DNA as a double helix polymer

Summary

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.

Key Points

• 1

  DNA consists of two antiparallel polynucleotide chains forming a double helix.

• 2

  Complementary base pairing (A–T and C–G) links the strands via hydrogen bonds, while base stacking and the sugar–phosphate backbone stabilize the structure.

• 3

  The double helix’s geometry (e.g., right-handed B-DNA twist and ~3.4 nm pitch) supports a reversible separation/rejoining mechanism essential for replication.