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Biol 200, Autumn 2014
Exam 2 Key
This portion of the exam is worth 80 points. Carefully examine key as your version may vary from
this one.
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1. (20 pts) Imagine the following gene codes for "fertilizin", the receptor that binds to "bindin" in a sea
urchin. Urchins are diploid, have autosomes and XY chromosomes, similar to humans. E =
enhancer, P = promoter, T = terminator.
5'
3'
E
P Exon1 Intron1 Exon2
Intron2
Exon3
T
3'
5'
'i'
(2 points each)
a. Who has the fertilizin gene? (Circle ONE)
- males
- females
- both
Answer b-e with the name of an organelle or a location in a cell:
b. Where is the fertilizin gene found in the cell? nucleus
c. Where will fertilizin mRNA be spliced? nucleus
d. Where is fertilizin protein primarily synthesized (be specific)? Rough ER
e. Where does the completed fertilizin go to do its job? Plasma membrane (oocyte membrane)
f. Which of the following will be found in the processed mRNA of the fertilizin gene? (Circle ALL
correct answers)
promoter
Intron 2
Exon 3
enhancer
g. Name a type of bond that forms between fertilizin and bindin when they bind. H-bond, ionic bond
h. List two events during normal fertilization that change some aspect of fertilizin's protein structure:
Fast block/mem. pot. change, bindin binding, cleavage by enzymes from cortical granules
"Slow block" will now be accepted for full points (turn in to Dr. Mandy)
i. Imagine there is an early-stop (nonsense) mutation in the fertilizin gene at the location shown by
the arrow marked 'i'. This results in the protein missing its intracellular portion. Which of the
following processes will be changed by this mutation? (Write an "X" in the blank if the process will
be changed.)
______ The number of times the fertilizin gene is transcribed.
______ The ability of the oocyte to splice fertilizin mRNA
______ The number of times the fertilizin mRNA is translated
___X___ The ability of the oocyte to prevent polyspermy
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Biol 200, Autumn 2014
Exam 2 Key
2. (10 pts) For each event, write the stage of the cell cycle (G1, S, G2 or M) in which it occurs. (Write
the ONE BEST answer) (2 points each)
a. __S__ Helicase is catalyzing its reaction
b. ___M__ Microtubules bind kinetochores
c. __G1__ Cells check for sufficient nutrient levels
d. ___M___ HAT enzymes at lowest activity
e. __G2___ Phosphodiester backbones of sister chromatids checked for breaks
3. (10 pts) The diagram below shows a small portion of a chromosome during DNA replication. The
template (parent) strand is on TOP. Look CAREFULLY at the molecular structures of all molecules.
a. (2 pts) An enzyme just removed this nucleotide
to create the gap seen in the bottom strand.
What is the name of the enzyme?
DNA polymerase I
d
b. (3 pts) Circle the nucleotide below that will be ADDED by this enzyme immediately after the
nucleotide in 'a' is removed.
c. (2 pts) The enzyme you named in 'a' is able to add nucleotides to the (Circle ONE) 3' end
d. (3 pts) What enzyme catalyzed the formation
of the phosphodiester bond labeled 'd'? primase
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5' end
Biol 200, Autumn 2014
Exam 2 Key
4. (8 pts) Each description below describes a lab experiment performed with sea urchin gametes.
Place an X in the box for every event that you expect will occur in that experiment. If NONE of the
events occur, then put an X in the column labeled "none". (2 pts each)
Experiment
Acrosome Cortical Granule Increase in
None
Reaction
fusion
Polyspermy
a. Normal gametes are mixed in normal
X
X
seawater
b. Sperm that have no enzymes in their
X
acrosomes are mixed with normal oocytes
c. Oocytes with cortical granules that don't
have any digestive enzymes in them are
X
X
X
mixed with normal sperm.
d. Oocytes with no jelly coat are mixed
X
with normal sperm.
5. (10 pts) Fill in the blank. For each, write TWO different answers that fit the description given. Be
as specific as possible (use names of molecules). It is acceptable to use the same answer for different
descriptions.
a. (4 pts) A specific DNA sequence that is not
PPE…
a template for RNA polymerase.
promoter, origin, centromere, enhancer,
b. (4 pts) A specific cell type found in gonads that
follicle cell, oogonium, spermatogonium
goes through mitosis. (We gave 1 pt each for Sertoli and Leydig – they are cells in the testes
that are not in meiosis, but they don't go through mitosis, either)
c. (2 pts) A specific molecule that is used as a
(r)ATP, (r)CTP, (r)GTP, UTP
substrate for both DNA replication and transcription.
