Bio-251 Final Exam Quiz Questions
and All Correct Answers 2025-2026
Updated
1) Actin is one of the most evolutionarily conserved proteins. What does this tell you about the
structure and function of this protein in eukaryotic cells? - Answer Evolutionarily conserved-
hasn't undergone much change since existence of eukaryotes.
Tells us that this protein is vital for cellular function. Tells us it can do a variety of functions in
cells.
Its so conserved because it is very important for cells. Multifunctional makes it more well-
conserved because monomers that make it. Up are very simple, and are able to combine in. a
complex way to perform its many functions. It has to be something to do with the fact that so
many other proteins can bind to it. G-actin has lots of aa side chains that stick out. Sequence of
aa side chains make it multifunctional. Every different protein that binds to actin has to bind to a
specific place on actin. Mutation in aa side chains could negatively impact the binding.
1) I mentioned phalloidin is a toxic produced by the deathcap mushroom. It is toxic because it
binds across two actin monomers and holds them together. In contrast, latrunculin, another
toxin - this one is made in sponges - tightly binds to G-actin monomers. Describe the effect each
drug would have on actin treadmilling. - Answer Phalloidin-filaments will continue to
elongate, unable to take filaments off of chain. Prevents depolymerization. Polymerization
would stop for this when the cell reaches below some threshold of free G-actin.
Latrunculin- would bind. To. G-actin, prevent polymerization, stop force against PM, length of. F-
actin would get shorter because there would be depolymerization but not polymerization. Level
of monomers in cell may make depolymerization slow down
1) Let's think about the stages of death. If you were a crime scene investigator and you were
examining a body at ambient temperature, you'd probably know the following:
If the body feels warm and no rigor is present, death occurred under 3 hours before.
If the body feels warm and stiff, death occurred 3-8 hours earlier.
If the body feels cold and stiff, death occurred 8-36 hours earlier.
If the body is cold and not stiff, death occurred more than 36 hours earlier.
Rigor refers to rigor mortis, which is the stiffening of a body after it dies. Knowing what you
know about cellular respiration and myosin's ATPase cycle and muscle function, please answer
the following questions: What is the basis of rigor mortis? Why is it referred to as stiffening and
not contracting? Why does it take 3 hours for stiffening to occur? Why does the rigor retreat? -
Answer Basis of rigor mortis- after ATP hydrolyzed, and after everything else takes place, ATP
stops. In death, ATP runs out, causes muscles to be stiff because no ATP to release myosin head
from actin.
Stiffening instead of contracting-myosin is not moving actin filaments along, myosin is. Bound to
actin and is immovable. No cycle of contraction, so will only be stiff
Why 3 hours-there will still be some ATP left over which will be used up until it is gone.
,Why does rigor finally go away?- the body begins to decay so the proteins will be degraded,
releasing the myosin from the actin. actin fibers fall apart, myosin heads degrade, becomes
pliable again
1) You know that cancer cell migration is a key part of metastasis, which is the deadliest aspect
of cancer. So, you want to develop a drug that can inhibit cancer cell migration. You know that
myosin-2-based contraction is required for cell migration and so you specifically want to
develop a drug to inhibit it, myosin-2. (Note: the myosin-2 that promotes migration is very
similar to but ultimately a different isoform from muscle myosin-2. The non-muscle myosin-2
forms smaller filaments that are more dynamic, whereas the myosin-2 thick filaments in
muscles are very stable and larger structures.)
a) What aspect of migration is myosin-2 involved in. Think about the steps of migration
discussed in lecture (and see figure 17-33 in your text). Where is the force being produced that
involves myosin-2? - Answer Myosin-2 is involved in retraction (back end of cell). Force being
produced is where actin brings up end of cell back into middle
a) You develop a drug that seems to inhibit myosin-2, but you are unsure of its mechanism of
action. In general, how might you know whether the drug blocks ATP from binding to myosin
versus allowing ATP to bind but blocking hydrolysis? Can you think of a way to test this in a
biochemical assay with purified actin and myosin-2? - Answer You could track the amount of
free ATP, if there was a comparatively low amt it would be binding and if there was a high
amount, it wouldn't be. If ATP cannot bind, youd have the same amount of ATP in your solution
as you started with. Control-without drug
Your control, the ATP would decrease after going through the cycle
Experimental-ATP levels would stay higher if ATP binding was prevented by the drug.
