Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Replication ensures each daughter cell receives the same hereditary information.
- It is semiconservative: the parent strands separate, each acts as a template, and each new molecule keeps one parent strand.
- Evidence (Meselson and Stahl, 1958): after one round in a new isotope, all DNA was intermediate (not conservative); after two rounds, two bands appeared (not dispersive).
- DNA polymerase adds nucleotides only to a 3′ end, so DNA is made 5′ → 3′. It cannot start a chain, so it needs an RNA primer.
- Replication starts at an origin, which opens two forks moving apart. Bacteria typically have one origin; each eukaryotic chromosome has many.
Key relationships
| Enzyme or part | Where | Job |
|---|---|---|
| Topoisomerase | ahead of the fork | relieves supercoiling (overwinding) |
| Helicase | at the fork | separates strands by breaking hydrogen bonds between bases |
| RNA primer | start of each new strand or fragment | provides a 3′ end to build on |
| DNA polymerase | behind the fork | adds complementary nucleotides to the 3′ end; also replaces primers with DNA |
| Ligase | lagging strand | seals the backbone between fragments |
| Strand | Direction relative to the fork | Synthesis | Primers |
|---|---|---|---|
| Leading | towards the fork | continuous | one per fork |
| Lagging | away from the fork | discontinuous, in fragments | one per fragment |
| Model | After round 1 | After round 2 |
|---|---|---|
| Conservative | old and new bands, 50 : 50 | old 25%, new 75% |
| Semiconservative | all hybrid | hybrid 50%, all-new 50% |
| Dispersive | one hybrid band | one band, shifted further |
Assumptions behind the numbers
- Band predictions assume every molecule replicates exactly once per round.
- Replication-time calculations assume constant fork speed, two forks per origin and all origins starting together.
Mistakes to avoid
- Saying the lagging strand is made 3′ → 5′.
- Saying helicase breaks covalent bonds, or that topoisomerase acts behind the fork.
- Forgetting that DNA polymerase needs a primer.
- Saying ligase forms hydrogen bonds.
- Writing a new strand without reversing it to read 5′ → 3′.
- Confusing “fraction of molecules containing an old strand” with “fraction of strands that are old”.
Quick self-check
- A template strand reads 5′-ATTGCC-3′. Write the new strand 5′ → 3′. (5′-GGCAAT-3′)
- DNA with both strands labelled replicates three times in unlabelled medium. What percentage of molecules contain a labelled strand? (8 molecules, 2 contain one labelled strand: 25%)
- A drug blocks ligase. Which strand is affected, and how? (The lagging strand stays as separate fragments; the leading strand is largely unaffected.)
Next: practice questions.