DNA Replication

• 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.

• Each strand can serve as a template for the synthesis of another strand.

• DNA replication begins at specific sites called origins of replication.

• 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 to the freshly built strand is DNA polymerase. But DNA polymerase can add nucleotides only to the 3′ end of an existing strand.

• To start off replication, an enzyme called RNA primase adds a short strand of RNA nucleotides called an RNA primer.

• After replication, the primer is degraded by enzymes and replaced with DNA so that the final strand contains only DNA.

• During DNA replication, one DNA strand is called the leading strand, and it is made continuously. 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.

• Nucleotides are added only in the 5′ to 3′ direction since nucleotides can be added only to the 3′ end of the growing chain.

• 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, the strand needs to be built in pieces.

• The lagging strand is built in the opposite direction of the way the helix is opening, so it can build only until it hits a previously built stretch. Once the helix unwinds a bit more, it can build another Okazaki fragment.

• 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 a way that conserves half of the original molecule in each of the two new ones, it is said to be semiconservative.

• The bits of unimportant DNA are at the ends of a molecule. These ends are called telomeres.

• 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.

• DNA 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.