How DNA Copies Itself Before a Cell Divides

DNA is shaped like a twisted ladder. The sides are made from repeating sugar and phosphate units, while the rungs are pairs of chemical bases. Adenine pairs with thymine, and cytosine pairs with guanine. These matching rules are the key to copying DNA.

When copying begins, an enzyme called helicase moves along the DNA and separates the two strands. It breaks the weak attractions between paired bases, creating an open region called a replication fork. Each original strand can now act as a template for building a new partner.

The cell does not unzip an entire chromosome in one dramatic movement. Copying starts at many points and spreads along the DNA. This lets the cell finish the enormous job more efficiently, instead of waiting for one tiny copying team to crawl from one end to the other.

An enzyme called DNA polymerase adds new DNA building blocks beside the exposed bases. Wherever the template has adenine, the new strand receives thymine. Wherever it has cytosine, the new strand receives guanine. The order on the old strand therefore determines the order on the new one.

The two template strands point in opposite directions, and DNA polymerase can build in only one direction. One new strand is made smoothly. The other is built in short pieces that are later joined by another enzyme called ligase. The result looks tidy at the end, although the work was carried out in two slightly different ways.

Each finished DNA molecule contains one original strand and one newly built strand. This is called semiconservative replication. The old strand is not thrown away. It remains as half of the new double helix and guides the construction of its matching partner.

DNA polymerase checks much of its own work as it moves. Other repair systems inspect the DNA afterward and fix many remaining errors. A few changes can still escape, and these changes are called mutations. Most have little effect, some are harmful and a small number can create useful variation. Cells also pause at checkpoints before division, giving repair systems more time to detect damage that could cause larger problems later.

You can remember the copying process as a calm sequence:

• •The double helix opens.
• •Each old strand becomes a template.
• •Matching bases are added.
• •Short pieces are joined where needed.
• •Repair systems check the finished copy.

DNA replication works because each base has a reliable partner. The strands separate, enzymes build matching strands and repair systems check the result. Slow, careful copying gives dividing cells the instructions they need.