ESSENTIAL CELL BIOLOGY CHAPTER
#17 EXAM QUESTIONS WITH VERIFIED
ANSWERS
Suppose that the actin molecules in a cultured skin cell have been randomly labeled in
such a way that 1 in 10,000 molecules carries a fluorescent marker. What would you
expect to see if you examined the lamellipodium (leading edge) of this cell through a
fluorescence microscope? Assume that your microscope is sensitive enough to detect a
single fluorescent molecule - ANSWER-The unpolymerized actin molecules would
cause a faintly dim background because they're diffused in the cytosol. I would expect
to see the fluorescent actin molecules more clearly in the lamellipodium, where there is
a greater and more organized concentration of actin filaments. Additionally, since actin
molecules are added on to the leading edge, I would expect to see individual monomers
begin at the leading edge and make their way towards the interior of the cell, as the plus
end of the actin filament is built, and the minus end is broken down.
What do you suppose holds the actin filaments at their other ends to prevent them from
just being pushed into the cell's interior? - ANSWER-They are anchored to the cell's
cortex by actin-binding proteins
If both the actin and myosin filaments of muscle are made up of subunits held together
by weak non covalent bonds, how is it possible for a human being to lift heavy objects?
- ANSWER-Strength in numbers, combined with a fairly even distribution of the
mechanical forces
Compare the structure of intermediate filaments with that of the myosin-II filaments in
skeletal muscle cells. Major similarities and major differences? How do the differences
relate to their structure? - ANSWER-Made of protein subunits that are held together by
coiled-coil interactions. Dimers polymerize through their coiled-coil interactions into
filaments. Intermediate filaments have no polarity, but myosin-II does.
A. Note that troponin molecules are evenly spaced along an actin filament, every 7th
actin molecule. How do you suppose that troponin molecules can be positioned this
regularly? What does this tell you about the binding of troponin to actin filaments?
B. What do you suppose would happen if you mixed actin filaments with (i) troponin
alone, (ii) tropomyosin alone, (iii) troponin plus myosin and then added actin filaments.
Would the effects be dependent on Ca2+? - ANSWER-A. Tropomyosin spans about 7
monomers, like a mile marker on a highway and is the location where troponin should
bind.
B. Calcium ions influence force generation in the actin-myosin system only if both
troponin and tropomyosin are both present. So, (i) the troponin can't bind to the actin
without tropomyosin, so the system would be continuously active because the actin
filament would be permanently exposed to the myosin. (ii) The system would be
, continuously deactivated because the actin would never be exposed to the myosin.
Ca2+ doesn't matter here. (iii) The system would contract when Ca2+ is added
T/F Kinesin moves ER membranes alone microtubules so that the network of ER
tubules becomes stretched throughout the cell - ANSWER-True
T/F Without actin, cells can form a functional mitotic spindle and pull their chromosomes
apart but cannot divide - ANSWER-True. Actin is involved in the cleavage of daughter
cells
T/F
Lamellipodia and filopodia are "feelers" that a cell extends to find anchor points on the
substratum that it will then crawl over - ANSWER-True.
T/F - GTP is hydrolyzed by tubulin to to cause the bending of flagella - ANSWER-False.
ATP is hydrolyzed
T/F cells having an intermediate filament network that cannot be depolymerized would
die - ANSWER-False. Some cells have stable intermediate filaments that are not known
to depolymerize (like nerve cells)
T/F The plus ends of microtubules grow faster because they
have a larger GTP cap. - ANSWER-False. Rate of growth doesn't depend on cap size.
The plus ends grow faster because they have physically distinct binding sites
T/F The transverse tubules in muscle cells are an extension of the plasma membrane,
with which they are continuous; similarly, the sarcoplasmic reticulum is an extension of
the ER - ANSWER-True
T/F Activation of myosin movement on actin filaments is triggered by the
phosphorylation of troponin in some situations and by Ca2+ binding to troponin in others
- ANSWER-False. It's activated by the phosphorylation of myosin.
Why do eukaryotic cells, and especially animal cells, have such large and complex
cytoskeletons? List the differences between animal cells and bacteria that depend on
the eukaryotic cytoskeleton - ANSWER-1. Animal cells are much larger and have more
diverse shapes, but do not have a cell wall. Cytoskeletal elements provide mechanical
strength and shape.
