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DNA is a double-helix polymer made of two intertwined polynucleotide chains that carry genetic instructions for living organisms and many viruses. Each chain is built from repeating nucleotides, where every nucleotide contains a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (A, T, C, or G). The sugars and phosphates form an alternating sugar–phosphate backbone, while the bases on opposite strands pair specifically through hydrogen bonds according to Watson–Crick rules: A pairs with T and C pairs with G. Because the strands are complementary, each strand stores the same biological information, which can be duplicated when the helix separates during replication. The two DNA strands run in opposite directions (antiparallel), giving each strand polarity with a 5′ end (terminal phosphate) and a 3′ end (terminal hydroxyl). The double helix is stabilized mainly by hydrogen bonding between paired bases and by base-stacking interactions between neighboring aromatic bases. DNA also contains structural features such as major and minor grooves, which provide accessible binding sites for proteins (e.g., transcription factors) that recognize specific DNA sequences. In addition, DNA can adopt different conformations (most commonly B-DNA in cells, but also A-DNA and Z-DNA under certain conditions), and its strand separation can be studied via melting behavior, which depends on factors like GC content, sequence, and length.
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