Chapter 16: The Molecular Basis of Inheritance

Concept 16.1: DNA is the Genetic Material

The identification of DNA as the genetic material was a major milestone in biology. Early 20th-century research focused on determining whether DNA or protein carried genetic information. Key experiments with bacteria and viruses provided evidence for DNA's role in heredity.

• Genes and Chromosomes: Genes are located on chromosomes, which consist of DNA and protein.

• Griffith's Experiment: Demonstrated transformation, where harmless bacteria became pathogenic by assimilating DNA from dead pathogenic cells.

• Transformation: Defined as a change in genotype and phenotype due to assimilation of foreign DNA.

• Avery, McCarty, MacLeod: Identified DNA as the transforming substance, confirming its genetic role.

• Bacteriophages: Viruses that infect bacteria, used to study genetic material.

• Hershey-Chase Experiment: Showed that DNA, not protein, is the genetic material of phage T2.

• Chargaff's Rules: DNA composition varies between species; in any species, the amount of adenine (A) equals thymine (T), and guanine (G) equals cytosine (C).

Example: The Hershey-Chase experiment used radioactive labeling to distinguish DNA from protein, demonstrating that only DNA enters the bacterial cell during infection.

Building a Structural Model of DNA

Understanding DNA's structure was crucial for explaining its function in inheritance. X-ray crystallography and chemical analysis led to the discovery of the double helix.

• X-ray Crystallography: Used by Franklin and Wilkins to study DNA's molecular structure.

• Double Helix: Watson and Crick deduced that DNA consists of two antiparallel strands forming a double helix.

• Base Pairing: Purines (A, G) pair with pyrimidines (T, C), resulting in uniform width. Specific pairing: A with T, G with C.

• Antiparallel Strands: The two DNA strands run in opposite directions (5' to 3' and 3' to 5').

Example: The Watson-Crick model explains Chargaff’s rules and the mechanism for accurate DNA replication.

Concept 16.2: Proteins in DNA Replication and Repair

DNA replication ensures the faithful transmission of genetic information. Multiple proteins and enzymes coordinate the process, which is highly accurate and efficient.

• Replication: The process of copying DNA before cell division.

• Semiconservative Model: Each new DNA molecule consists of one old strand and one new strand.

• Origins of Replication: Specific sites where DNA replication begins, forming replication bubbles.

• Replication Fork: Y-shaped region where DNA is unwound.

• Key Enzymes:

  • Helicase: Unwinds the DNA helix.

  • Single-Strand Binding Proteins: Stabilize unwound DNA.

  • Topoisomerase: Relieves strain from unwinding.

  • Primase: Synthesizes RNA primer.

  • DNA Polymerase: Adds nucleotides to the growing DNA strand.

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

• Leading vs. Lagging Strand: Leading strand is synthesized continuously; lagging strand is synthesized in fragments (Okazaki fragments).

• Directionality: DNA polymerase adds nucleotides only to the 3' end; synthesis occurs in the 5' → 3' direction.

Example: The Meselson-Stahl experiment confirmed the semiconservative model of DNA replication.

Table: Bacterial DNA Replication Proteins and Their Functions

Proofreading and Repairing DNA

DNA polymerases proofread newly synthesized DNA, correcting errors. Additional repair mechanisms fix mismatches and damage caused by environmental factors.

• Proofreading: DNA polymerases replace incorrect nucleotides during replication.

• Mismatch Repair: Repair enzymes correct mismatched bases.

• Nucleotide Excision Repair: Nuclease removes damaged DNA, which is replaced by new nucleotides.

• Mutations: Permanent changes in DNA sequence; source of genetic variation.

Example: Exposure to X-rays can cause DNA damage, which is repaired by nucleotide excision repair.

Replicating the Ends of DNA Molecules

Linear DNA molecules in eukaryotes face challenges in replicating their ends. Specialized structures and enzymes help maintain chromosome integrity.

• Telomeres: Repetitive nucleotide sequences at chromosome ends; postpone gene erosion.

• Telomerase: Enzyme that extends telomeres in germ cells.

• Chromosome Shortening: Occurs with each cell cycle; telomeres protect essential genes.

• Telomerase and Cancer: Telomerase activity is observed in cancer cells, allowing unlimited cell division.

Example: Telomere shortening is associated with aging and limits the number of cell divisions.

Concept 16.3: Chromosome Structure and DNA Packing

Chromosomes consist of DNA molecules tightly packed with proteins. The organization of chromatin affects gene expression and cell division.

• Bacterial Chromosomes: Double-stranded, circular DNA with minimal protein; found in the nucleoid.

• Eukaryotic Chromosomes: Linear DNA with substantial protein; organized as chromatin.

• Histones: Proteins responsible for DNA packing; form nucleosomes.

• Nucleosome: DNA wrapped around a core of eight histones; basic unit of chromatin structure.

• Chromatin Packing: Chromatin can be loosely packed (euchromatin) or highly condensed (heterochromatin).

• Gene Expression: Dense packing in heterochromatin makes gene expression difficult.

• Chromatin Changes: Chromatin undergoes structural changes during the cell cycle, condensing into metaphase chromosomes for mitosis.

• Histone Modification: Chemical changes to histones affect chromatin condensation and gene expression.

Example: Chromatin "painting" techniques reveal the organization of chromosomes within the nucleus.

Key Equations and Concepts

• Chargaff's Rule:

• Semiconservative Replication:

• Directionality of DNA Synthesis:

Relevant Images

• (Directly relevant as the source textbook for these notes)