Overview of DNA Replication
DNA replication occurs during the S phase of the cell cycle, prior to cell division, through a semi-conservative mechanism where each strand of the double helix serves as a template for a new complementary strand. This process ensures that daughter cells receive identical genetic information. The replication begins at specific sites called origins of replication, where the DNA helix unwinds, forming replication forks that progress bidirectionally.
Key Steps and Enzymes Involved
The process involves several key steps: helicase unwinds the DNA double helix, creating single-stranded templates; single-strand binding proteins stabilize these strands; primase synthesizes short RNA primers; DNA polymerase adds nucleotides to the primers in the 5' to 3' direction, synthesizing the leading strand continuously and the lagging strand in Okazaki fragments; ligase joins the fragments. Proofreading by DNA polymerase corrects errors, maintaining fidelity.
Practical Example in Mitosis
In human cells undergoing mitosis, replication starts at multiple origins along each chromosome during the S phase. For instance, at one origin, helicase opens a segment of DNA, allowing two replication forks to form. The leading strand is synthesized smoothly toward the fork, while the lagging strand is built in short segments away from it, ensuring both new DNA molecules are complete before the cell proceeds to divide into two identical daughter cells.
Importance in Cell Division and Beyond
Accurate DNA replication is crucial for cell division, enabling growth, repair, and reproduction by preserving genetic integrity across generations of cells. Errors in replication can lead to mutations, contributing to diseases like cancer, while its study informs biotechnology applications such as PCR for DNA amplification and gene therapy, highlighting its foundational role in biology.