okazaki fragments

Meaning: DNA polymerase adds nucleotides only to a free 3' end, so every new strand grows 5'→3'. The two template strands are antiparallel. The leading strand is copied continuously in the same direction the fork opens. The lagging strand template must be copied in the direction away from the fork, so synthesis goes in short stretches called Okazaki fragments. Each fragment begins with an RNA primer made by primase. In prokaryotes, DNA polymerase III extends the primer, DNA polymerase I removes the RNA and fills the gap with DNA, and DNA ligase seals the remaining nick. Eukaryotic fragments are shorter than prokaryotic ones.

On the MCAT: Passages show a replication fork and ask which strand is lagging, which way polymerase moves, or where primers are needed. The lagging strand needs many primers, while the leading strand typically needs one per fork. A common prediction question is what happens if ligase or primase is missing. Without ligase, unjoined fragments with nicks accumulate. Without primase, polymerase has no 3' end to extend and replication fails.

Common confusion: Students say the lagging strand is made 3'→5'. It is not. All DNA synthesis runs 5'→3'. The lagging strand is only "backward" relative to the direction the fork moves, and that mismatch is why it is made in fragments.