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Deoxyribonucleic acid (DNA) is the blueprint that carries genetic information from one generation to the next. Every cell contains at least one complete set of this code, organized into 23 chromosome pairs—most cells are diploid, holding one set from each parent. Before a cell divides, it must faithfully duplicate its DNA so that each daughter cell receives an exact copy of the genome. This process relies on multiple layers of quality control to prevent mutations.
DNA is a long polymer composed of a sugar–phosphate backbone with four nucleotide bases—adenine (A), guanine (G), cytosine (C), and thymine (T)—projecting from each sugar. The sequence of these bases encodes the instructions for protein synthesis. Two complementary strands pair via hydrogen bonds, forming the classic double‑helix: A pairs exclusively with T, and C pairs exclusively with G. Maintaining these base‑pairing rules during replication is essential to avoid errors.
Replication is semi‑conservative: each new DNA double helix contains one original strand and one newly synthesized strand. Helicase enzymes unwind the helix, exposing the two template strands. DNA polymerase reads each nucleotide on the template and adds the complementary base to the growing strand. For example, when the polymerase encounters a G on the template, it incorporates a C on the new strand.
DNA polymerase is not only a polymerizing machine; it also performs real‑time proofreading. If it inserts an incorrect base, the polymerase’s exonuclease activity excises the mistake and replaces it with the correct nucleotide. This built‑in error‑checking yields an accuracy rate of about 99 % during synthesis.
To catch errors that slip past polymerase proofreading, cells deploy a second line of defense: mismatch repair. Mut proteins scan the DNA helix for distortions caused by mismatched bases. Once detected, the machinery identifies the newly synthesized strand, cleaves a segment containing the error, and excises it. DNA polymerase then resynthesizes the removed segment, restoring the correct sequence. Unlike the single‑base corrections performed by polymerase, mismatch repair can replace thousands of bases in a single repair event, ensuring genomic stability.
