DNA Structure & Chromosomal Organisation

Week 6: DNA Structure & Chromosomal Organization

  • Overview of Key Terms:
    • Cell: Basic unit of life.
    • Nucleus: Organelle that contains DNA.
    • Base pair: Two nucleotides bound together by hydrogen bonds.
    • Chromosome: Structure of tightly coiled DNA.
    • DNA (Double Helix): The helical structure of DNA.
    • Histone proteins: Proteins that package and order DNA into structural units called nucleosomes.
    • Nucleosomes: DNA wrapped around histone proteins.

DNA Discovery: Humble Beginnings

  • Friedrich Miescher (1860s): First to purify nuclei from pus cells.

    • Extracted a substance named nuclein composed of Hydrogen, Carbon, Oxygen, Nitrogen, and Phosphorous.

  • Early 1900s: Chromosomes recognized for carrying genetic information containing both DNA and proteins (20 amino acids).

Transformation Experiment

  • Frederick Griffith: Notable experiments using Streptococcus pneumoniae strains.

    • Strain S (with capsule): Causes infection.
    • Strain R (no capsule): No infection.
    • Mice injected with live S strain died; live R strain did not cause death.

  • Heat-killed S strain + live R strain: Mice died, indicating transformation occurred - genetic information passed from dead S strain to live R strain.

Nucleic Acids as Genetic Material

  • Avery, MacLeod & McCarty (1944): Proved nucleic acids, not proteins, are genetic material.

  • Hershey & Chase (1952): Used bacteriophages to prove DNA carries genetic information.

    • Radioactive tracers used to trace whether proteins or DNA entered bacterial cells.

Nucleic Acid Structure

  • Types of Nucleic Acids:

    • DNA (Deoxyribonucleic acid): Double-stranded, contains A, T, C, G.
    • RNA (Ribonucleic acid): Single-stranded, contains A, U, C, G.

  • Nucleotide Structure:

    • Consists of a phosphate group, sugar (deoxyribose in DNA, ribose in RNA), and nitrogen-containing bases (purines: A and G; pyrimidines: C and T/U).

Chargaff’s Rules

  • Equal quantities of adenine (A) and thymine (T); cytosine (C) and guanine (G).
  • Total purines (A + G) equal total pyrimidines (C + T).

Rosalind Franklin

  • Conducted X-ray crystallography studies indicating:
    • DNA is a helix, phosphates on the exterior, and has a consistent diameter.

Watson and Crick Model

  • Proposed structure of DNA in 1953:
    • Two antiparallel chains of nucleotides forming a double helix with complementary base pairing (A-T, C-G).
  • Genetic information is stored in the sequence of bases.
  • DNA is compacted into tightly coiled chromosomes in the nucleus.

DNA Replication

  • Semi-conservative Replication: Each daughter DNA molecule contains one old strand and one new strand.
  • Key Enzymes:
    • Helicase: Unwinds the DNA helix.
    • Primase: Synthesizes RNA primer for DNA polymerase.
    • DNA Polymerase III: Synthesizes the new DNA strand, adding nucleotides in a 5’ to 3’ direction.
    • Ligase: Joins Okazaki fragments on the lagging strand.

Chromosome Organization

  • Each chromosome made up of highly coiled double-stranded DNA.
  • Nucleosomes formed by DNA winding around histones, condensing DNA significantly.
  • Centromere: Region where sister chromatids attach; forms kinetochores during cell division.
  • Telomeres: Repetitive DNA sequences that protect chromosome ends; shorten with cell division but are restored in germ cells by telomerase.

Summary

  • DNA is essential for genetic information storage and transmission.
  • Replication is semi-conservative and involves several critical enzymes ensuring accuracy.
  • Chromosomes organize DNA into a compact structure necessary for cell division and functionality.