• Chromosomes are highly organized structures that package DNA efficiently within the cell.