Exam 3

Question 1

The cytoskeletal system characterized as being comprised of rope-like fibers that give the cell mechanical strength and distribute mechanical stress are:
a. intermediate filaments
b. myosin thick filament
c. actin filaments
d. microtubules

Answer 1

a. intermediate filaments

Question 2

Which of the following represents the placement of microtubules in an epithelial cell?
A. none of these
B. C
C. A
D. B

Answer 2

D. B

Question 3

(Q001) Which of the following statements about the cytoskeleton is FALSE?
a. Covalent bonds between protein monomers hold together cytoskeletal filaments.
b. The cytoskeleton is made up of three types of protein filaments.
c. The cytoskeleton of a cell can change in response to the environment.
d. The cytoskeleton controls the location of organelles in eukaryotic cells.

Answer 3

a. Covalent bonds between protein monomers hold together cytoskeletal filaments.

Question 4

(Q027) For both actin and microtubule polymerization, nucleotide hydrolysis is important for
a. increasing the rate at which subunits are added to the filaments.
b. promoting nucleation of filaments.
c. stabilizing the filaments once they are formed.
d. decreasing the binding strength between subunits on filaments.

Answer 4

d. decreasing the binding strength between subunits on filaments.

Question 5

(Q010) Which of the following statements about the structure of microtubules is FALSE?
a. The two ends of a protofilament are chemically distinct, with α-tubulin exposed at one end and β-tubulin exposed at the other end.
b. α-Tubulin and β-tubulin are covalently bound to make the tubulin dimer that then assembles into protofilaments.
c. Microtubules are built from protofilaments that come together to make a hollow structure.
d. Within a microtubule, all protofilaments are arranged in the same orientation, giving the microtubule structural polarity.

Answer 5

b. α-Tubulin and β-tubulin are covalently bound to make the tubulin dimer that then assembles into protofilaments.

Question 6

(Q013) The hydrolysis of GTP to GDP carried out by tubulin molecules
a. occurs because the pool of free GDP has run out.
b. provides the energy needed for tubulin to polymerize.
c. tips the balance in favor of microtubule assembly.
d. allows the behavior of microtubules called dynamic instability.

Answer 6

d. allows the behavior of microtubules called dynamic instability.

Question 7

(Q012) Which of the following statements about microtubules is TRUE?
a. The centromere nucleates the microtubules of the mitotic spindle.
b. ATP hydrolysis by a tubulin heterodimer is important for controlling the growth of a microtubule.
c. Motor proteins move in a directional fashion along microtubules by using the inherent structural polarity of a protofilament.
d. Because microtubules are subject to dynamic instability, they are used only for transient structures in a cell.

Answer 7

c. Motor proteins move in a directional fashion along microtubules by using the inherent structural polarity of a protofilament.

Question 8

(Q025) Which of the following statements about actin is FALSE?
a. Actin filaments are nucleated at the side of existing actin filaments in lamellipodia.
b. ATP hydrolysis decreases actin filament stability.
c. Linkage of actin to the desmosome provides structural integrity to the tissue.
d. Actin at the cell cortex helps govern the shape of the plasma membrane.

Answer 8

c. Linkage of actin to the desmosome provides structural integrity to the tissue.

Question 9

(Q036) Which of the following statements about skeletal muscle contraction is FALSE?
a. When a muscle cell receives a signal from the nervous system, voltage-gated channels open in the T-tubule membrane.
b. During muscle contraction, the Z discs move closer together as the myosin heads walk toward the plus ends of the actin filaments.
c. A change in the conformation of troponin leads to changes in tropomyosin such that it no longer blocks the binding of myosin heads to the actin filament.
d. The changes in voltage across the plasma membrane that occur when a muscle cell receives a signal from the nervous system cause an influx of Ca into the sarcoplasmic reticulum, triggering a muscle contraction.

Answer 9

d. The changes in voltage across the plasma membrane that occur when a muscle cell receives a signal from the nervous system cause an influx of Ca into the sarcoplasmic reticulum, triggering a muscle contraction.