If you carried out the same experiment, could you tell whether it was able to bind but not able
to hydrolyze? No- ATP levels would stay the same as compared to when you first put it in.
depends on ratio concentration of ATP to actin and myosin.
Another experiment- generate michaelis menten curve-keep myosin levels the same, change
levels of ATP, see if drug competitively inhibits it. Kd value would increase if it affected ATP
binding. But cannot analyze whether atp was bound or not to myosin
a) After thinking more about this, you decide that giving a person a pill that inhibits myosin-2
might not be a good idea, even if it does stop tumor cell metastasis really well. Why is that? -
Answer Not a good idea because it would affect healthy cells as well as cancer cells. Would
cause a lot of issues for the person, including muscle contraction and cell motility; drug sounds
like the death cap mushroom toxins.
1) Microtubules can be formed in vitro from tubulin that is bound to GTP analogues that (unlike
GTP) cannot be hydrolyzed. What properties would you expect these microtubules to possess?
, Could such microtubules still show dynamic instability? - Answer These MTs would
polymerize but not depolymerize, so there would be none or less dynamic instability. GTP cap
stabilizes the ends of the. MTS and hydrolyzes after a little while. These MTs would not show
dynamic instability because they wouldn't be depolymerizing, only polymerizing—exceptionally
stable
1) If an individual kinesin molecule can move at a rate of 800 nm/sec in an in vitro motility
assay, what is the maximal turnover rate (ATP molecules hydrolyzed per second) by one of the
molecule's motor domains. What information do you need to answer this? Think about how the
kinesins walk. Note that a- and b-tubulin are each about 4 nm in length. - Answer 8nm/step.
100 steps/sec. ATPmolec per second would be 100/sec for two domains, so 50/sec for ONE
domain.
1) Because cytoplasmic vesicles are seen to move in both directions within the axon of a
neuron, can you conclude that some microtubules are oriented with their plus end facing the
axon terminus and others oriented with the opposite polarity? Why or why not? - Answer
We cannot conclude this because MTs only grow out of one spot in the cell, plus side grows
outward but vesicles are allowed to move back and forth because they are kinesins and dyneins
1) How does signaling by hydrophobic molecules like the steroid hormones differ from signaling
by hydrophilic molecules like peptide hormones? - Answer Hydrophobic molecules like
steroid hormones have intracellular signaling receptors-hydrophobic allows it to diffuse through
membrane. Additionally, signaling pathway is different. Peptide signaling pathway transduced
by binding to some cell membrane receptor which requires more of a signal cascade than
steroid signaling pathway. Steroid hormones go right to nucleus where receptor is, allows
steroid to go into nucleus—shorter pathway. Speed of response in cell signaling can tell us a lot-
two ways we think of this: binding of molec to receptor-cascade results in activation or
deactivation of proteins already there-very fast responses. Binding of molecule to receptor that
involves gene expression take a lot longer-because transcription and. Translation have to take
place. Steroid hormones have slower response because they require the transcription and
translation of genes.
1) Glucagon is a hormone that binds to its receptor on the liver and triggers release of glucose
in essentially the exact same way as adrenaline does. What might be the effect on liver function
of a mutation in a gene that encodes a cAMP phosphodiesterase? Of a mutation in a gene
encoding a glucagon receptor? Of a mutation in a gene encoding phosphorylase kinase? Of a
mutation that altered the active site of the GTPase of a Ga subunit? (Assume in all cases that
the mutation causes a loss of function of the gene product.) - Answer Mutation in gene that
encodes a cAMP phosphodiesterase: phosphodiesterase resets cAMP into regular AMP which
then would be able to be phosphorylated into ATP at some point. Getting rid of this would affect
liver function by continuing to activate everything downstream, which would increase liver
function
Mutation in gene encoding glucagon receptor: prevent pathway from activating because no
binding of G protein to receptor if ligand cant bind.
and All Correct Answers 2025-2026
Updated
1) Actin is one of the most evolutionarily conserved proteins. What does this tell you about the
structure and function of this protein in eukaryotic cells? - Answer Evolutionarily conserved-
hasn't undergone much change since existence of eukaryotes.
