DNA replication
DNA replication is the cell process that copies genetic information before cell division, using each strand of the double helix as a template to build a new complementary strand with high accuracy.
What DNA replication does
Before a cell divides, it must copy its DNA so each daughter cell receives a full set of genetic instructions. DNA replication is the process that makes that copy. It is essential for growth, tissue repair, reproduction, and heredity. In humans, every dividing cell must copy billions of DNA base pairs, while bacteria and viruses copy smaller genomes using related but sometimes different machinery.
Why the double helix matters
DNA has two strands wound into a double helix. The bases pair in predictable ways: adenine pairs with thymine, and cytosine pairs with guanine. Because of this base-pairing rule, each original strand can serve as a template for rebuilding the missing partner strand. Replication is called semiconservative because each finished DNA molecule contains one original strand and one newly made strand.
Where replication begins
Replication starts at specific DNA sequences called origins of replication. Proteins recognize these origins, open the double helix, and create replication bubbles. At each end of a bubble is a replication fork, the Y-shaped region where DNA is being unwound and copied. Bacteria often have one main origin on a circular chromosome, while eukaryotic cells such as plant and animal cells use many origins across long linear chromosomes.
The replication machinery
Several enzymes work together. Helicase unwinds the double helix. Single-strand binding proteins keep the separated strands from snapping back together. Topoisomerase relieves twisting stress ahead of the fork. Primase makes short RNA primers because DNA polymerase cannot start a new strand by itself. DNA polymerase extends from the primer by adding nucleotides. DNA ligase seals gaps so the sugar-phosphate backbone becomes continuous.
Leading and lagging strands
DNA polymerase can add nucleotides only to the 3-prime end of a growing strand, so new DNA is made in the 5-prime to 3-prime direction. Because the two template strands run in opposite directions, one new strand is copied continuously toward the fork; this is the leading strand. The other is copied away from the fork in short Okazaki fragments; this is the lagging strand. Later, primers are removed, replaced with DNA, and fragments are joined by ligase.
Proofreading and repair
Replication is accurate because DNA polymerases choose bases according to template pairing and many polymerases proofread as they work. If a wrong base is added, proofreading can remove it before synthesis continues. Additional repair systems can fix mismatches or damage after replication. These systems are not perfect, so some changes become mutations. Mutations can be neutral, harmful, or occasionally useful for evolution, but in body cells they can also contribute to cancer.
Telomeres and replication stress
Eukaryotic chromosomes are linear, which creates a problem at the ends because ordinary replication cannot fully copy the very end of the lagging strand. Telomeres are repeated DNA sequences that protect chromosome ends, and telomerase can extend them in some cells. Replication stress happens when copying slows or stalls because of DNA damage, difficult sequences, low nucleotide supply, or collisions with other cell processes. Cells must manage this stress to preserve genome stability.
Why it matters
DNA replication matters because life depends on copying genetic information accurately enough to preserve identity, but flexibly enough for evolution over long timescales. Understanding replication helps explain inheritance, development, aging, cancer, antibiotic targets, viral infection, genetic disease, biotechnology, and DNA sequencing. When replication works, cells pass on instructions. When it fails, the consequences can range from harmless variation to serious disease.