Biology
DNA replication — quick study summary
DNA replication is how cells copy their entire genome before division. It's semi-conservative: each new DNA molecule has one original strand and one newly synthesised strand. Helicase unwinds the double helix, DNA polymerase III pairs complementary bases (A–T, G–C), and ligase seals fragments together. The leading strand is built continuously; the lagging strand in Okazaki fragments. Replication is fast (~1000 bases/sec in bacteria) and accurate (error ~1 in 10⁹) thanks to proofreading.
Key points
- Semi-conservative — each new helix has one parent strand + one new strand
- Helicase unwinds DNA; primase lays RNA primers; DNA polymerase III extends
- Leading strand: continuous 5' → 3' synthesis toward the fork
- Lagging strand: Okazaki fragments joined by DNA ligase
- Proofreading by DNA polymerase keeps the error rate ~1 in 10⁹ bases
Practice quiz
Click each question to reveal the answer.
1. Which enzyme unwinds the DNA double helix at the replication fork?
- DNA polymerase
- Helicase
- Ligase
- Primase
Answer: Helicase
Helicase breaks the hydrogen bonds between base pairs to separate the two strands.
2. What does it mean that DNA replication is 'semi-conservative'?
Answer: Each new DNA molecule contains one parent strand and one new strand
Meselson and Stahl's 1958 experiment confirmed this — each new helix is half old, half newly synthesised.
3. Why is the lagging strand built in Okazaki fragments?
Answer: DNA polymerase only synthesises 5' → 3', so the strand opposite the fork direction is built in short backward pieces
Each fragment starts with its own primer; DNA ligase joins them into a continuous strand.
Last reviewed: May 2026