Question 10

(Q034) Which of the following structures shorten during muscle contraction?
a. myosin filaments
b. flagella
c. actin filaments
d. sarcomeres

Answer 10

d. sarcomeres

Question 11

(Q008) You are studying nuclear lamins and use recombinant DNA technology to alter the coding sequence of a nuclear lamin gene. The alteration you make creates a situation such that the gene now codes for a nuclear lamin protein that can no longer be phosphorylated when the nuclear envelope is broken down during mitosis. What do you predict would happen if the cell only had the altered nuclear lamin gene (and not the unaltered version)?
a. Disassembly of the nuclear lamins will occur prematurely because the lamins cannot be phosphorylated.
b. Nuclear lamins will be unable to produce dimers, as the coiled-coil formation will be disrupted.
c. Nuclear lamins will no longer disassemble properly during mitosis.
d. Mitosis should proceed as usual because the dephosphorylation of the lamin is what is important for nuclear lamina assembly during mitosis, so phosphorylation will not be necessary.

Answer 11

c. Nuclear lamins will no longer disassemble properly during mitosis.

Question 12

(Q011) Which of the following statements about the function of the centrosome is FALSE?
a. Centrosomes typically contain a pair of centrioles, which is made up of a cylindrical array of short microtubules.
b. Centrosomes are the major microtubule-organizing center in animal cells.
c. Centrosomes contain hundreds of copies of the gamma-tubulin ring complex important for microtubule nucleation.
d. Microtubules emanating from the centrosome have alternating polarity such that some have their plus end attached to the centrosome while others have their minus end attached to the centrosome.

Answer 12

d. Microtubules emanating from the centrosome have alternating polarity such that some have their plus end attached to the centrosome while others have their minus end attached to the centrosome.

Question 13

What must happen at the end of the microtubule in order for it to stop shrinking and start growing again?
a. Interaction with ARP in order to seed a branch point
b. Once a microtubule starts shrinking, it must completely disassemble before a new microtubule can start growing again.
c. Phosphorylation of tau protein so it will bind and stabilize the microtubule
d. Addition of a sufficient number of GTP-loaded tubulin subunits to the growing end to quickly cover up the GDP-containing subunits

Answer 13

d. Addition of a sufficient number of GTP-loaded tubulin subunits to the growing end to quickly cover up the GDP-containing subunits

Question 14

Question 14: Shown below is the structure of a bipolar mitotic spindle. Which element(s) is/are the astral microtubules?
a. none of these
b. B
c. A
d. C

Answer 14

c. A

Question 15

What is tensegrity?
a. A structural principle based on a system of isolated components under compression inside a network of continuous tension.
b. The stabilizing of a microtubule network due to end-binding proteins inhibiting disassembly.
c. The homeostasis that a cell achieves when it has the correct complement of transporters and channels.
d. The state of “rigor” that occurs in a muscle after death.

Answer 15

a. A structural principle based on a system of isolated components under compression inside a network of continuous tension.

Question 16

(Q005) Which of the following statements about the cell cycle is FALSE?
a. An unfavorable environment can cause cells to arrest in G1.
b. A cell has more DNA during G2 than it did in G1.
c. The cleavage divisions that occur in an early embryo have short G1 and G2 phases.
d. Once a cell decides to enter the cell cycle, the time from start to finish is the same in all eukaryotic cells.

Answer 16

d. Once a cell decides to enter the cell cycle, the time from start to finish is the same in all eukaryotic cells.

Question 17

(Q007) Progression through the cell cycle requires a cyclin to bind to a Cdk because
a. without cyclin binding, a cell-cycle checkpoint will be activated.
b. cyclin binding inhibits Cdk activity until the appropriate time in the cell cycle.
c. the cyclins are the molecules with the enzymatic activity in the complex.
d. the binding of a cyclin to Cdk is required for Cdk enzymatic activity.

Answer 17

d. the binding of a cyclin to Cdk is required for Cdk enzymatic activity.

Question 18

(Q032) Which of the following statements about kinetochores is TRUE?
a. Kinetochores contain DNA-binding proteins that recognize sequences at the telomere of the chromosome.
b. Kinetochores assemble on chromosomes that lack centromeres.
c. Kinetochores assemble onto chromosomes during late prophase.
d. Kinetochore proteins bind to the tubulin molecules at the minus end of microtubules.

Answer 18

c. Kinetochores assemble onto chromosomes during late prophase.

