Binghamton University - BIOL 311 -
Test 4 – RVB Final Exam and Actual
Detailed Answers 2025-2026 Updated.
Cytosekeleton - Answer Network of filaments that run throughout the cytoplasm, is dynamic
and not static, can be a target for cancer (microtubules)
What does the cytoskeleton do - Answer Gives the cell its shape and aids in movement,
guide for vesicular transport and orientation of organelles
where is Internal cytoskeleton - Answer Specific to Eukaryotes
3 Major components - Answer 1.) Actin filaments (microfilaments, smallest)
2.) Intermediate filaments (Medium size)
3.) Microtubules (largest)
Type of intermediate filaments - Answer Keratin filaments that interconnect desmosomes
Where are actin filaments found - Answer In cortex and microvilli
Microtubules - Answer Involved in mitosis and in non-dividing cell often radiate out from a
MTOC (microtubule organizing center)
Experiment with force of filaments - Answer Increasing force, measure when it breaks and
how much it stretches, intermediate filaments are the toughest and don't break
How much of proteins do actin filaments make up - Answer 10% in muscle cells, 1-5% in
normal cells, most abundant protein in muscle cells
Molecular weight of actin filaments - Answer 42000 - G-actin (globular)
How many actin genes are there - Answer 6 in humans that each encode an isoform
Alpha actin - Answer Associated with contractile structures
Beta actin - Answer Front or leading edge of moving cells
,Gamma actin - Answer Accounts for filaments in stress fibers
F-actin - Answer consists of units G-actin arranged in a tight helix, binds either ATP or ADP
How to examine F to G actin change - Answer In vitro, by changing the concentrations of F-
actin and ion concentration (add Mg, K, or Na - induce polymerization)
Myosin S1 - Answer Can be used to decorate F-actin and looks like a series of arrow heads
pointing in one direction -> suggests polarity at ends
Polarity of F-actin - Answer Has a (+) and (-) end that are different, (+) is barbed, (-) is
pointed
How can F-actin polarity be monitored (3) - Answer 1.) Viscometry
2.) Sedimentation
3.) Fluorescence
3 phases of F-actin - Answer 1.) Nucleation
2.) Elongation
3.) Steady State
Steady state - Answer Shows no net change in length, solution has reached the critical
concentration
Critical concentration of G-actin - Answer The concentration of actin at which there is no
change in length at (+) or (-) ends
Difference between CC of (+) and (-) - Answer (+) = 0.12 uM
(-) = 0.60 uM
5 fold difference = G-actin adds 5 times faster at (+) end than (-) end
Capping in vivo vs in vitro - Answer In vivo = see "capping" of one end so the rates are
different, in vitro = theoretically possible for actin filaments to treadmill
, Treadmill - Answer Occurs at CC values intermediate between the (+) and (-) ends values,
responsible for some cell movement
What can toxins do - Answer Alter actin polymerization
Cytochalasin D - Answer Fungal product that inhibits polymerization by binding to (+) end
(inhibits F-actin)
Phalloidins - Answer Toxins from mushroom (Amanita/Angel of Death) that promote
polymerization
Use of phalloidins and remedy - Answer Remedy - eat lots of raw meat to saturate and can
be used to fluorescently tag actin
What regulates actin polymerization in vivo - Answer Actin-binding proteins
Thymosin beta4 - Answer Regulates G-actin polymerization, calculations suggest all G-actin
should be F-actin but 40% is in G form
What specifically does Thymosin beta4 do - Answer Sequesters ATP-G-actin and maintains
active G-actin pool, more Thymosin Beta4 more G-actin
Profilin - Answer Regulated G-actin polymerization, binds to ADP-actin in a 1:1 ratio
How does Profilin promote F-actin assembly - Answer Encouraging the ATP for ADP exchange
(only protein known to do this), aids in addition of monomers to the (+) end of F-actin
Cofilin - Answer Binds to filaments containing ADP-actin, inducing them to fragment and
thus enhancing depolymerization, one of the severing proteins
Proteins to sever actin filaments and where did discovery come from - Answer Gelsolin and
cofilin, discovery from amoeba during movement (cytosol moves from center to edge)
How do severing proteins work - Answer Cap the (+) end and prevents addition of units, (-)
end shortens and increases the solubility of filaments
CapZ - Answer Binds to the (+) end of actin and prevents addition or loss of units
Test 4 – RVB Final Exam and Actual
Detailed Answers 2025-2026 Updated.
