DNA Structure, Properties, and Functions

DNA as a double helix polymer

DNA is a polymer made of two polynucleotide strands that coil around each other to form a double helix. Each strand has an alternating sugar–phosphate backbone, while the nitrogenous bases project inward and pair with bases on the opposite strand. The two strands are antiparallel (one runs 5′→3′ while the other runs 3′→5′), and base pairing follows complementary rules: adenine pairs with thymine (A–T) and cytosine pairs with guanine (C–G), held together primarily by hydrogen bonds. The double helix is stabilized not only by hydrogen bonding between base pairs but also by base-stacking interactions between aromatic bases. The helix has characteristic geometry in its common B-DNA form, including a right-handed twist and a repeating pitch of about 3.4 nm per turn (with ~10 base pairs per turn). Because the strands are complementary, each strand contains the information needed to recreate the other during replication when the helix separates and DNA polymerases rebuild complementary sequences.

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Nucleotides and sugar-phosphate backbone

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