DNA double helix and nucleotide pairing rules

Summary

DNA is made of two polynucleotide strands twisted into a double helix, often compared to a ladder spiraled around its length. The sides of the ladder consist of alternating deoxyribose sugars and phosphate groups, while the rungs are pairs of nitrogenous bases. The four bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Each nucleotide contains a sugar, a phosphate group, and one nitrogenous base. The bases pair according to fixed complementary rules: adenine pairs only with thymine (A–T), forming two hydrogen bonds, while cytosine pairs only with guanine (C–G), forming three hydrogen bonds. Pairing can be written in either direction, such as A–T or T–A. These complementary rules allow DNA to store genetic information in the sequence of its bases and enable accurate replication, because each original strand serves as a template for building a matching new strand.

Key Points

• 1

  DNA consists of two complementary polynucleotide strands forming a double helix.

• 2

  The sides of the DNA structure are made of deoxyribose sugars and phosphate groups; the rungs are paired nitrogenous bases.

• 3

  Adenine pairs exclusively with thymine (A–T) through two hydrogen bonds.

• 4

  Cytosine pairs exclusively with guanine (C–G) through three hydrogen bonds.

• 5

  Complementary base pairing allows DNA to encode information and be copied accurately during replication.