Question 19

(Q043) Apoptosis differs from necrosis in that necrosis
a. causes cells to swell and burst, whereas apoptotic cells shrink and condense.
b. involves a caspase cascade.
c. causes DNA to fragment.
d. requires the reception of an extracellular signal.

Answer 19

a. causes cells to swell and burst, whereas apoptotic cells shrink and condense.

Question 20

(Q029) Which word or phrase below best describes the phase in mitosis depicted in Figure 18-29?
a. prometaphase
b. S-phase checkpoint
c. metaphase
d. anaphase

Answer 20

c. metaphase

Question 21

(Q006) Which of the following descriptions is consistent with the behavior of a cell that lacks a protein required for a checkpoint mechanism that operates in G2?
a. The cell would be unable to enter G .2
b. The cell would enter M phase under conditions when normal cells would not.
c. The cell would be unable to enter M phase.
d. The cell would pass through M phase more slowly than normal cells.

Answer 21

b. The cell would enter M phase under conditions when normal cells would not.

Question 22

(Q003) Which of the following events does NOT usually occur during interphase?
a. The centrosomes are duplicated.
b. DNA is replicated.
c. Cells grow in size.
d. The nuclear envelope breaks down.

Answer 22

d. The nuclear envelope breaks down.

Question 23

(Q034) Which of the following statements about the anaphase-promoting complex (APC) is FALSE?
a. It inhibits M-Cdk activity.
b. M-Cdk stimulates its activity.
c. It is continuously active throughout the cell cycle.
d. It promotes the degradation of proteins that regulate M phase.

Answer 23

c. It is continuously active throughout the cell cycle.

Question 24

(Q042) Programmed cell death occurs
a. by means of an intracellular suicide program.
b. only in unhealthy or abnormal cells.
c. rarely and selectively only during animal development.
d. only during embryonic development.

Answer 24

a. by means of an intracellular suicide program.

Question 25

(Q017) The G1 DNA damage checkpoint a. causes cells to proceed through S phase more quickly. b. is activated by errors caused during DNA replication. c. involves the inhibition of cyclin–Cdk complexes by p21. d. activates APC.

Answer 25

c. involves the inhibition of cyclin–Cdk complexes by p21.

Question 26

Which of the following changes takes place when a skeletal muscle contracts? a. Myosin filaments contract. b. Actin filaments contract. c. Sarcomeres become shorter. d. Z discs move further apart.

Answer 26

c. Sarcomeres become shorter.

Question 27

How are spindle microtubules attached to chromosomes? a. The plus ends of the microtubule bind directly to DNA b. The microtubules bind to the kinetochore complexes through a connecting protein c. The microtubules bind to cohesin complexes on the DNA d. The minus ends of the microtubules bind directly to a protein complex on DNA

Answer 27

b. The microtubules bind to the kinetochore complexes through a connecting protein

Question 28

The expression levels of different __________ fluctuate throughout the cell cycle. a. cyclins b. Cdks c. cyclins and Cdks d. phosphates

Answer 28

a. cyclins

Question 29

What determines the position of the cleavage furrow of the dividing cell? a. The interpolar microtubules send signals to form a cleavage furrow between the poles. b. The two spindle poles send signals to the plasma membrane so that the cleavage furrow forms in the same plane as the two poles. c. Astral microtubules contact the membrane and activate proteins to form a central furrow. d. The cleavage furrow position is determined randomly.

Answer 29

a. The interpolar microtubules send signals to form a cleavage furrow between the poles.

Question 30

In which phase of mitosis does the nuclear envelope break down? a. telophase b. metaphase c. prometaphase d. anaphase

Answer 30

c. prometaphase

Question 31

Consider the events that lead to the formation of the new nucleus at telophase. How do nuclear and cytosolic proteins become properly re-sorted so that the new nucleus contains nuclear proteins but not cytosolic proteins? a. Because the nuclear envelope reassembles on the surface of the chromosomes, cytosolic proteins are excluded from the reforming nucleus. Nuclear proteins are selectively imported through the NPC by nuclear transport receptors that recognize the NLS they contain.. b. Nuclear proteins bind chromosomes during mitosis. Because the nuclear envelope reassembles on the surface of the chromosomes, nuclear proteins end up inside the nucleus and cytoplasmic proteins end up on the outside of the nuclear envelope. c. Proteins are degraded during mitosis, so cytoplasmic and nuclear proteins are targeted via their normal targeting mechanisms. d. Cytosolic proteins are excluded from the reforming nucleus due to their cytoplasmic signal recognition sequence. Nuclear proteins are selectively imported through the NPC due to their nuclear localization sequence..