Tells us that this protein is vital for cellular function. Tells us it can do a variety of functions in
cells.
Its so conserved because it is very important for cells. Multifunctional makes it more well-
conserved because monomers that make it. Up are very simple, and are able to combine in. a
complex way to perform its many functions. It has to be something to do with the fact that so
many other proteins can bind to it. G-actin has lots of aa side chains that stick out. Sequence of
aa side chains make it multifunctional. Every different protein that binds to actin has to bind to a
specific place on actin. Mutation in aa side chains could negatively impact the binding.
1) I mentioned phalloidin is a toxic produced by the deathcap mushroom. It is toxic because it
binds across two actin monomers and holds them together. In contrast, latrunculin, another
toxin - this one is made in sponges - tightly binds to G-actin monomers. Describe the effect each
drug would have on actin treadmilling. - Answer Phalloidin-filaments will continue to
elongate, unable to take filaments off of chain. Prevents depolymerization. Polymerization
would stop for this when the cell reaches below some threshold of free G-actin.
Latrunculin- would bind. To. G-actin, prevent polymerization, stop force against PM, length of. F-
actin would get shorter because there would be depolymerization but not polymerization. Level
of monomers in cell may make depolymerization slow down
1) Let's think about the stages of death. If you were a crime scene investigator and you were
examining a body at ambient temperature, you'd probably know the following:
If the body feels warm and no rigor is present, death occurred under 3 hours before.
If the body feels warm and stiff, death occurred 3-8 hours earlier.
If the body feels cold and stiff, death occurred 8-36 hours earlier.
If the body is cold and not stiff, death occurred more than 36 hours earlier.
Rigor refers to rigor mortis, which is the stiffening of a body after it dies. Knowing what you
know about cellular respiration and myosin's ATPase cycle and muscle function, please answer
the following questions: What is the basis of rigor mortis? Why is it referred to as stiffening and
not contracting? Why does it take 3 hours for stiffening to occur? Why does the rigor retreat? -
Answer Basis of rigor mortis- after ATP hydrolyzed, and after everything else takes place, ATP
stops. In death, ATP runs out, causes muscles to be stiff because no ATP to release myosin head
from actin.
Stiffening instead of contracting-myosin is not moving actin filaments along, myosin is. Bound to
actin and is immovable. No cycle of contraction, so will only be stiff
Why 3 hours-there will still be some ATP left over which will be used up until it is gone.
,Why does rigor finally go away?- the body begins to decay so the proteins will be degraded,
releasing the myosin from the actin. actin fibers fall apart, myosin heads degrade, becomes
pliable again
1) You know that cancer cell migration is a key part of metastasis, which is the deadliest aspect
of cancer. So, you want to develop a drug that can inhibit cancer cell migration. You know that
myosin-2-based contraction is required for cell migration and so you specifically want to
develop a drug to inhibit it, myosin-2. (Note: the myosin-2 that promotes migration is very
similar to but ultimately a different isoform from muscle myosin-2. The non-muscle myosin-2
forms smaller filaments that are more dynamic, whereas the myosin-2 thick filaments in
muscles are very stable and larger structures.)
a) What aspect of migration is myosin-2 involved in. Think about the steps of migration
discussed in lecture (and see figure 17-33 in your text). Where is the force being produced that
involves myosin-2? - Answer Myosin-2 is involved in retraction (back end of cell). Force being
produced is where actin brings up end of cell back into middle
a) You develop a drug that seems to inhibit myosin-2, but you are unsure of its mechanism of
action. In general, how might you know whether the drug blocks ATP from binding to myosin
versus allowing ATP to bind but blocking hydrolysis? Can you think of a way to test this in a
biochemical assay with purified actin and myosin-2? - Answer You could track the amount of
free ATP, if there was a comparatively low amt it would be binding and if there was a high
amount, it wouldn't be. If ATP cannot bind, youd have the same amount of ATP in your solution
as you started with. Control-without drug
Your control, the ATP would decrease after going through the cycle
Experimental-ATP levels would stay higher if ATP binding was prevented by the drug.
