Overview of DNA Replication
DNA replication is the biological process by which a cell makes an exact copy of its DNA before cell division. It occurs during the S phase of the cell cycle and is semi-conservative, meaning each new DNA molecule consists of one original strand and one newly synthesized strand. This ensures genetic continuity across generations of cells.
Key Steps in DNA Replication
The process begins with initiation, where the enzyme helicase unwinds the DNA double helix at specific origins of replication, creating replication forks. Primase synthesizes short RNA primers to start synthesis. DNA polymerase then adds nucleotides in the 5' to 3' direction: continuously on the leading strand and discontinuously (forming Okazaki fragments) on the lagging strand. Finally, DNA ligase seals the fragments, and proofreading enzymes correct errors.
Practical Example: Replication in Prokaryotes
In bacteria like E. coli, replication starts at a single origin and proceeds bidirectionally around the circular DNA. Helicase unwinds the strands, and DNA polymerase III extends the new strands. For instance, if the original DNA sequence is ATGC, the complementary strand synthesized would be TACG, demonstrating base-pairing rules (A-T, G-C) in action during elongation.
Biological Importance and Applications
Accurate DNA replication is crucial for cell growth, repair, and reproduction, preventing mutations that could lead to diseases like cancer. In applications, understanding this process aids biotechnology, such as PCR for amplifying DNA in research and forensics, and informs treatments targeting replication errors in pathogens.