DNA Replication

You’ll be able to

Explain why DNA replication is semiconservative and the evidence behind it
Identify the enzymes of replication and the specific job of each
Relate antiparallel strands to continuous leading and discontinuous lagging synthesis

One old strand, one new strand

Before a cell divides it must copy its entire genome, and it does so semiconservatively: the double helix unzips, and each original strand serves as a template for building a fresh complementary partner. Every daughter molecule therefore ends up with one parental strand and one newly made strand — half old, half new. Meselson and Stahl proved this with density labeling: after one round of replication every molecule was a hybrid of heavy (old) and light (new) DNA, exactly what the semiconservative model predicts and what the "conservative" and "dispersive" models could not.

The enzyme crew at the replication fork

Replication begins at an origin of replication and opens outward, forming a Y-shaped replication fork. Each enzyme has one job. Helicase unwinds and separates the two strands, breaking the hydrogen bonds between bases. Single-strand binding proteins hold the separated strands apart, and topoisomerase relieves the twisting strain ahead of the fork. Primase lays down a short RNA primer to give synthesis a starting point. DNA polymerase then adds DNA nucleotides one at a time, following complementary base pairing (A–T, G–C). Finally ligase seals the remaining gaps in the sugar-phosphate backbone.

Direction of synthesis

DNA polymerase builds 5′ → 3′ only

New nucleotides can only be added to a free 3′ end, so the new strand grows 5′→3′ while it is read off a 3′→5′ template.

Leading vs. lagging strand

The two template strands are antiparallel (one runs 5′→3′, the other 3′→5′), but DNA polymerase can only build in the 5′→3′ direction. On the template exposed 3′→5′, polymerase runs smoothly toward the fork in one continuous piece — the leading strand. On the opposite template, polymerase must work away from the fork, so it can only synthesize in short backward pieces called Okazaki fragments as the fork keeps opening — the lagging strand. Each fragment needs its own primer, and ligase later stitches the fragments into one continuous strand.

Worked example

A template strand reads 3′–T A C G G A–5′. Using it as a template for DNA polymerase, write the new complementary DNA strand in the correct orientation.

1.Apply DNA base pairing to each base: T→A, A→T, C→G, G→C, G→C, A→T.
2.Reading straight across the template (3′→5′), the new bases are A T G C C T.
3.The new strand is antiparallel, so it runs 5′→3′ across from the 3′→5′ template: 5′–A T G C C T–3′.

Answer: The new strand is 5′–A T G C C T–3′ (antiparallel and complementary to the 3′–T A C G G A–5′ template).

Checkpoint

After a molecule of DNA replicates semiconservatively, what does each of the two daughter double helices contain?

Watch out

DNA polymerase can only add to a 3′ end, so it builds every strand 5′→3′ and always needs a primer to start. This single rule is why the lagging strand must be made in backward Okazaki fragments — the direction constraint is the whole reason for the leading/lagging split.

Checkpoint

On the lagging strand, DNA is made in many short Okazaki fragments. Which enzyme joins these fragments into one continuous strand?

On the exam

A reliable free-response earner: match each enzyme to its job in one clean chain — helicase unwinds → primase primes → polymerase extends 5′→3′ → ligase seals. Naming the enzyme and its specific action is what scores the point.

Answer the 2 checkpoints as you read.