If you carried out the same experiment, could you tell whether it was able to bind but not able
to hydrolyze? No- ATP levels would stay the same as compared to when you first put it in.
depends on ratio concentration of ATP to actin and myosin.
Another experiment- generate michaelis menten curve-keep myosin levels the same, change
levels of ATP, see if drug competitively inhibits it. Kd value would increase if it affected ATP
binding. But cannot analyze whether atp was bound or not to myosin
a) After thinking more about this, you decide that giving a person a pill that inhibits myosin-2
might not be a good idea, even if it does stop tumor cell metastasis really well. Why is that? -
Answer Not a good idea because it would affect healthy cells as well as cancer cells. Would
cause a lot of issues for the person, including muscle contraction and cell motility; drug sounds
like the death cap mushroom toxins.
1) Microtubules can be formed in vitro from tubulin that is bound to GTP analogues that (unlike
GTP) cannot be hydrolyzed. What properties would you expect these microtubules to possess?
, Could such microtubules still show dynamic instability? - Answer These MTs would
polymerize but not depolymerize, so there would be none or less dynamic instability. GTP cap
stabilizes the ends of the. MTS and hydrolyzes after a little while. These MTs would not show
dynamic instability because they wouldn't be depolymerizing, only polymerizing—exceptionally
stable
1) If an individual kinesin molecule can move at a rate of 800 nm/sec in an in vitro motility
assay, what is the maximal turnover rate (ATP molecules hydrolyzed per second) by one of the
molecule's motor domains. What information do you need to answer this? Think about how the
kinesins walk. Note that a- and b-tubulin are each about 4 nm in length. - Answer 8nm/step.
100 steps/sec. ATPmolec per second would be 100/sec for two domains, so 50/sec for ONE
domain.
1) Because cytoplasmic vesicles are seen to move in both directions within the axon of a
neuron, can you conclude that some microtubules are oriented with their plus end facing the
axon terminus and others oriented with the opposite polarity? Why or why not? - Answer
We cannot conclude this because MTs only grow out of one spot in the cell, plus side grows
outward but vesicles are allowed to move back and forth because they are kinesins and dyneins
1) How does signaling by hydrophobic molecules like the steroid hormones differ from signaling
by hydrophilic molecules like peptide hormones? - Answer Hydrophobic molecules like
steroid hormones have intracellular signaling receptors-hydrophobic allows it to diffuse through
membrane. Additionally, signaling pathway is different. Peptide signaling pathway transduced
by binding to some cell membrane receptor which requires more of a signal cascade than
steroid signaling pathway. Steroid hormones go right to nucleus where receptor is, allows
steroid to go into nucleus—shorter pathway. Speed of response in cell signaling can tell us a lot-
two ways we think of this: binding of molec to receptor-cascade results in activation or
deactivation of proteins already there-very fast responses. Binding of molecule to receptor that
involves gene expression take a lot longer-because transcription and. Translation have to take
place. Steroid hormones have slower response because they require the transcription and
translation of genes.
1) Glucagon is a hormone that binds to its receptor on the liver and triggers release of glucose
in essentially the exact same way as adrenaline does. What might be the effect on liver function
of a mutation in a gene that encodes a cAMP phosphodiesterase? Of a mutation in a gene
encoding a glucagon receptor? Of a mutation in a gene encoding phosphorylase kinase? Of a
mutation that altered the active site of the GTPase of a Ga subunit? (Assume in all cases that
the mutation causes a loss of function of the gene product.) - Answer Mutation in gene that
encodes a cAMP phosphodiesterase: phosphodiesterase resets cAMP into regular AMP which
then would be able to be phosphorylated into ATP at some point. Getting rid of this would affect
liver function by continuing to activate everything downstream, which would increase liver
function
Mutation in gene encoding glucagon receptor: prevent pathway from activating because no
binding of G protein to receptor if ligand cant bind.