Cytosekeleton - Answer Network of filaments that run throughout the cytoplasm, is dynamic
and not static, can be a target for cancer (microtubules)
What does the cytoskeleton do - Answer Gives the cell its shape and aids in movement,
guide for vesicular transport and orientation of organelles
where is Internal cytoskeleton - Answer Specific to Eukaryotes
3 Major components - Answer 1.) Actin filaments (microfilaments, smallest)
2.) Intermediate filaments (Medium size)
3.) Microtubules (largest)
Type of intermediate filaments - Answer Keratin filaments that interconnect desmosomes
Where are actin filaments found - Answer In cortex and microvilli
Microtubules - Answer Involved in mitosis and in non-dividing cell often radiate out from a
MTOC (microtubule organizing center)
Experiment with force of filaments - Answer Increasing force, measure when it breaks and
how much it stretches, intermediate filaments are the toughest and don't break
How much of proteins do actin filaments make up - Answer 10% in muscle cells, 1-5% in
normal cells, most abundant protein in muscle cells
Molecular weight of actin filaments - Answer 42000 - G-actin (globular)
How many actin genes are there - Answer 6 in humans that each encode an isoform
Alpha actin - Answer Associated with contractile structures
Beta actin - Answer Front or leading edge of moving cells
,Gamma actin - Answer Accounts for filaments in stress fibers
F-actin - Answer consists of units G-actin arranged in a tight helix, binds either ATP or ADP
How to examine F to G actin change - Answer In vitro, by changing the concentrations of F-
actin and ion concentration (add Mg, K, or Na - induce polymerization)
Myosin S1 - Answer Can be used to decorate F-actin and looks like a series of arrow heads
pointing in one direction -> suggests polarity at ends
Polarity of F-actin - Answer Has a (+) and (-) end that are different, (+) is barbed, (-) is
pointed
How can F-actin polarity be monitored (3) - Answer 1.) Viscometry
2.) Sedimentation
3.) Fluorescence
3 phases of F-actin - Answer 1.) Nucleation
2.) Elongation
3.) Steady State
Steady state - Answer Shows no net change in length, solution has reached the critical
concentration
Critical concentration of G-actin - Answer The concentration of actin at which there is no
change in length at (+) or (-) ends
Difference between CC of (+) and (-) - Answer (+) = 0.12 uM
(-) = 0.60 uM
5 fold difference = G-actin adds 5 times faster at (+) end than (-) end
Capping in vivo vs in vitro - Answer In vivo = see "capping" of one end so the rates are
different, in vitro = theoretically possible for actin filaments to treadmill
, Treadmill - Answer Occurs at CC values intermediate between the (+) and (-) ends values,
responsible for some cell movement
What can toxins do - Answer Alter actin polymerization
Cytochalasin D - Answer Fungal product that inhibits polymerization by binding to (+) end
(inhibits F-actin)
Phalloidins - Answer Toxins from mushroom (Amanita/Angel of Death) that promote
polymerization
Use of phalloidins and remedy - Answer Remedy - eat lots of raw meat to saturate and can
be used to fluorescently tag actin
What regulates actin polymerization in vivo - Answer Actin-binding proteins
Thymosin beta4 - Answer Regulates G-actin polymerization, calculations suggest all G-actin
should be F-actin but 40% is in G form
What specifically does Thymosin beta4 do - Answer Sequesters ATP-G-actin and maintains
active G-actin pool, more Thymosin Beta4 more G-actin
Profilin - Answer Regulated G-actin polymerization, binds to ADP-actin in a 1:1 ratio
How does Profilin promote F-actin assembly - Answer Encouraging the ATP for ADP exchange
(only protein known to do this), aids in addition of monomers to the (+) end of F-actin
Cofilin - Answer Binds to filaments containing ADP-actin, inducing them to fragment and
thus enhancing depolymerization, one of the severing proteins
Proteins to sever actin filaments and where did discovery come from - Answer Gelsolin and
cofilin, discovery from amoeba during movement (cytosol moves from center to edge)
How do severing proteins work - Answer Cap the (+) end and prevents addition of units, (-)
end shortens and increases the solubility of filaments
CapZ - Answer Binds to the (+) end of actin and prevents addition or loss of units
