Overview
The transcript explains DNA damage from UV-induced thymine dimers and the enzymatic repair process during replication, highlighting roles of nucleases, DNA polymerase III, and DNA ligase.
DNA Damage and Thymine Dimers
- UV light can damage DNA by creating thymine dimers on the same strand.
- Thymine dimers cause a kink or distortion in the DNA helix.
- This distortion can block replication and transcription if not repaired.
Nucleotide Excision Repair (Concept)
- Damaged DNA region is recognized and cut out to remove distortion.
- The gap left by removal is refilled with correct nucleotides.
- The strand is sealed to restore the original DNA structure.
Enzymes and Functions
- Nuclease: Cuts and removes the damaged DNA segment.
- DNA polymerase: Fills the gap with correct nucleotides.
- DNA ligase: Seals the repaired section to the existing strand.
DNA Polymerase III and Proofreading
- DNA polymerase III is the main enzyme adding nucleotides during replication.
- It also proofreads newly synthesized DNA to correct errors.
- Proofreading uses exonuclease activity to remove incorrect bases.
- Separate exonucleases can also remove wrong bases when needed.
- Using DNA polymerase III for repair is efficient since it is already synthesizing the strand.
Structured Summary
| Process/Item | Cause/Role | Effect/Function | Outcome |
|---|
| Thymine dimer | UV light links adjacent thymine bases on one strand | Creates a kink/distortion in DNA | Blocks replication and transcription |
| Nuclease | Recognizes damaged region | Cuts out damaged DNA segment | Prepares gap for repair synthesis |
| DNA polymerase (III) | Main synthesizing enzyme; proofreading | Adds correct nucleotides; exonuclease removes errors | Restores correct sequence during repair |
| Exonuclease (separate) | Error correction | Removes wrong bases independently of polymerase III | Supports proofreading and repair |
| DNA ligase | Post-synthesis sealing | Forms final phosphodiester bond | Restores continuous DNA strand |
Key Terms & Definitions
- Thymine dimer: Covalent linkage of two adjacent thymine bases on the same DNA strand.
- Nuclease: Enzyme that cuts nucleic acids, removing damaged regions.
- Exonuclease: Enzyme activity that removes nucleotides from DNA ends to correct errors.
- DNA polymerase III: Main DNA-synthesizing enzyme with proofreading capability.
- DNA ligase: Enzyme that seals nicks by forming phosphodiester bonds in DNA.
Action Items / Next Steps
- Understand how UV light leads to thymine dimer formation.
- Memorize the roles of nuclease, DNA polymerase III, and DNA ligase in repair.
- Practice tracing the repair steps: damage recognition → excision → synthesis → sealing.