BIOL 240 - DNA Structure & Replication Notes

DNA Structure & Replication

Key Contributions to DNA Knowledge

  • James Watson and Francis Crick:

    • Proposed the double helix model of DNA structure based on earlier works.
    • Suggested mechanisms for replication and encoding genetic information.

  • Rosalind Franklin:

    • Provided critical X-ray diffraction images of DNA, which indicated its helical structure.
    • Her work helped reveal the distance between bases in the DNA helix.

  • Maurice Wilkins:

    • Assisted in the X-ray diffraction studies.
    • Collaborated with Franklin, leading to key insights about DNA structure.

Nucleotide Structure

  • Three parts:

    1. Phosphate Group - attached to the 5' carbon of the sugar.
    2. Deoxyribose Sugar - a five-carbon sugar molecule.
    3. Nitrogenous Base - one of four (Adenine, Thymine, Cytosine, Guanine).

  • 5’ and 3’ Ends:

    • 5’ End: Phosphate group at the fifth carbon.
    • 3’ End: Hydroxyl group at the third carbon.

DNA Strand Orientation

  • New nucleotides are always added to the 3' end of a DNA strand during replication.
  • Antiparallel Structure:
    • Refers to the orientation of the two DNA strands; one runs 5’ to 3’ while the other runs 3’ to 5’. This is crucial for their complementary base pairing.

DNA Composition in Chromosomes

  • A single DNA molecule (chromosome) is composed of two strands of DNA, forming a double helix.

Semiconservative DNA Replication

  • Description of Process:
    • Each new DNA molecule consists of one old (template) strand and one newly synthesized strand.
  • Identifying Newly Synthesized Strands:
    • In subsequent rounds of replication, the newly produced strands can be recognized through labeling or sequencing.

DNA Replication in Bacteria

  • Circular DNA Replication:
    • Bacterial DNA is circular, replicating from a single origin of replication, creating replication forks.

Key Terms in Replication

  • Origin of Replication: The specific location where the DNA replication begins.
  • Replication Bubble: The region where the double-stranded DNA has been unwound to allow replication.
  • Replication Fork: The Y-shaped structure that forms during DNA replication where the DNA strands are separated.

Key Proteins and Their Functions

  • DNA Polymerase: Enzyme that synthesizes new DNA strands by adding nucleotides.
  • Primase (RNA Polymerase): Synthesizes a short RNA primer needed by DNA polymerase to initiate DNA synthesis.

Consequences of Protein Mutations

  • If a key protein involved in DNA replication is mutated or nonfunctional, it may lead to replication failures or mutations in the DNA resulting in cellular malfunction or disease.

DNA Replication in Eukaryotes

  • Starts at the origin of replication, forming a replication bubble as the DNA unwinds. Paired strands separate at the replication fork.

Role of Telomeres

  • Telomeres help protect the ends of linear chromosomes from degradation.
  • They shorten with each round of replication, playing a role in aging and cellular division limits.

Leading vs Lagging Strand

  • Leading Strand: Synthesized continuously in the same direction as the replication fork.
  • Lagging Strand: Synthesized discontinuously in sections (Okazaki fragments) opposite to the fork direction.

Direction of Nucleotide Addition

  • Both DNA polymerase and primase add nucleotides in the 5’ to 3’ direction.
  • Primer Requirement: DNA polymerase requires a primer to initiate synthesis, while primase does not need a prior strand to begin RNA synthesis.

Relevant Reading References

  • Campbell Biology, 11th Ed., Chapter 16 discusses:
    • Concept 16.1: DNA as Genetic Material
    • Concept 16.2: Proteins in DNA Replication and Repair
    • Includes figures outlining key processes and structures in DNA replication.