Answer 31

a. Because the nuclear envelope reassembles on the surface of the chromosomes, cytosolic proteins are excluded from the reforming nucleus. Nuclear proteins are selectively imported through the NPC by nuclear transport receptors that recognize the NLS they contain..

Question 32

All of the following is true EXCEPT: a. Apoptosis is mediated by special intracellular proteases, one of which cleaves nuclear lamins. b. Cells do not pass directly from G1 to M unless there are sufficient nutrients to complete an entire cell cycle. c. The enzymatic activity of a Cdk protein is determined both by the presence of a bound cyclin and by the phosphorylation state of the Cdk. d. Developing neurons compete for limited amounts of survival factors.

Answer 32

b. Cells do not pass directly from G1 to M unless there are sufficient nutrients to complete an entire cell cycle.

Question 33

Which of the following is TRUE regarding the role of Ca2+ in muscle contraction? a. Ca2+ binds troponin, causing it to move tropomyosin and thereby expose actin filaments to myosin heads. b. Ca2+ release is required for the refractory stage that returns the muscle to its pre-contraction state. c. Ca2+ binding to myosin causes it to release from actin. d. Ca2+ binding is required to maintain the structure of the actin filament.

Answer 33

a. Ca2+ binds troponin, causing it to move tropomyosin and thereby expose actin filaments to myosin heads.

Question 34

What is the function of condensins? a. to shorten the contractile ring b. to break down the nuclear envelope c. to coil sister chromatids into a compact form d. to hold sister chromatids together

Answer 34

c. to coil sister chromatids into a compact form

Question 35

In the graph below, Peak B would be expected to contain cells that are in which phase(s) of the cell cycle? a. prophase b. G1 c. G2 and prophase d. G2

Answer 35

c. G2 and prophase

Question 36

In the EM images below, which cell(s) are undergoing apoptosis? a. Only A b. Only B c. B and C d. Only C

Answer 36

c. B and C

Question 37

The slow rise of S cyclin levels throughout G1 phase is due to ________, and the abrupt decrease is caused by __________. a. synthesis; mitosis b. phosphorylation; translation c. transcription; proteolysis d. import; export

Answer 37

c. transcription; proteolysis

Question 38

What is a reasonable explanation for why cells have evolved a special G0 phase to exit from the cell cycle, rather than just stopping in G1 and not moving on to S phase? a. Because it takes too much energy to keep synthesizing G1 cyclins when they are not needed. b. Because the G1 state leaves the cell in a vulnerable state since it has not yet duplicated its genome and thus is more sensitive to radiation damage. c. The G0 state offers protection from aberrant activation of cell division because the cell-cycle control system is largely dismantled. d. Because the G0 state is more stable since the dynamic instability of microtubules is paused since spindles are not needed anymore.

Answer 38

c. The G0 state offers protection from aberrant activation of cell division because the cell-cycle control system is largely dismantled.

Question 39

Where is Ca2+ stored inside a muscle cell? a. Golgi apparatus b. mitochondria c. sarcoplasmic reticulum d. neurosecretory vesicles

Answer 39

c. sarcoplasmic reticulum

Question 40

Cyclins are targeted for destruction in a cell-cycle-dependent manner by: a. ubiquitination b. phagocytosis c. phosphorylation d. G-protein binding

Answer 40

a. ubiquitination

Question 41

PDGF is encoded by a gene that can cause cancer when expressed inappropriately. Why do cancers not arise at wounds in which PDGF is released from platelets? A. PDGF is only released during normal blood clotting and not at wounds. B. PDGF has no association with cancer development. C. The limited release of PDGF at a wound site triggers cell division of neighboring cells for a limited amount of time, until the PDGF is degraded. D. Wounds are naturally resistant to the oncogenic effects of PDGF.

Answer 41

C. The limited release of PDGF at a wound site triggers cell division of neighboring cells for a limited amount of time, until the PDGF is degraded.

