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Lecture 12

BIOB11H3 Lecture 12: 2017 OP12


Department
Biological Sciences
Course Code
BIOB11H3
Professor
Dan Riggs
Lecture
12

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OP12: DNA Replication I
Smart Pages: Figs 13-2, 3, 4, 7, 8, 9, 10, 11
Vocabulary words: conservative/semiconservative/dispersive/density label/topoisomerase/
gyrase/DNA polymerase/leading vs. lagging strand/Okazaki fragment/primase
Central Ideas:
1. DNA is a double stranded, antiparallel structure, where the amount of A=T; G=C.
2. There are three possible replication mechanisms: conservative, semi-conservative,
and dispersive. Based on considerations of time, energy, and the potential for
errors, the semi-conservative mechanism is probably the most feasible.
3. Meselson & Stahl used density labeling and ultracentrifugation to show that
bacterial replication is semi-conservative. J.H. Taylor used radiolabelling to show
that eukaryotic cells also employ semi-conservative replication.
4. The mechanism of replication is very much like that of transcription, and problems
such as torsional stress are shared. Topoisomerases and/or gyrases change the
superhelical density of DNA to alleviate/prevent problems.
5. Because the two strands of DNA are antiparallel, this presents a problem for a
replication machine, which can only work 5’ to 3’ (answer next lecture)
6. The leading strand is continuously synthesized 5’ to 3’, whereas the lagging strand
is made in a discontinuous fashion by synthesis of small ‘Okazaki’ fragments.
Questions a scientist might ask:
1. What is the energy source for DNA replication?
2. Could Meselson & Stahl have used radioactive labeling in their experiments?
Sample multiple choice question:
If one repeated Meselson & Stahls experiment beginning with unlabelled DNA and then
added a radioactive label, what proportion of the double stranded molecules would have
one labeled and one unlabelled strand after the third round of replication?
a. 1 of 8.
b. 2 of 8.
c. 1 of 16.
d. 2 of 16.
e. 2 of 32.
Analytical questions:
See questions 1-3 page 515; questions 3 and 4, page 521; questions 1, 3, 5, page 537.
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