Molecular Basis of Inheritance

DNA Replication and Genetic Material

  • DNA Replication
    • Genetic information is inherited through DNA replication during mitosis and meiosis.
    • Each gene is a hereditary unit consisting of a specific DNA sequence.
    • Replication starts at multiple sites, transforming unduplicated to duplicated chromosomes.

Concept 16.1: DNA is the Genetic Material

  • Historical Context

    • The structure of DNA was elucidated in 1953 by Watson and Crick, presenting the double helix model.
    • Early 20th century biologists faced challenges in identifying inheritance molecules.

  • Experimentation

    • T.H. Morgan established that genes are on chromosomes; DNA and protein were candidates for genetic material.
    • Griffith’s Experiment (1928):
    • Studied bacterial strains; discovered that heat-killed pathogenic bacteria could transform harmless bacteria into pathogenic forms, termed transformation.
    • Transformation: change in genotype and phenotype due to foreign DNA assimilation.

Evidence That DNA Can Transform Bacteria

  • Avery-MacLeod-McCarty Experiment

    • Identified the transforming substance as DNA.
    • Skepticism existed as the understanding of DNA was still rudimentary.

  • Hershey-Chase Experiment (1952):

    • Used bacteriophage T2 to demonstrate that DNA is the genetic material in viruses.
    • Concluded that only DNA enters the bacterial cell, providing genetic information for virus replication.

Chargaff's Rules

  • Base Composition: Varies among species; A=T and G=C.
  • Base Pairing:
    • A pairs with T (2 hydrogen bonds).
    • G pairs with C (3 hydrogen bonds).

Concept 16.2: DNA Replication and Repair

  • Semiconservative Model of Replication:
    • New DNA helix consists of one old strand and one new strand.
    • Competing models: conservative (parent strands rejoin) and dispersive (strands mix).
  • Replication Overview:
    • Initiates at origins of replication, creating replication bubbles.
    • Each bubble's ends form replication forks.

Mechanisms of DNA Replication

  • Protein Involvement:
    • Helicases unwind DNA, single-strand binding proteins stabilize strands, and topoisomerase alleviates strain.
    • DNA Polymerases: Required for adding nucleotides to a growing strand.
  • Leading and Lagging Strands:
    • The leading strand synthesizes continuously; the lagging strand in Okazaki fragments, later joined by DNA ligase.

Proofreading and Repair Mechanisms

  • DNA Polymerases: Proofread newly synthesized DNA, correcting mismatched nucleotides.
  • Nucleotide Excision Repair: Cuts out and replaces damaged DNA segments.

Telomeres and Aging

  • Telomerase Role: Extends telomeres in germ cells to prevent DNA loss during replication.
  • Shortening telomeres are proposed to connect to aging processes and limit cancer cell growth, as active telomerase is found in many cancer cells.

Chromosomal Structure

  • Chromatin: DNA wrapped around histone proteins, forming nucleosomes.
    • Loose packing (euchromatin) allows gene expression, while dense packing (heterochromatin) restricts it.
  • Interphase and Mitosis: Chromatin condenses into chromosomes during mitosis, ensuring proper DNA segregation.

Summary of Key Concepts

  • Know how Griffith’s work led to the understanding of transformation.
  • Explain Hershey & Chase, and Watson & Crick’s significance in molecular biology.
  • Understand the rules of base pairing and the processes of DNA replication.