Notes on Molecular Basis of Inheritance

Overview of DNA and Its Role in Inheritance

  • Genetic Components
    • Main components of chromosomes: DNA and protein
    • The role of DNA in heredity and genetic information.

Evidence Supporting DNA as Genetic Material

  • Griffith's Experiment (1928)

    • Showed that a substance from heat-killed bacteria could transform living bacteria.
    • Identified DNA as the transforming substance through the work of Oswald Avery.

  • Hershey-Chase Experiment (1952)

    • Conducted by Alfred Hershey and Martha Chase.
    • Demonstrated that only DNA (not protein) entered E. coli during viral infection, confirming DNA as genetic material.

Structure of DNA

  • Watson and Crick Model (1953)

    • DNA is structured as a double helix, allowing for the replication of genetic information.
    • Hereditary information drives the biochemical, anatomical, and physiological traits.

  • Role of Rosalind Franklin's X-ray Crystallography

    • Provided images that suggested DNA was helical, the width of the helix, and spacing of nitrogenous bases.
    • Contributed crucial data leading to Watson and Crick's model.

  • Chargaff's Rules

    • Base composition of DNA varies among species.
    • In any species, the amount of adenine (A) equals that of thymine (T) and guanine (G) equals that of cytosine (C).

DNA Replication

  • Semiconservative Model

    • Proposed by Watson and Crick: each daughter DNA strand contains one old and one new strand.
    • Supported by Meselson and Stahl's experiments.

  • Mechanism of Replication

    • DNA strands replicated in an antiparallel fashion: new strands elongate from the 5' to 3' direction.
    • Key Enzymes:
    • DNA Polymerases: Extend DNA strands by adding nucleotides.
    • DNA Ligase: Joins Okazaki fragments on lagging strands.

DNA Repair Mechanisms

  • Nucleotide Excision Repair

    • A system that removes damaged DNA segments and fills in the gaps, crucial for maintaining DNA integrity.
    • Important for repairing damage caused by UV light.

  • Mutations and Evolution

    • Proofreading mechanisms result in low but non-zero error rates, leading to mutations.
    • Mutations provide genetic variation, which is essential for natural selection.

Telomeres

  • Function
    • Composed of repetitive nucleotide sequences, protecting chromosomes from degradation during replication.
    • Without telomerase, chromosomes shorten with each cell division, leading to aging and cellular senescence.

Chromatin Structure

  • Types:
    • Heterochromatin: tightly packed, largely inactive.
    • Euchromatin: loosely packed, accessible for transcription.
  • Chromosomes exhibit dynamic behavior, changing structure during various cell processes.