DNA Replication
The semiconservative process by which cells duplicate their DNA before division, driven by helicase, polymerases, and Okazaki fragments.
DNA replication is the process by which a cell copies its genome before division, so that each daughter cell receives a complete set of genetic instructions. The mechanism is semiconservative: each new double helix consists of one parental strand and one newly synthesized strand. This was proven by the Meselson–Stahl experiment of 1958, which grew bacteria in heavy nitrogen and then light nitrogen and showed that DNA bands at intermediate density after one generation — exactly what semiconservative replication predicts.
Replication begins at origins of replication, where the double helix is locally unwound, and proceeds bidirectionally, creating two replication forks. The enzyme helicase separates the strands, single-strand binding proteins keep them apart, and DNA polymerase synthesizes new DNA in the 5′ → 3′ direction only. This directionality forces asymmetry at each fork: the leading strand is synthesized continuously toward the fork, while the lagging strand is made in short, discontinuous pieces called Okazaki fragments, which are later joined by DNA ligase. Primase lays down short RNA primers that polymerase extends.
Copying a genome of billions of base pairs with high fidelity requires proofreading. DNA polymerase checks each added nucleotide and removes mismatches, and additional mismatch-repair systems scan freshly replicated DNA; together they bring the error rate down to roughly one mistake per billion base pairs. Errors that survive become mutations, the raw material of evolution and the cause of many diseases.
Replication is tightly coupled to the cell cycle and regulated by checkpoints; failures can lead to genomic instability and cancer. Because the machinery is deeply conserved across life — the polymerases of bacteria, archaea, and eukaryotes share a common ancestor — replication is also a tool: PCR and sequencing exploit polymerases in the laboratory, and the copied DNA is what gene expression later reads to build proteins, passing traits between generations as described by Mendelian inheritance.
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cell division dna genetics molecular biology