Question 42

Which of the following types of cells would you expect to contain a high density of intermediate filaments in their cytoplasm? (Can pick multiple) A. Amoeba proteus (a free-living amoeba) B. Skin epithelial cell C. Smooth muscle cell in the digestive tract D. Escherichia coli E. Nerve cell in the spinal cord F. Sperm cell G. Plant cell

Answer 42

B. Skin epithelial cell C. Smooth muscle cell in the digestive tract E. Nerve cell in the spinal cord

Question 43

Which statement about intermediate filaments is NOT true? A. They lack polarity. B. They are wider than actin filaments. C. Their disruption can lead to premature aging. D. They rupture under stress.

Answer 43

D. They rupture under stress.

Question 44

Why do you suppose it is much easier to add tubulin to existing microtubules than to start a new microtubule from scratch? A. Existing microtubules act as a template, guiding the formation of new ones through a process known as "microtubule replication." B. Two tubulin dimers have a lower affinity for each other than a tubulin dimer has for the end of a microtubule C. Starting a new microtubule from scratch requires a special enzyme called "microtubulinase," which is not always readily available in cells.

Answer 44

B. Two tubulin dimers have a lower affinity for each other than a tubulin dimer has for the end of a microtubule

Question 45

How would the dynamics of microtubule polymerization change if cells were incubated with a nonhydrolyzable analog of GTP? A. Microtubules would polymerize more and grow longer. B. Microtubules would depolymerize and shrink. C. Dynamic instability would increase as microtubules switch more frequently between growing and shrinking states. D. Microtubule dynamics would not change.

Answer 45

A. Microtubules would polymerize more and grow longer.

Question 46

Dynamic instability in microtubules stems from the intrinsic capacity of tubulin molecules to hydrolyze what? A. tubulin dimers B. ATP C. water D. GTP

Answer 46

D. GTP

Question 47

What statement describes the behavior of an actin filament undergoing treadmilling at the leading edge of a lamellipodium? A. The filament grows exponentially. B. The filament depolymerizes and disappears. C. The filament adds actin monomers to its minus end while losing them from its plus end. D. The filament remains about the same length, although the subunits that comprise it are replaced.

Answer 47

D. The filament remains about the same length, although the subunits that comprise it are replaced.

Question 48

Which does NOT contain both actin and myosin? A. a muscle cell sarcomere B. the contractile ring that carries out cytokinesis C. a contractile bundle in a nonmuscle cell D. the lamellipodium at the leading edge of a crawling cell

Answer 48

D. the lamellipodium at the leading edge of a crawling cell

Question 49

When a muscle is stimulated to contract, what does Ca2+ bind to, and what effect does that binding have? A. tropomyosin, which moves the troponin that otherwise blocks the interaction of actin and myosin B. actin, allowing it to associate with myosin C. myosin, allowing it to associate with actin D. troponin, which moves the tropomyosin that otherwise blocks the interaction of actin and myosin

Answer 49

D. troponin, which moves the tropomyosin that otherwise blocks the interaction of actin and myosin

Question 50

Which of the following correctly matches a phase of the eukaryotic cell cycle with an event that takes place in that phase? A. S phase—preparations for entry into cell cycle B. G1 phase—DNA synthesis C. G2 phase—mitosis D. M phase—cytokinesis

Answer 50

D. M phase—cytokinesis

Question 51

Which of the following statements about the cell-cycle control system is NOT true? A. It ensures that the environment is favorable for the cell to initiate DNA replication. B. It ensures that all chromosomes are attached to the mitotic spindle before the chromosomes are segregated to the 2 daughter cells. C. It ensures that all DNA is replicated before M phase begins. D. It ensures that M phase is completed before cytokinesis begins.

Answer 51

D. It ensures that M phase is completed before cytokinesis begins.

Question 52

How do cyclin–Cdk complexes trigger different stages of the cell cycle? A. The concentration of the Cdk subunit increases at different stages of the cycle. B. The Cdk subunits are degraded at different stages of the cycle. C. The kinase activity of the Cdk subunit increases at different stages of the cycle. D. All of the above are correct.

Answer 52

C. The kinase activity of the Cdk subunit increases at different stages of the cycle.

Question 53

The cell-cycle control system initiates chromosome segregation only after which of the following has occurred? A. DNA has been completely replicated and the chromosomes have decondensed. B. Cytokinesis is complete. C. M phase is complete. D. The duplicated chromosomes are correctly aligned on the mitotic spindle.