2. The nucleus of eukaryotic cells is dependent on filaments
3. Animal cells need more organization during mitosis because they have larger
genomes
4. Animal cells have organelles that depend on the cytoskeleton (and motor proteins) for
movement/organization
Does a intermediate filament have polarity? Explain. - ANSWER-No. The filaments are
built from symmetrical tetramers
#17 EXAM QUESTIONS WITH VERIFIED
ANSWERS
Suppose that the actin molecules in a cultured skin cell have been randomly labeled in
such a way that 1 in 10,000 molecules carries a fluorescent marker. What would you
expect to see if you examined the lamellipodium (leading edge) of this cell through a
fluorescence microscope? Assume that your microscope is sensitive enough to detect a
single fluorescent molecule - ANSWER-The unpolymerized actin molecules would
cause a faintly dim background because they're diffused in the cytosol. I would expect
to see the fluorescent actin molecules more clearly in the lamellipodium, where there is
a greater and more organized concentration of actin filaments. Additionally, since actin
molecules are added on to the leading edge, I would expect to see individual monomers
begin at the leading edge and make their way towards the interior of the cell, as the plus
end of the actin filament is built, and the minus end is broken down.
What do you suppose holds the actin filaments at their other ends to prevent them from
just being pushed into the cell's interior? - ANSWER-They are anchored to the cell's
cortex by actin-binding proteins
If both the actin and myosin filaments of muscle are made up of subunits held together
by weak non covalent bonds, how is it possible for a human being to lift heavy objects?
- ANSWER-Strength in numbers, combined with a fairly even distribution of the
mechanical forces
Compare the structure of intermediate filaments with that of the myosin-II filaments in
skeletal muscle cells. Major similarities and major differences? How do the differences
relate to their structure? - ANSWER-Made of protein subunits that are held together by
coiled-coil interactions. Dimers polymerize through their coiled-coil interactions into
filaments. Intermediate filaments have no polarity, but myosin-II does.
A. Note that troponin molecules are evenly spaced along an actin filament, every 7th
actin molecule. How do you suppose that troponin molecules can be positioned this
regularly? What does this tell you about the binding of troponin to actin filaments?
B. What do you suppose would happen if you mixed actin filaments with (i) troponin
alone, (ii) tropomyosin alone, (iii) troponin plus myosin and then added actin filaments.
Would the effects be dependent on Ca2+? - ANSWER-A. Tropomyosin spans about 7
monomers, like a mile marker on a highway and is the location where troponin should
bind.
B. Calcium ions influence force generation in the actin-myosin system only if both
troponin and tropomyosin are both present. So, (i) the troponin can't bind to the actin
without tropomyosin, so the system would be continuously active because the actin
filament would be permanently exposed to the myosin. (ii) The system would be
, continuously deactivated because the actin would never be exposed to the myosin.
Ca2+ doesn't matter here. (iii) The system would contract when Ca2+ is added
T/F Kinesin moves ER membranes alone microtubules so that the network of ER
tubules becomes stretched throughout the cell - ANSWER-True
T/F Without actin, cells can form a functional mitotic spindle and pull their chromosomes
apart but cannot divide - ANSWER-True. Actin is involved in the cleavage of daughter
cells
T/F
Lamellipodia and filopodia are "feelers" that a cell extends to find anchor points on the
substratum that it will then crawl over - ANSWER-True.
T/F - GTP is hydrolyzed by tubulin to to cause the bending of flagella - ANSWER-False.
ATP is hydrolyzed
T/F cells having an intermediate filament network that cannot be depolymerized would
die - ANSWER-False. Some cells have stable intermediate filaments that are not known
to depolymerize (like nerve cells)
T/F The plus ends of microtubules grow faster because they
have a larger GTP cap. - ANSWER-False. Rate of growth doesn't depend on cap size.
The plus ends grow faster because they have physically distinct binding sites
T/F The transverse tubules in muscle cells are an extension of the plasma membrane,
with which they are continuous; similarly, the sarcoplasmic reticulum is an extension of
the ER - ANSWER-True
T/F Activation of myosin movement on actin filaments is triggered by the
phosphorylation of troponin in some situations and by Ca2+ binding to troponin in others
- ANSWER-False. It's activated by the phosphorylation of myosin.
Why do eukaryotic cells, and especially animal cells, have such large and complex
cytoskeletons? List the differences between animal cells and bacteria that depend on
the eukaryotic cytoskeleton - ANSWER-1. Animal cells are much larger and have more
diverse shapes, but do not have a cell wall. Cytoskeletal elements provide mechanical
strength and shape.
2. The nucleus of eukaryotic cells is dependent on filaments
3. Animal cells need more organization during mitosis because they have larger
genomes
4. Animal cells have organelles that depend on the cytoskeleton (and motor proteins) for
movement/organization
Does a intermediate filament have polarity? Explain. - ANSWER-No. The filaments are
built from symmetrical tetramers
