Genetics 320 Homework Weinert Due October 7, 2005.
You Must Type Out Your Answers where your answers involve words.
List the three reasons given in class for
drug side effects
(This is a “regurgitation” question, of which there will be very few on an exam!)
Refer to a Table of the Genetic Code (there is one in your text book. pg240)
A. Start with an “ATG” (the nearly-universal “start” codon). Not using the same codon twice,
write out 30 consecutive “sense” codons. You have just generated a new “open reading frame”.
Underline consecutive triplet bases, each encoding an amino acid, in this open reading frame.
B. Calculate the probability that, if you wrote down bases in random order (A,T,C,G) , that you
would generate an open reading frame (i.e. one having no stop or non-sense codons”) that was
30 codons long.
C. Now delete the 4th base from the left.
Underline the new reading frame. Is it
.encoding amino acids? Circle the first (if any) nonsense codon. By my calculation,
about 4 in 5 of you should have sequences that end in a nonsense codon. Write down the amino
acids that this new reading frame encodes. If your sequence did not result in an in-frame
nonsense codon, go back to your original sequence, now add a base after the third base (the G of
ATG), underline the new reading frame and circle the first (if any) nonsense codons.
D. Without considering your specific sequence, what is the probability that you will
stop codon by the 10th triplet?
Hopefully you now understand why a “frameshift mutation”
frequently eliminates a proteins function (as opposed to a missense mutation that may change
one amino acid), either because of a nonsense codon or different amino acids.
E. If genome #1 is “AT rich’ compared to genome #2 (say an obscure yeast versus cow). That
means that genome #1 has, say, 60% AT and 40% GC, while genome #2 is 50%AT and 50%
GC. If a frameshift mutation occurs, is a nonsense codon more likely, less likely, or just as
likely to occur in the new reading frame in a gene in the AT-rich genome #1 compared to
genome#2. Explain briefly.
On the structure of mutational intermediates caused by base modification
A. See Slide 23. Draw a G and a C base-paired, complete with their correct chemical structures
and hydrogen bonding. Draw the ethylated G base-paired with T see in slide 23. The
mechanisms of how ethylation causes a mutation by altering base pairing should now be clear.
B. Now look up and write down a second example that illustrates the chemistry of how a base