Answer 53

D. The duplicated chromosomes are correctly aligned on the mitotic spindle.

Question 54

Most of the diversity in rates of cell division in the adult body is due to variations in the length of which cell-cycle phase(s)? A. S phase B. M phase C. G1 phase and G2 phase D. G1 phase and G0 phase

Answer 54

D. G1 phase and G0 phase

Question 55

Which of the following statements is false? A. The activation of S-Cdk initiates DNA replication. B. DNA replication occurs only in S phase. C. The activation of S-Cdk helps prevent DNA replication from occurring more than once during each cycle. D. The activation of S-Cdk helps prevent the onset of M phase.

Answer 55

D. The activation of S-Cdk helps prevent the onset of M phase.

Question 56

What does the phosphorylation of Cdc25 by M-Cdk do? A. It activates Cdc25, allowing the cell to exit mitosis. B. It inactivates Cdc25, which promotes activation of more M-Cdk. C. It activates Cdc25, which inactivates M-Cdk. D. It activates Cdc25, which in turn activates more M-Cdk.

Answer 56

D. It activates Cdc25, which in turn activates more M-Cdk.

Question 57

Which of the following statements is true? A. The mitotic spindle is made primarily of microtubules and actin filaments. B. The dynamic instability of microtubules decreases at the start of mitosis. C. The assembly of the spindle is driven, in part, by myosins associated with the non-kinetochore microtubules. D. New microtubules grow out in random directions from the centrosomes.

Answer 57

D. New microtubules grow out in random directions from the centrosomes.

Question 58

The anaphase-promoting complex or cyclosome (APC/C) triggers the onset of anaphase by doing which of the following? A. triggering the destruction of the condensins that hold the duplicated chromosomes in a condensed state B. phosphorylating the motor proteins that pull the spindle poles apart C. cleaving spindle microtubules D. triggering the cleavage of the cohesins that hold the sister chromatids together

Answer 58

D. triggering the cleavage of the cohesins that hold the sister chromatids together

Question 59

What is the contractile ring made of? A. astral and non-kinetochore microtubules B. cohesins and condensins C. actin and integrins D. actin and myosin

Answer 59

D. actin and myosin

Question 60

Which of the following statements about intermediate filaments is FALSE? A. Each filament is made of eight strands, and each strand is made from staggered tetramers linked end to end. B. Intermediate filaments protect cells from mechanical stress because they have high tensile strength and resist stretching. C. Intermediate filaments are constructed of identical subunits found in all eukaryotic cells. D. Intermediate filaments can connect cells at cell–cell junctions called desmosomes.

Answer 60

C. Intermediate filaments are constructed of identical subunits found in all eukaryotic cells.

Question 61

Mutations that disrupt desmin intermediate filament structure or function could explain the symptoms of desmin-related myopathy. Possible mutations that may result in this disease include all of the following EXCEPT. A. Alteration in head groups, so tetramers are unable to link end to end. B. Disruption of the polarity of the final desmin strands. C. Mutation such that formation of dimers is blocked. D. Alteration in the twist or coiling of the dimers, blocking formation of staggered tetramers.

Answer 61

B. Disruption of the polarity of the final desmin strands.

Question 62

Which of the following would increase the level of muscle contraction? A. blockage of the Ca2+ pump B. mutation in troponin such that it no longer binds tropomyosin C. inactivation of the myosin motor D. addition of a molecule to bind free Ca2+

Answer 62

A. blockage of the Ca2+ pump

Question 63

Rigor mortis occurs in muscle cells after death and is characterized by muscles being locked in a contracted state where myosin is bound to actin. What could be added to the muscle cell to release the rigor state? A. ATP B. ADP C. Pi D. ADP + Pi

Answer 63

A. ATP

Question 64

Which of the following would be a consequence of mutations that disrupt the interaction between cross-linking, stabilizing proteins and keratin filaments? A. neurodegeneration caused by an interference with normal axonal transport B. defects in muscle development caused by improper organization of sarcomeres C. corneal damage caused by cell rupture from mechanical trauma D. skeletal and cardiac abnormalities caused by a weakened nuclear envelope

Answer 64

C. corneal damage caused by cell rupture from mechanical trauma

Question 65

How would an increase in tubulin concentration affect a microtubule in its shrinking phase? A. It would speed up microtubule disassembly since the presence of a tubulin dimer promotes conversion of other tubulin to its dimer form. B. It would have no effect since the rate of microtubule elongation is independent of tubulin subunit concentration. C. The rate of addition of GTP-tubulin would be greater at higher tubulin concentrations, so it would be more likely to "rescue" the microtubule so it would begin growing again. D. It would have no effect unless there were also an increase in a GEF that could increase the GTP concentration, in which case "rescue" would occur and the microtubule would grow.

