Molecular Basis of Inheritance

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A nucleotide has three components: (1) Deoxyribose sugar (5-carbon), (2) Phosphate group, (3) Nitrogenous base (A, T, G, or C). Nucleotides are connected by 3'–5' phosphodiester bonds.

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James D. Watson and Francis Crick proposed the double helix model in 1953, based on X-ray diffraction data from Rosalind Franklin and Maurice Wilkins. Published in Nature.

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In double-stranded DNA: A = T and G = C (base pairing rule). Also: A + G = T + C (purine = pyrimidine). Chargaff observed constant A/T and G/C ratios across species but varying G+C content.

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Hydrogen bonds between complementary base pairs: A–T (2 hydrogen bonds) and G–C (3 hydrogen bonds). The strands are antiparallel (one runs 5'→3', the other 3'→5').

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B-DNA (most common form): pitch = 3.4 nm per complete turn, diameter = 2 nm. Each turn has 10 base pairs, spaced 0.34 nm apart.

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Purines (double ring): Adenine and Guanine. Pyrimidines (single ring): Thymine, Cytosine (DNA), and Uracil (RNA). Purine always pairs with pyrimidine.

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A nucleosome is the basic unit of chromatin packaging. DNA wraps ~1.65 times around a histone octamer (2 each of H2A, H2B, H3, H4). Linked by H1 histone (linker histone). ~200 bp per nucleosome.

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DNA → nucleosome (11 nm "beads on string") → 30 nm solenoid fibre → 300 nm loops → 700 nm condensed coils → 1400 nm metaphase chromosome. Total compaction ratio ~10,000-fold.