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Okazaki Fragments

by Greg Crowther


The leading strand elongates toward
The moving replication fork;
Continuously it extends
(Okazaki joined by ligase!)
Out from the primer to the end.
(Okazaki joined by ligase!)

Reiji Okazaki, working in Japan,
Studied replicating DNA.
Tritiated thymidine helped him understand
The two new strands are built in different ways.


The labeled DNA strands were long and short as well
Following a fifteen-second pulse.
The replication process occurring in the cells
Must be semidiscontinuous!

CHORUS (twice)


This song, originally written for Genetics students at the University of Puget Sound, is about DNA replication by the enzyme DNA polymerase. Its music underscores the difference between the leading strand (synthesized as one long, continuous piece of DNA, and sung as one long, continuous vocal line) and the Okazaki fragments of the lagging strand (synthesized and sung in pieces until joined together by DNA ligase).

Questions: (1) What is tritiated thymidine? (2) What does it mean to say that DNA replication is "semidiscontinuous"? (3) What was the experimental evidence that DNA replication is semidiscontinuous? Answers: (1) Thymidine (one of the four nucleotide building blocks of DNA) that has been radioactively labeled with "heavy hydrogen" (tritium or H-3). (2). "Semidiscontinous" means half discontinuous. One of the two new DNA strands is synthesized continuously; the other new strand is synthesized discontinuously. (3) If all DNA replication was continuous, Okazaki would have found that all just-made pieces of DNA were long. However, he found that some pieces were short, presumably as a result of discontinuous replication.