6.2 Replication

Semiconservative copying

DNA replication is semiconservative: each new double helix keeps one original strand and one new strand. First helicase unwinds the double helix; each old strand then serves as a template for building a new complementary strand.

The enzymes

DNA polymerase adds nucleotides only in the 5′→3′ direction. The leading strand is built continuously; the lagging strand is made in pieces called Okazaki fragments, which DNA ligase later joins. Primase lays down a short RNA primer to get polymerase started.

Key terms

Semiconservative replication

Each new DNA molecule has one original strand and one new strand.

Helicase

The enzyme that unwinds and separates the DNA double helix.

DNA polymerase

Builds the new DNA strand, adding nucleotides 5′→3′.

Okazaki fragments

Short pieces that make up the lagging strand, later joined by ligase.

DNA ligase

Joins DNA fragments into a continuous strand.

Try an AP-style question

Which model does this result rule out?

1. Conservative replication
2. Semiconservative replication
3. No model can be ruled out
4. Dispersive replication