Overview of DNA Replication
DNA replication is the biological process by which a cell duplicates its deoxyribonucleic acid (DNA) to produce two identical copies. This occurs during the S phase of the cell cycle and is essential for cell division, ensuring each daughter cell receives a complete set of genetic information. The process is semi-conservative, meaning each new DNA molecule consists of one original strand and one newly synthesized strand.
Key Stages of DNA Replication
DNA replication involves three main stages: initiation, elongation, and termination. Initiation begins at specific sites called origins of replication, where the enzyme helicase unwinds the double helix, creating a replication fork. Single-strand binding proteins stabilize the unwound strands. During elongation, DNA polymerase adds nucleotides to the 3' end of the growing strand, synthesizing the leading strand continuously and the lagging strand in short Okazaki fragments. Primase provides RNA primers to start synthesis. Termination occurs when replication forks meet, and enzymes like ligase seal the fragments.
Practical Example: Replication in Bacteria
In prokaryotes like E. coli, replication starts at a single origin of replication (oriC). Helicase unwinds the DNA, and two replication forks move bidirectionally around the circular chromosome. DNA polymerase III synthesizes new strands, with the leading strand formed continuously and the lagging strand via Okazaki fragments joined by DNA ligase. This process completes in about 40 minutes, producing two identical bacterial genomes for daughter cells.
Importance and Applications
Accurate DNA replication is crucial for genetic stability, enabling growth, repair, and reproduction. Errors, or mutations, can lead to diseases like cancer if not corrected by proofreading mechanisms in DNA polymerase. Understanding this process aids in biotechnology applications, such as PCR for amplifying DNA in research and forensics, and developing antiviral drugs targeting viral replication.