Explain the process of DNA replication. Draw a labelled replication fork.

1. 1 · Define the type. DNA replicates semiconservatively: each daughter molecule keeps one parental strand.
2. 2 · Start and open. Replication starts at the origin of replication; the strands separate at a small replication fork.
3. 3 · Name the enzyme and substrates. DNA-dependent DNA polymerase adds deoxyribonucleoside triphosphates, which also give energy.
4. 4 · Explain polarity. Polymerase works only 5′→3′: continuous on the 3′→5′ template, discontinuous on the 5′→3′ template.
5. 5 · Join the pieces. DNA ligase joins the short pieces into one strand.
6. 6 · Draw the fork. [Diagram: Y-shaped fork, leading and lagging strands, Okazaki fragments, polymerase, ligase, 5′ and 3′ ends]
7. 7 · Give the evidence. Meselson and Stahl's ¹⁵N/¹⁴N experiment proved semiconservative replication.
8. 8 · Close in one line. So replication copies DNA exactly, one old strand guiding each new one.

Scoring tip: Label 5′ and 3′ on every strand of the fork. A fork without polarity loses marks.