34 DNA MODEL WATSON CRICK

The DNA Model—Watson & Crick

Background

  • Importance of understanding DNA structure:

    • Reveals how DNA is copied during the S phase of interphase.

    • Explains how DNA stores and produces genetic information affecting an organism’s genotype.

  • Historical context:

    • Knowledge of DNA accumulated since the 1940s, waiting to be pieced together.

Historical Interludes

Interlude One: Building Blocks of Biological Macromolecules

  1. General name for building blocks of biological macromolecules: Monomers

  2. Long string of monomers: Polymer

  3. Meaningful segments of DNA: Genes

Interlude Two: Chargaff's Findings

  • Molecular Components Analysis (Linus Pauling):

    • DNA consists of three types of molecules: phosphate, deoxyribose sugar, and nitrogenous bases.

    • Each type is present in equal numbers (e.g., 85 phosphates = 85 sugars = 85 bases).

  1. Name for the three-part building block of DNA: Nucleotide

  • Types of nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T)

    • Ratio patterns found by Irwin Chargaff:

      • A, G, C, T percentages in sample species:

        • Snouters: A 35%, G 15%, C 15%, T 35%

        • Flounders: A 19.5%, G 30.5%, C 30.5%, T 19.5%

        • Barbaloots: A 22%, G 28%, C 28%, T 22%

Chemical Classifications of Bases

  • Purines: Adenine and Guanine

    • Defined by presence of two fused rings (six-sided + five-sided).

  • Pyrimidines: Cytosine and Thymine

    • Defined by presence of a single six-sided ring.

Hydrogen Bonds

  1. Definition of a hydrogen bond: A weak bond formed by the attraction between a hydrogen atom that is covalently bonded to a larger atom (typically O or N) and another atom.

DNA Structure Insights

  • Nucleotide Models: Models representing four bases

    • Components of nucleotides: Red blobs = Oxygen, Blue = Nitrogen, White = Hydrogen, Gray = Carbon

  • Complementary base pairing:

    • Purine-pyrimidine combinations form base pairs via hydrogen bonds.

Watson and Crick Contributions

  • Collaboration with Rosalind Franklin:

    • Utilized Franklin's X-ray diffraction photograph, suggesting the DNA structure was made of two strands twisted in a double helix.

  • Key Features of DNA Structure:

    • Long string molecule made of nucleotides.

    • Specific ratios and pairing of bases due to hydrogen bonding.

    • Two twisted strands providing stability and structure.

Watson and Crick's Model Proposal

  • Structural details presented in Nature (April 25, 1953):

    • Novel features of DNA structure and biological implications.

    • Proposal of a double helical structure with purine-pyrimidine base pairing, revealing mechanisms for genetic information copying.

  • Key findings:

    • Identified existence of two helical strands, specific base pair combinations (A-T and G-C), with implications for genetic replication.