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DNA double helix deoxyribonucleic acid nucleic acid nucleotide base sugar phosphate backbone spiral helix turn pitch rise twist conformation structure molecular biology biochemistry genetics chromosomes genes proteins nucleotides bases deoxyribose phosphate adenine thymine guanine cytosine uracil nitrogenous bases purines pyrimidines double hydrogen bond major groove minor groove Watson-Crick Franklin-Wilkins Rosalind Franklin James Watson Francis Crick Maurice Wilkins