Library · Cellular process
How DNA is copied
Make it clearer
Before a cell divides in two, it has to make a complete copy of its DNA, the molecule where all of its information is written. That copy is called replication, and it has one peculiarity: it is semiconservative. That means each of the two molecules left at the end carries one old strand, which served as the template, and one new strand paired with it.
It all happens at one specific point in the cell's life, S phase. First comes helicase, an enzyme that breaks the hydrogen bonds, the weak links holding the two strands together. As they separate, a Y shape called the replication fork opens up: it is like opening a zipper and leaving the two rows of teeth exposed.
The two strands are not copied the same way, because they run in opposite directions. One is copied continuously, moving toward the fork. The other, because of its opposite orientation, has to be copied in separate little pieces, the Okazaki fragments, which an enzyme called ligase later stitches together.
And the copy does not come out perfect. That is why the process comes with proofreading mechanisms, and after G2 phase, another point in the cell cycle, fewer than ten errors remain.
In brief
Replication is semiconservative: each new molecule carries one old strand, the template, and one newly made strand.
It happens in S phase: helicase breaks the hydrogen bonds and opens the double helix into a fork.
A zipper that opens and leaves two rows exposed.
One strand is copied continuously; the other, running the opposite way, comes out as Okazaki fragments that ligase joins.
The copy is not perfect; proofreading follows, and after G2 phase fewer than ten errors remain.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingBiochemistry, DNA Replication, StatPearlsPublished
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