DNA is double helix โ two polynucleotide chains wound around each other. Each nucleotide: deoxyribose sugar + phosphate + nitrogenous base. Bases: Purines (Adenine, Guanine) and Pyrimidines (Cytosine, Thymine). Base pairing: A=T (2 H-bonds), GโกC (3 H-bonds). Chargaff's rule: %A=%T, %G=%C. Chains are antiparallel: one 5'โ3', other 3'โ5'.
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