DNA replication is the biological process that produces copies of DNA, enabling a cell to transmit its genetic information during cell division. Each strand of an existing DNA double helix serves as a template for a complementary new strand. The resulting DNA molecules normally preserve the original sequence, although copying errors can occur. Replication depends on coordinated enzymes that separate the parental strands, synthesize new DNA, and complete and check the products. (ncbi.nlm.nih.gov)
Semiconservative replication
Replication is described as semiconservative because each daughter double helix contains one parental strand and one newly synthesized strand. Complementary base pairing supplies the copying rule: adenine pairs with thymine, and guanine with cytosine. Consequently, either parental strand specifies the sequence of its new partner. The two strands are antiparallel, with their sugar–phosphate backbones running in opposite directions. (ncbi.nlm.nih.gov)
The Meselson–Stahl experiment, published in 1958, provided decisive evidence for this mechanism in Escherichia coli. Researchers grew the bacteria with nitrogen-15, then transferred them to nitrogen-14 medium. Density-gradient centrifugation revealed intermediate-density DNA after one generation and both intermediate- and light-density DNA after two. These patterns distinguished semiconservative replication from conservative and dispersive alternatives. The nitrogen labels were stable isotopes, not radioactive tracers. (pmc.ncbi.nlm.nih.gov)
Origins and initiation
Chromosomal replication begins at regions called origins of replication, where initiator proteins recruit and assemble the copying machinery. Local strand separation creates a replication bubble, generally bounded by two replication forks moving in opposite directions. Each fork is a Y-shaped junction between unreplicated DNA and newly forming daughter molecules. (ncbi.nlm.nih.gov)
Many bacteria have a circular chromosome replicated from a single origin. Eukaryotes generally use numerous origins along their larger, linear chromosomes, allowing different regions of the genome to be copied simultaneously. Origin specification differs among organisms: some origins depend on recognizable DNA sequences, whereas others are strongly influenced by surrounding chromatin and associated proteins. (ncbi.nlm.nih.gov)
In a typical eukaryotic cell cycle, nuclear DNA replication occurs during S phase. Origins are licensed before S phase through loading of replicative helicase complexes; activation subsequently creates working forks. Separating licensing from activation helps prevent a chromosome segment from being replicated repeatedly within the same cycle. Chromatin must also be reorganized as the machinery advances, and rebuilt behind the forks. (ncbi.nlm.nih.gov)
Synthesis at the replication fork
DNA helicase separates the parental strands, while single-stranded DNA-binding proteins stabilize exposed templates. DNA topoisomerases relieve torsional strain generated ahead of advancing forks by transiently breaking and rejoining DNA. These activities cooperate with polymerases and other proteins in a molecular assembly called the replisome. (ncbi.nlm.nih.gov)
DNA polymerase adds nucleotides to a pre-existing strand’s free 3′ hydroxyl group. New DNA therefore grows in the 5′-to-3′ direction, while its template is read in the opposite direction. The substrates are deoxyribonucleoside triphosphates; incorporation releases pyrophosphate and forms the sugar–phosphate backbone. Replicative polymerases cannot normally initiate a strand without a primer. Primase supplies short RNA primers that provide the required starting end. (ncbi.nlm.nih.gov)
Because the templates are antiparallel, synthesis has two contrasting arrangements:
- The leading strand is synthesized largely continuously in the direction of fork movement.
- The lagging strand is synthesized discontinuously as Okazaki fragments, each initiated by a new primer and extended away from the advancing fork.
Primer removal and replacement with DNA leave adjacent fragments that DNA ligase joins by sealing breaks in the backbone. Thus, both daughter strands are synthesized 5′ to 3′, despite their different relationships to fork movement. (ncbi.nlm.nih.gov)
The particular enzymes differ between organisms. In eukaryotic nuclear replication, polymerase α–primase makes RNA–DNA primers, polymerase ε performs most leading-strand elongation, and polymerase δ performs most lagging-strand synthesis. This division is not absolute: polymerase δ also contributes to establishing leading-strand synthesis. Sliding clamps help retain polymerases on DNA, supporting sustained copying without repeated dissociation. (pmc.ncbi.nlm.nih.gov)
Fidelity and repair
Accurate replication depends on several successive safeguards. Polymerases preferentially incorporate correctly paired nucleotides. Many replicative polymerases also perform proofreading through 3′-to-5′ exonuclease activity, removing an incorrectly incorporated terminal nucleotide before synthesis resumes. These mechanisms make copying substantially more accurate than base pairing alone. (ncbi.nlm.nih.gov)
Mismatch repair provides an additional safeguard by recognizing mismatched bases that escape proofreading. Repair machinery removes a section of the newly synthesized strand containing the error, after which DNA synthesis and ligation restore the sequence. Reconstitution with purified human proteins has demonstrated coordinated mismatch recognition, excision, and resynthesis. An error that escapes correction can become a stable mutation in a later replication cycle; replication fidelity therefore complements other forms of DNA repair. (pubmed.ncbi.nlm.nih.gov)
Completion and chromosome ends
Replication finishes when converging forks complete intervening DNA. Daughter molecules must also be disentangled so that they can separate during division. Circular chromosomes can remain interlinked after copying, requiring topoisomerase-mediated separation. (ncbi.nlm.nih.gov)
Linear chromosomes present an additional end-replication problem: ordinary primer-dependent synthesis cannot fully maintain their termini. Telomeres are specialized DNA–protein structures at these ends. In cells expressing telomerase, this enzyme extends the chromosome’s 3′ end using an internal RNA template, providing additional sequence for complementary-strand synthesis. Its mechanism differs from ordinary DNA-dependent replication because the template directing telomeric repeat addition is part of the enzyme complex itself. (ncbi.nlm.nih.gov)