DNA Structure and Composition

Watson and Crick's DNA Model (1953)

  • James Watson (American, 24 years old)
  • Francis Crick (English, 36 years old)

Chromosomes and Genetic Material

  • Chromosomes contain genetic material, composed of a mixture of proteins and deoxyribonucleic acid (DNA) nucleotides.
  • Proteins consist of 20 different amino acids, all present in chromosomes.
  • There are only four kinds of nucleotides present in chromosomes.
  • The genetic information would be in either the protein or nucleotide component, but not both.

Nucleotide Components

  • Four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).
    • Pyrimidines: Cytosine (C) and Thymine (T) (single-ring structures)
    • Purines: Adenine (A) and Guanine (G) (double-ring structures)
  • Each nucleotide includes:
    • Nitrogenous base
    • Pentose sugar
    • Phosphate group

Chargaff's Rule

  • Base Compositions in DNA from Selected Organisms:
    • In chromosomal material:
      • Proportion of adenine = proportion of thymine
      • Proportion of cytosine = proportion of guanine
  • Examples of A/T and G/C ratios in different organisms' DNA:
    • E. coli: A/T = 1.09, G/C = 0.99
    • Yeast: A/T = 0.95, G/C = 1.09
    • Sea urchin sperm: A/T = 1.02, G/C = 0.96
    • Herring sperm: A/T = 1.01, G/C = 0.98
    • Human liver: A/T = 1.00, G/C = 0.98
    • Corn (zea mays): A/T = 1.01, G/C = 1.00