Discussion 11: Problem Solving III
MCB 150
Problem Solving III
Exam III Material
1. The cell nucleus is highly organized into distinct regions. Why were these
regions referred to as functional domains during class? Describe two
such domains of the nucleus and their respective roles.
2. Pick a sequence of 3 bases read in the template strand of a DNA
molecule.
• What is the directionality of this sequence of bases?
• If this template is transcribed into mRNA, what is the codon in the
mRNA and what amino acid does it specify?
• One at a time, change each base to some other base.
o For each change (mutation), specify the amino acid that
would now be coded for and the type of mutation that has
occurred, both in general and specifically.
o What is the likely effect on the protein product that
encounters this change?
3. If an E. coli cell makes one mistake during DNA replication, how many
mistakes is that same cell likely to make if its DNA polymerases do not
proofread?
How many mistakes would it be likely to make if there were no mismatch
repair system?
What if it was lacking both of them?
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Discussion 11: Problem Solving III
MCB 150
4. Why are errors in transcription:
• more frequent than errors in DNA replication?
• less critical than errors in DNA replication?
5. Why is the semi-permeability of phospholipid bilayers essential for
lysosomal function?
6. Protein P was recently characterized as an exclusively nuclear protein. It
is a large protein of 120 kDa that does not have a nuclear localization
signal. Propose a mechanism by which protein P is recruited to the
nucleus. Draw a schematic representation of your proposed pathway and
indicate the source of energy if one is needed.
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Discussion 11: Problem Solving III
MCB 150
7. Protein P (from the previous question) was shown to have a nuclear
export signal and an RNA binding domain. Propose a role for protein P.
How could you reconcile the fact that protein P has an NES but is only
detected in the nucleus?
8. Cells can internalize foreign material by phagocytosis. However, when the
foreign material cannot be degraded, like asbestos fibers (a material that
was once used extensively for the insulation of buildings), it accumulates
in one type of organelle within the cell cytoplasm. Which one? Explain
why.
9. In the intestinal track of mammals, specialized epithelial cells secrete large
quantities of digestive enzymes. Which organelle(s) would you expect to
be very prominent in these cells? Justify your hypothesis.
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Discussion 11: Problem Solving III
MCB 150
10. Secreted proteins are usually about 20 amino acids longer when they are
synthesized by free ribosomes in a test tube (RER is absent from the test
tube) than when synthesized in vivo. Yet, free ribosomes function
identically to membrane-bound ribosomes. What hypothesis can you
propose to explain this experimental observation?
11. Let’s consider a trans-membrane protein, TMP, destined to the plasma
membrane. Suppose that you could link a fluorescent dye to the
cytoplasmic domain of a few TMP molecules still associated with the
rough ER. You then follow the fate of the labeled TMPs using
fluorescence microscopy. As the labeled TMPs distribute among cellular
compartments, where would you expect the dye to be located? Draw a
schematic representation of your answer, in which you should indicate
which side(s) of the cellular compartments (cytoplasm vs. lumen/outside of
the cell) are labeled.
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