DNA Replication

The big idea

DNA replication is how a cell copies its DNA before it divides. The copy has to be almost perfect, because each new cell needs the same genes. The old DNA does not disappear. Instead, each old strand serves as a pattern for a new partner strand.

Think of DNA as a zipper that can unzip down the middle. Each half is the pattern for a new matching half. When the zipper closes again, you get two zippers, and each one contains one old side and one new side. That is semi-conservative replication, meaning every daughter DNA molecule keeps one original strand and one newly made strand.

What you'll learn

• Why DNA replication makes two DNA molecules that are half old and half new
• What helicase, primase, DNA polymerase, and ligase each do
• Why DNA can only be built in the 5′→3′ direction
• Why the leading strand is continuous and the lagging strand is broken into Okazaki fragments

Where it starts: the replication fork

1. Unzip the helix. Helicase is the enzyme that breaks the hydrogen bonds between base pairs and separates the two strands. This opens the double helix so each strand can be copied.
2. Expose the templates. A template strand is an old DNA strand used as the pattern for a new one. Because bases only match in one way—A with T, and C with G—the template gives the cell the right next base every time.
3. Make a fork. When the DNA opens, it forms a Y-shaped replication fork, the place where the DNA is being unwound and copied. The picture further down shows that fork and the two different ways new DNA is built.

Stage 1: Primers and DNA polymerase

1. Start with a primer. Primase is the enzyme that builds a short RNA primer, a starter piece that gives DNA polymerase something to attach to. DNA polymerase cannot begin a brand-new strand from nothing.
2. Extend the chain. DNA polymerase is the enzyme that adds DNA nucleotides to the 3′ end of the growing strand. That means new DNA is always made in the 5′→3′ direction. The enzyme reads the template strand in the opposite direction, 3′→5′.
3. Match the bases. The cell follows base-pairing rules: A pairs with T, and C pairs with G. This is why the copy is accurate. If the template base is A, the new strand gets T; if the template base is C, the new strand gets G.
4. Keep going. As polymerase adds nucleotides, the sugar-phosphate backbone of the new strand grows longer. The energy for joining nucleotides comes from the incoming nucleotides themselves.

This stage is the same on both new strands. The difference is the direction the template strands run.

Stage 2: Leading strand and lagging strand

1. Read the strand directions. The two DNA templates are antiparallel, meaning they run in opposite directions. Because DNA polymerase only works 5′→3′, the two sides of the fork cannot be copied the same way.
2. Build the leading strand. The leading strand is the new strand made continuously toward the replication fork. One primer is enough, and DNA polymerase can follow the fork as it opens.
3. Build the lagging strand. The lagging strand is the new strand made away from the fork in short pieces, because polymerase still has to build 5′→3′ even though the template runs the other way. Each short piece is an Okazaki fragment, a short stretch of newly made DNA on the lagging strand.
4. Swap out the primers. The RNA primers can't stay in DNA. Another DNA polymerase removes each primer and fills the gap with DNA nucleotides.
5. Seal the gaps. DNA ligase is the enzyme that joins the pieces — including the Okazaki fragments — by sealing the sugar-phosphate backbone. It does not add new bases; it only connects already-made pieces.

This is why one strand looks smooth and the other looks patchy. The leading strand can keep up with the fork, but the lagging strand has to be started over and over.

This picture shows the fork where one DNA molecule is being copied.

Helicase opens the parent strands at the fork. Primase makes primers, DNA polymerase builds new DNA 5′→3′, the leading strand grows continuously, and the lagging strand grows as Okazaki fragments that ligase joins.

Putting it together

Old DNA → helicase unzips the helix → primase lays primers → DNA polymerase extends new DNA 5′→3′ → primers are replaced with DNA → ligase seals the gaps → two DNA molecules

Read it left to right: the cell opens the original DNA, starts each new strand with a primer, builds the new strands by matching bases, and then joins everything into finished DNA.

Worked example: Template strand: 3′-A T G C C A-5′
New strand: 5′-T A C G G T-3′

Step by step, A pairs with T, T pairs with A, G pairs with C, C pairs with G, C pairs with G, and A pairs with T. The new strand is built 5′→3′ because DNA polymerase can only add to the 3′ end. If this template were on the lagging side, the cell would still use the same base-pair rules, but it would make the DNA in short Okazaki fragments that ligase later joins. The final result is two identical DNA molecules, and each one has one old strand and one new strand.

Check yourself

1. Why is DNA replication called semi-conservative? Answer Each daughter DNA molecule keeps one old strand and makes one new strand.
2. Why does primase matter? Answer It makes an RNA primer so DNA polymerase has a starting point.
3. Why is the lagging strand made in fragments? Answer DNA polymerase can only build 5′→3′, so one side of the fork has to be copied in pieces.

Common mistakes

• “DNA polymerase can start a new strand by itself.” It cannot. It needs a primer with a free 3′ end.
• “Ligase adds nucleotides.” It does not. It seals the gaps between fragments.
• “Leading and lagging mean one strand is copied faster and one is copied slower.” The real difference is direction: one is built continuously toward the fork, the other in pieces away from it.

Remember this

• DNA replication is semi-conservative: one old strand + one new strand in each DNA molecule.
• Helicase unzips DNA, primase starts it, DNA polymerase builds it (and swaps the RNA primers for DNA), and ligase seals it.
• DNA is always built 5′→3′; that is why you get a leading strand and a lagging strand.