Cracking the AP Biology Exam
Because the DNA molecule is twisted over on itself, the first step in replication is to unwind the double helix by breaking the hydrogen bonds. This is accomplished by an enzyme called helicase. The exposed DNA strands now form a y-shaped replication fork:
Now each strand can serve as a template for the synthesis of another strand. DNA replication begins at specific sites called origins of replication. Because the DNA helix twists and rotates during DNA replication, another class of enzymes, called DNA topoisomerases, cuts and rejoins the helix to prevent tangling. The enzyme that performs the actual addition of nucleotides alongside the naked strand is DNA polymerase. But DNA polymerase, oddly enough, can only add nucleotides to the 3′ end of an existing strand. Therefore, to start off replication at the 5′ end, DNA polymerase must add nucleotides to an RNA primase—a short strand of RNA nucleotides. The primer is later degraded by enzymes and the space is filled with DNA.
One strand is called the leading strand, and it is made continuously. That is, the nucleotides are steadily added one after the other by DNA polymerase. The other strand—the lagging strand—is made discontinuously. Unlike the leading strand, the lagging strand is made in pieces of nucleotides known as Okazaki fragments. Why is the lagging strand made in small pieces?
You’ll recall from the diagrams that a molecule of DNA has two ends: a 3′ end and a 5′ end. Normally, nucleotides are added only in the 5′ to 3′ direction. However, when the double-helix is “unzipped,” one of the two strands is oriented in the opposite direction—3′ to 5′. Because DNA polymerase doesn’t work in this direction, nucleotides—the Okazaki fragments—need to be added in pieces. These fragments are eventually linked together by the enzyme DNA ligase to produce a continuous strand. Finally, hydrogen bonds form between the new base pairs, leaving two identical copies of the original DNA molecule.
When DNA is replicated, we don’t end up with two entirely new molecules. Each new molecule has half of the original molecule. Because DNA replicates in this way, by conserving half of the original molecule in each of the two new ones, it is said to be semiconservative.
Many enzymes and proteins are involved in DNA replication. The ones you’ll need to know for the AP Biology Exam are DNA helicase, DNA polymerase, DNA ligase, topoisomerase, and RNA primase.
- Helicase unwinds our double helix into two strands.
- Polymerase adds nucleotides to an existing strand.
- Ligase brings together the Okazaki fragments.
- Topoisomerase cuts and rejoins the helix.
- RNA primase catalyzes the synthesis of RNA primers.