Overview
This lecture explains DNA replication, covering where and when it occurs, the main enzymes involved, the step-by-step process, and key structural features of DNA.
DNA Replication: When and Where
- DNA replication is the process of making more DNA before cell division.
- In eukaryotic cells, replication occurs in the nucleus during interphase, before mitosis or meiosis.
- Prokaryotic cells, which lack a nucleus, also perform DNA replication with some differences.
Key Enzymes Involved
- Helicase "unzips" the two DNA strands by breaking hydrogen bonds.
- DNA polymerase builds new DNA strands by adding nucleotides.
- Primase makes RNA primers to help DNA polymerase begin replication.
- Ligase joins DNA fragments together.
- Topoisomerase prevents DNA supercoiling during replication.
- SSB (single-stranded binding) proteins keep DNA strands separated during replication.
DNA Structure and Directionality
- DNA is composed of two complementary, anti-parallel strands.
- Strands are labeled as 5’ to 3’ or 3’ to 5’ based on the carbon numbering on their sugar backbone.
- DNA polymerase can only build new DNA in the 5’ to 3’ direction.
Steps of DNA Replication
- Replication starts at specific DNA sequences called origins.
- Helicase unwinds DNA, SSB proteins bind to keep strands apart, and topoisomerase reduces supercoiling.
- Primase places RNA primers on both strands.
- DNA polymerase builds new strands in the 5’ to 3’ direction.
- The leading strand is synthesized continuously; the lagging strand is synthesized in Okazaki fragments.
- Ligase seals gaps between Okazaki fragments.
- RNA primers are replaced with DNA bases.
- The result is two identical DNA molecules, each with one original and one new strand (semi-conservative replication).
Proofreading and Significance
- DNA polymerase proofreads to minimize errors in base pairing.
- Accurate replication is essential, as mistakes can lead to incorrect genes or proteins.
- Understanding DNA replication has enabled the development of treatments targeting replication in harmful cells.
Key Terms & Definitions
- Helicase — enzyme that unwinds and separates DNA strands.
- DNA polymerase — enzyme that synthesizes new DNA strands.
- Primase — enzyme that creates RNA primers to start replication.
- Ligase — enzyme that connects DNA fragments.
- Okazaki fragments — short DNA pieces made on the lagging strand.
- Semi-conservative replication — process where each new DNA molecule has one old and one new strand.
- Topoisomerase — enzyme that prevents DNA from over-winding (supercoiling).
- SSB proteins — proteins that keep DNA strands single during replication.
Action Items / Next Steps
- Review further reading suggestions to deepen your understanding of DNA replication.