Okazaki fragment
/ oh-kah-ZAH-kee /
Because the lagging strand cannot be made in one continuous stretch, the cell builds it as a series of short, separate pieces, like laying a sidewalk one slab at a time instead of pouring it all at once. Each of those short pieces of newly made lagging-strand DNA is called an Okazaki fragment, after Reiji and Tsuneko Okazaki, the scientists who first found evidence for them.
Each fragment begins with a short RNA primer made by primase, which DNA polymerase then extends with DNA in the 5'-to-3' direction until it reaches the start of the previous fragment. In bacteria these fragments are roughly 1,000 to 2,000 nucleotides long; in eukaryotes they are much shorter, around 100 to 200 nucleotides, partly because each one corresponds roughly to the DNA wrapped on a nucleosome. So a single replicating chromosome can spawn thousands or millions of Okazaki fragments along its lagging strands.
An Okazaki fragment is only temporary in its raw form. Before the lagging strand is finished, the RNA primer at the front of each fragment is removed and replaced with DNA, and then DNA ligase seals the nick between neighbouring fragments, turning the patchwork into one unbroken strand. The Okazakis discovered these fragments by pulse-labelling replicating DNA for very short times and finding the newest DNA in short pieces — direct experimental proof that the lagging strand really is built discontinuously.
Maturing one fragment: a new fragment is extended up to the RNA primer of the older fragment ahead of it; a nuclease strips out that RNA; polymerase fills the gap with DNA; finally ligase seals the last nick. Repeat for every fragment, and a continuous strand emerges from many pieces.
Short pieces, primers removed, gaps filled, nicks sealed.
Only the lagging strand is made of Okazaki fragments. The leading strand is essentially continuous (one piece per origin), so it has no Okazaki fragments — a frequent exam mix-up.