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How does complementary base pairing allow DNA to replicate?

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Diagram of a DNA polynucleotide chain showing linked deoxyribose sugars, phosphate groups, and nitrogenous bases (adenine, guanine, cytosine, thymine), with an inset comparing ribose and uracil in RNA. — Nucleic acid A portion of the polynucleotide chain of deoxyribonucleic acid (DNA), with an inset showing the corresponding pentose sugar and pyrimidine base in ribonucleic acid (RNA).

Complementary base pairing is the fundamental mechanism that allows DNA to replicate accurately, ensuring that genetic information is preserved and passed down through generations.12

The process relies on the specific chemical structure of DNA, which consists of two antiparallel strands held together by hydrogen bonds between nitrogenous bases. According to Watson-Crick base-pairing rules, adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C).134

During replication, the double helix unwinds and the two strands separate, exposing the bases on each strand. Each original strand then serves as a template for the synthesis of a new, complementary strand. Enzymes known as DNA polymerase move along these templates, pairing free nucleotides with their complementary partners on the existing strand. Because A only bonds with T and G only with C, the sequence of the new strand is precisely dictated by the original, resulting in two identical double-stranded DNA molecules. This semiconservative replication ensures that each daughter cell receives an exact copy of the parent's genetic code.1235