DNA Replication
DNA replication is the biological process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules, each containing one original and one newly synthesized strand. This semi-conservative process is essential for cell division, ensuring that each daughter cell receives a complete copy of the genetic information. It is carried out by a complex of enzymes including DNA polymerase, helicase, primase, and ligase, and occurs during the S phase of the cell cycle.
Key Enzymes in DNA Replication and Their Functions
| Enzyme | Function | Direction | Location |
|---|---|---|---|
| Helicase | Unwinds the double helix | Both strands | Replication fork |
| Primase | Synthesizes RNA primers | 5' to 3' | Leading & lagging strand |
| DNA Polymerase III | Adds new nucleotides | 5' to 3' | Replication fork |
| DNA Polymerase I | Removes RNA primers | 5' to 3' | Lagging strand |
| DNA Ligase | Joins Okazaki fragments | N/A | Lagging strand |
| Topoisomerase | Relieves supercoiling ahead of helicase | N/A | Ahead of fork |
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Related Terms
Transcription (biology)
Transcription is the first step of gene expression in which a specific segment of DNA is copied into RNA (messenger RNA, mRNA) by the enzyme RNA polymerase. The process occurs in the nucleus of eukaryotes and the cytoplasm of prokaryotes, and involves three stages: initiation at the promoter, elongation of the RNA strand, and termination at a specific sequence. The resulting pre-mRNA in eukaryotes undergoes processing (5' capping, polyadenylation, and splicing) before being exported to the cytoplasm for translation.
Gene Expression
Gene expression is the process by which the information encoded in a gene is used to synthesize a functional gene product — most commonly a protein, but also functional RNA molecules such as tRNA, rRNA, and microRNA. It encompasses two main stages: transcription (DNA to mRNA) and translation (mRNA to protein), along with all associated regulatory and processing steps. Gene expression is tightly regulated at multiple levels — transcriptional, post-transcriptional, translational, and post-translational — allowing cells to respond dynamically to developmental cues, environmental signals, and metabolic needs.
Promoter (genetics)
A promoter is a regulatory DNA sequence located upstream (5') of a gene's transcription start site (+1) to which RNA polymerase and transcription factors bind to initiate transcription. In prokaryotes, the consensus promoter elements include the −10 (Pribnow box: TATAAT) and −35 (TTGACA) sequences; in eukaryotes, the core promoter often contains a TATA box (~−30), an initiator element (Inr) at +1, and downstream promoter elements (DPE). The strength of a promoter — determined by how closely its sequence matches the consensus — directly controls the frequency of transcription initiation and therefore the level of gene expression.
From Latin replicare, meaning "to fold back" or "to repeat." The term entered biological usage in the mid-20th century following the discovery of DNA structure by Watson and Crick in 1953, with the semi-conservative model confirmed by Meselson and Stahl in 1958.