| Enzyme | Direction | Key Function | NEET Trap |
|---|---|---|---|
| Helicase | 5'→3' along template | Unwinds double helix, breaks H-bonds | Does NOT synthesize DNA |
| SSB proteins | N/A | Stabilize single-stranded templates | Not enzymes — structural proteins |
| Topoisomerase (Gyrase) | N/A | Relieves positive supercoils ahead of fork | Distinct from helicase |
| Primase | 5'→3' | Synthesizes short RNA primer (5–10 nt) | RNA polymerase — no primer needed |
| DNA Pol III | 5'→3' synthesis (reads template 3'→5') | Main replication enzyme | Cannot initiate without primer |
| DNA Pol I | 5'→3' synthesis + 5'→3' exonuclease | Removes RNA primers; fills gaps | Has BOTH activities |
| DNA Ligase | N/A | Seals nicks; joins Okazaki fragments | Requires ATP (euk) or NAD+ (prok) |
| Telomerase | 5'→3' | Extends chromosome 3'-ends | Ribonucleoprotein; uses own RNA template |
Critical values:
- Both leading and lagging strands synthesized 5'→3'
- RNA primer removed by DNA Pol I (5'→3' exonuclease)
- Proofreading: 3'→5' exonuclease activity of DNA Pol I and III
- Error rate before proofreading: ~1/10^5; after: ~1/10^7