Biology 12

240 MHR • Unit 3 Molecular Genetics

SECTION REVIEW

1. Differentiate between conservative, semi-
   conservative, and dispersive theories of replication.
   Which theory was supported by experimental
   evidence?


2. Summarize the experiment conducted by
   Meselson and Stahl to establish the nature of
   DNA replication.


3. In a series of sketches, briefly outline the three
   phases of DNA replication.


4. Replication of DNA strands can only take place
   in one direction. Find some analogies that could be
   used to explain the significance of this for living cells.


5. Explain the role of the following enzymes in
   DNA replication.
   (a)helicase (b)DNA polymerase
   (c)DNA ligase (d)DNA primase


6. What is the purpose of the Okazaki fragments?
   What happens to them during replication?


7. Explain how replication errors are corrected.


8. Some scientists studying telomeres hope their
   research will eventually lead to a way of treating
   cancer. Give two examples of additional applications
   that could arise from a better understanding of these
   structures.
   9. Suppose mammalian cells are cultured in a
   medium containing radioactive thymine. They grow
   and divide many times, until eventually every
   chromosome contains radioactive thymine. The cells
   are then removed and allowed to replicate several
   more times in a culture medium containing normal
   thymine. Daughter chromosomes are tested with
   each successive generation to determine whether
   they carry the radioactive thymine.
   (a)Predict the radioactive status of the daughter
   chromosomes after one, two, and three rounds
   of division in the normal medium.
   (b)Explain how your predictions are consistent
   with the Watson-Crick explanation of semi-
   conservative DNA replication.


9. Lacking knowledge of Franklin’s X-ray analysis of
   the DNA molecule, Linus Pauling proposed a DNA
   structure in which the phosphate groups were tightly
   packed on the inside of the molecule, thus leaving
   the nitrogenous bases sticking outward. If DNA
   replication occurred in this structure, how do you
   think it would differ from what you know is the
   actual process?


10. Could you use what you have learned about
    the replication of DNA to develop a drug that kills
    bacteria but not eukaryotic cells? Explain your answer.

MC

I

I

MC

K/U

K/U

K/U

K/U

C

C

K/U

Figure 7.28As this illustration of the replication machine indicates, only a very

short region of either the parent or daughter DNA strand is ever left in a non-

base-paired form as the replication fork progresses.

5 ′

5 ′

5 ′

3 ′

3 ′

3 ′

helicase

primase

direction of replication fork movement

DNA polymerase

spliced by

DNA ligase

lagging

strand

leading

strand

DNA polymerase

adding DNA

nucleotides to

RNA primer

DNA polymerase

RNA primer

Okazaki fragment

RNA primer

removed and replaced

with DNA nucleotides