6. (8 pts) The diagram below shows a replication bubble. The thin lines represent DNA and the
thicker lines RNA. Synthesis has only been diagrammed on the right fork. Arrows represent the 3'
end of a strand.
lagging
leading
a. (1 pt) In which direction is helicase moving here? (Circle ONE)
to the left
to the right
b. (2 pts) Label the "leading" and "lagging" strands on the diagram.
c. (2 pts) On the left side of the bubble, draw in the new strands in the same format. Assume that the
same amount of replication has been completed.
d. (3 pts) Does telomerase need to have proofreading ability (like DNA pol III)? Explain in a few
words.
No, since the DNA that telomerase is adding doesn't code for anything, it does not matter if there
are some mismatches.
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Biol 200, Autumn 2014
Exam 2 Key
7. (10 pts) The diagram below shows the E. coli "UvrYC" operon. UvrY and UvrC are two genes that
code for DNA repair enzymes. The operator is the only regulatory DNA sequence in this operon.
"LexA" is a repressor protein that only has one conformation – it binds to the operator.
LexA
promoter
operator
UvrY
UvrC
terminator
a. (2 pts) In which of the following environmental conditions would you expect this operon to be
transcribed? (Choose ALL that apply)
- high lactose
- chemical that causes mutations
- radiation exposure
- low glucose
b. (2 pts) Should this operon have a strong or weak promoter? strong
c. RecB protein regulates LexA. RecB activity ultimately leads to an increase of transcription of the
UvrYC operon.
i. (2 pts) RecB should be (Circle ONE)
ii. (2 pts) RecB should (Circle ONE)
constitutively active
break LexA into pieces
active only in certain conditions
increase LexA expression
d. (2 pts) Imagine you have a mutant where LexA no longer binds the operator. Transcription of the
UvrYC operon would… (Circle ONE)
- never occur
- be regulated by RecB alone
- be constitutive
- be regulated by lactose
8. (4 pts) Imagine two cells. In Cell A, DNA polymerase III makes an error during DNA replication
that causes a missense (single amino acid change) mutation in a tumor suppressor gene. In Cell B,
RNA polymerase makes the exact same error in the same gene during transcription. Neither error is
corrected. The resulting protein does not work as well as the wild-type protein.
a. (1 pt) Which cell has a higher risk of causing cancer? (Circle ONE)
- Cell A
- Cell B
- the increased risk is the same for both cells
b. (3 pts) Explain your answer to 'a' in no more than 2 sentences:
In cell A, every mRNA will have the mutation, therefore every protein will as well (and all of the
daughter cells of this cell). In Cell B, only a few proteins will have the incorrect sequence, and
they will eventually degrade at the end of their natural "lifespan".
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Biol 200, Autumn 2014
Exam 2 Key
TAKE HOME PORTION – KEY:
Imagine you are creating a diagram to help teach a well-educated non-scientist (your grandmother,
your roommate, etc.) about the integration of spermatogenesis and meiotic chromosome
segregation. Create a visual diagram starting with a spermatogonium and ending with the
production of four spermatids. The diagram should include an aspect of testosterone signaling in at
least one cell. Assume the cells have a diploid chromosome number of 2N=4. Chromosome size can
be exaggerated for clarity.
Grading notes per section:
5 pts - Diagram contains all relevant cells and structures clearly labeled
We took off "integration" points if your cells were not in any sort of structural context (in the testes
or seminiferous tubules). This varied based on how weak the integration was. Complete lack of
context was a loss of 3 points. Also, if your diagram was messy or handwriting unclear, or there
was way too much writing, points were lost in this section.
5 pts - Telomeres, centromeres, kinetochores, and synaptonemal complexes should be clear and labeled
Each of these had to be clearly diagrammed, clearly labeled and be in the correct location.
3 pts - Maternal and paternal (homologous) chromosomes are clearly distinguished using color and/or shading
3 pts - Crossing over visually represented during meiosis, genetic variation illustrated in the spermatids
Crossing over had to be shown diagrammatically, not just "inferred"
3 pts – Clear illustration of testosterone signaling and cellular response
This is the section where the most points were lost. You had to show:
- testosterone entering a cell (any cell) across the plasma membrane (it is a lipid!!!)
- testosterone binding the intracellular androgen receptor
- the androgen receptor entering the nucleus and binding enhancers in the DNA to initiate
new gene expression
1 pt- Although a static diagram, being able to depict, in your own way, the dynamics of the process
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