DNAreplication

DNA replication is the cellular process by which a cell copies its genome before division. In all organisms, replication is semi-conservative: each of the two daughter DNA molecules contains one strand from the original molecule and one newly synthesized strand.

Initiation occurs at origins of replication. Bacteria typically have a single origin (oriC); eukaryotes have multiple origins per chromosome. Licensing restricts re-replication. The origin recognition complex and other factors recruit helicase to unwind DNA, aided by topoisomerases to relieve torsional stress.

During elongation, replication forks advance as helicase separates strands while primase synthesizes short RNA primers. DNA polymerases extend the new strands in the 5' to 3' direction. The leading strand is synthesized continuously; the lagging strand is synthesized in short segments called Okazaki fragments, later joined by DNA ligase. In bacteria, DNA polymerase III carries out most synthesis, while DNA polymerase I removes primers and fills gaps; in eukaryotes, polymerases alpha, delta, and epsilon perform these roles with PCNA and other factors.

Fidelity is maintained by proofreading exonuclease activity in DNA polymerases and by post-replication mismatch repair mechanisms. Topoisomerases and helicases help resolve supercoils and tangles; clamp proteins increase DNA polymerase processivity.

Termination concludes replication when forks meet; in linear chromosomes, ends pose a challenge that is addressed by telomeres and telomerase in certain cells. Replication timing and origin usage are tightly regulated to ensure complete and accurate genome duplication.