The Molecular Basis of Inheritance

The Molecular Basis of Inheritance

DNA Replication and Genetic Transmission

  • DNA Replication: It is the process by which genetic information is transmitted from a parent cell to daughter cells (via mitosis) and from one generation to the next (during meiosis).
  • Chromosomes:
    • Unduplicated Chromosome: Comprises one DNA molecule associated with proteins.
    • Duplicated and Condensed Chromosome: Composed of two DNA molecules and proteins resulting from the replication process.
  • Genes: Defined as units of hereditary information consisting of specific DNA sequences; replication starts from various checkpoints on the chromosome.
  • Process Overview: DNA replication concludes with two DNA molecules that are distributed to daughter cells, thereby ensuring inheritance.

Molecular Structure of DNA

  • Concept: DNA is the genetic material.
  • Composition:
    • DNA is a polymer of nucleotides.
    • Each nucleotide consists of a:
    • Nitrogenous base (A, T, G, C)
    • Sugar (deoxyribose)
    • Phosphate group
  • Base Composition Rules:
    • Varies between species.
    • Within any species, the number of adenine (A) is equal to thymine (T), and guanine (G) is equal to cytosine (C).

Structure of a DNA Strand

  • Backbone: Composed of sugar-phosphate structures.
  • Ends: Each DNA strand has a 5' end and a 3' end.
  • Diagram Features:
    • Nitrogenous bases exhibit distinct chemical properties:
    • Thymine (T)
    • Guanine (G)
    • Cytosine (C)
    • Adenine (A)

Building a Structural Model of DNA

  • Contributors: Watson and Crick conceptualized the double helix model based on X-ray diffraction data provided by Rosalind Franklin.
  • Model Characteristics: Indicates that there are two outer sugar-phosphate backbones with nitrogenous bases paired inside the structure.
  • Antiparallel Orientation: Both strands run in opposite directions, which is critical for replication and function.

Base Pairing Rules

  • Specific Pairing: Watson and Crick identified that:
    • Adenine (A) pairs exclusively with Thymine (T).
    • Guanine (G) pairs exclusively with Cytosine (C).
  • Chargaff’s Rules: Reinforce that the amount of A equals that of T, and G equals C within a given species.
  • Hydrogen Bonds: Base pairs are held together by hydrogen bonds, providing stability to the double helix structure.

DNA Replication Process

  • Accurate Replication: Necessary for the resemblance of offspring to parents. Ensures genetic information is preserved.
  • Base Pairing: Each strand serves as a template for constructing a new strand, producing two identical replicas of the original molecule.
  • Replication Models:
    • Semiconservative Model: Each daughter molecule retains one original (parental) strand and one newly synthesized strand.
    • Competing Models:
    • Conservative Model: Entire parental strand is conserved.
    • Dispersive Model: Each strand is a hybrid of old and new DNA.

Detailed DNA Replication Mechanics

  • Initiation: Begins at origins of replication, where strands are unwound to create replication bubbles which expand bidirectionally.
    • Eukaryotic chromosomes can have hundreds to thousands of origins.
  • Replication Forks: Create Y-shaped regions where the double helix is unwound, facilitated by:
    • Helicases: Unwind the DNA by breaking hydrogen bonds.
    • Single-strand Binding Proteins: Stabilize single-stranded DNA.
    • Topoisomerase: Relieves torsional strain by breaking and rejoining strands.

Synthesizing New DNA Strands

  • Role of Primers: DNA polymerases need a primer to initiate synthesis; the primer is created by primase and is typically short (5-10 nucleotides).
  • DNA Polymerases: Enzymes responsible for adding nucleotides during replication.
    • Elongation Rates: Approximately 500 nucleotides per second in bacteria, 50 nucleotides per second in humans.
  • Antiparallel Elongation: Strands elongate only in the 5' to 3' direction, influencing the synthesis of leading and lagging strands.
    • Leading Strand: Synthesized continuously towards the replication fork.
    • Lagging Strand: Synthesized in segments (Okazaki fragments) away from the replication fork, requiring RNA primers for each fragment.

Joining Okazaki Fragments

  • Lagging Strand Synthesis: Involves segments called Okazaki fragments, which are later joined by DNA ligase after the primer has been replaced by DNA.

Proofreading and Repair of DNA

  • DNA Repair Mechanisms: DNA polymerases proofreading capability ensures errors are corrected during synthesis.
    • Mismatch Repair: Repair enzymes correct incorrectly paired nucleotides post-replication.
    • Nucleotide Excision Repair: A mechanism where damaged DNA is cut out and replaced with the correct sequence.

Telomeres and Their Role in Replication

  • Function of Telomeres: Protect chromosomal ends and postpone the erosion of vital genes.
  • Telomerase Enzyme: Lengthens telomeres in germ cells, which otherwise shrink during DNA replication in somatic cells, potentially leading to gene loss in cells.
  • Cancer Relation: Telomerase activity is often found in cancer cells, allowing them to divide indefinitely.

Evolutionary Significance of DNA Mutations

  • Mutation Consequences: Changes in DNA sequences can become permanent, contributing to genetic variation, which is vital for the process of natural selection and the evolution of species.