Overview
This lecture explains the process of DNA replication, highlighting where and when it occurs, the key enzymes involved, and how new DNA strands are synthesized.
DNA Replication: Location and Timing
- DNA replication occurs in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells.
- Replication happens before cell division during interphase in eukaryotes, ensuring each daughter cell gets DNA.
Key Enzymes and Proteins in DNA Replication
- Helicase unzips the DNA double helix by breaking hydrogen bonds between bases.
- Single-stranded binding (SSB) proteins maintain strand separation during replication.
- Topoisomerase prevents DNA from supercoiling during unwinding.
- Primase makes short RNA primers so DNA polymerase can start replication.
- DNA polymerase builds new DNA strands by adding nucleotides in the 5' to 3' direction.
- Ligase seals the gaps between DNA fragments to create a continuous strand.
DNA Structure & Directionality
- DNA is composed of two antiparallel strands: one runs 5’ to 3’, the other 3’ to 5’.
- Base pairing: adenine pairs with thymine, guanine pairs with cytosine.
- The directionality is determined by the numbering of carbons on the DNA sugar backbone.
Synthesis of Leading and Lagging Strands
- The leading strand is synthesized continuously in the 5' to 3' direction.
- The lagging strand is synthesized in short fragments (Okazaki fragments), each requiring a new primer.
- Okazaki fragments are later joined together by DNA ligase.
Proofreading and Semi-Conservative Replication
- DNA polymerase proofreads new DNA, minimizing errors in replication.
- DNA replication is semi-conservative: each new DNA molecule has one old and one new strand.
Key Terms & Definitions
- Helicase — Enzyme that unwinds and separates the DNA strands.
- DNA Polymerase — Enzyme that synthesizes new DNA strands.
- Primase — Enzyme that creates RNA primers for DNA polymerase.
- Ligase — Enzyme that connects Okazaki fragments on the lagging strand.
- Okazaki Fragments — Short DNA pieces synthesized on the lagging strand.
- Topoisomerase — Enzyme that prevents DNA supercoiling during replication.
- SSB Proteins — Proteins that keep DNA strands apart during replication.
- Semi-Conservative Replication — Each new DNA molecule has one original and one new strand.
Action Items / Next Steps
- Review further reading on DNA replication for more details as suggested in the video.