Answer 65

C. The rate of addition of GTP-tubulin would be greater at higher tubulin concentrations, so it would be more likely to "rescue" the microtubule so it would begin growing again.

Question 66

The principal microtubule-organizing center in animal cells in the A. centromere. B. centrosome. C. cell cortex. D. kinetochore.

Answer 66

B. centrosome.

Question 67

For example, serum starvation deprives cells of mitogens and blocks cells in the G0/G1 phase of the cell cycle. Another method involves gently shaking culture flasks and harvesting the cells that release from the substratum. In what stage of the cell cycle are these cells most likely to be? A. M phase B. S phase C. G1 phase D. G2 phase

Answer 67

A. M phase

Question 68

The chemotherapy drug paclitaxel is used to treat ovarian and breast cancers. One action of Paclitaxel is to bind to and decrease the activity of Bcl-2. How does this result in the death of cancer cells, helping to treat the cancer? A. Paclitaxel disrupts the structure and integrity of the nucleus, leading to cell death. B. Paclitaxel blocks assembly of the kinetochore on the centromere. C. Paclitaxel promotes apoptosis of the cancer cell. D. Paclitaxel inhibits cytokinesis, leading to multinucleate cells.

Answer 68

C. Paclitaxel promotes apoptosis of the cancer cell.

Question 69

Which of the following statements is false? A. Attachment of sister chromatid pairs to opposite poles of the spindle creates tension in the kinetochores, which signals that the sister chromatids are ready to be separated. B. The cell-cycle control system monitors the tension on the kinetochores to ensure that anaphase does not occur until the sister chromatids are correctly attached to the spindle. C. The pairing of homologous chromosomes during prometaphase is called bi-orientation. D. Each sister chromatid in a pair must attach to the opposite pole of the spindle for the sister chromatids to be segregated properly.

Answer 69

C. The pairing of homologous chromosomes during prometaphase is called bi-orientation.

Question 70

Which of the following statements is FALSE? A. Phosphorylation of mitotic Cdk by the inhibitory kinase (Wee1) makes the Cdk inactive. B. The activating phosphatase (Cdc25) removes the phosphates from mitotic Cdk that were added by Wee1, so that M-Cdk will be active. C. Cdc25 dephosphorylation of Wee1 activates the kinase, promoting the G2/M transition. D. Inhibiting the Cdc25 phosphatase will delay the G2/M transition.

Answer 70

C. Cdc25 dephosphorylation of Wee1 activates the kinase, promoting the G2/M transition.

Question 71

To be effective, the artificial chromosome must be passed to daughter cells during mitosis, which requires that the artificial chromosome contain sequences on which kinetochores can assemble. With this in mind, what should designers of artificial chromosomes include to ensure proper kinetochore assembly on the artificial chromosome? A. origin of replication B. centromere C. centrosome D. condensin

Answer 71

B. centromere

Question 72

Cells in the G0 state A. do not divide. B. cannot reenter the cell cycle. C. have duplicated their DNA. D. have entered this arrest state from either G1 or G2.

Answer 72

A. do not divide.

Question 73

Mitogens are A. kinases that cause cells to grow in size. B. Transcription factors important for cyclin production. C. extracellular signals that stimulate cell division. D. produced by mitotic cells to keep nearby neighboring cells from dividing

Answer 73

C. extracellular signals that stimulate cell division.

Question 74

Characteristics of a cell undergoing programmed cell death include all of the following EXCEPT: A. breakdown of nuclear lamina B. cell lysis C. blebbing of large membrane-bound vesicles D. a relatively intact membrane

Answer 74

B. cell lysis

Question 75

The binding of ATP causes a conformational change in myosin that A. releases the myosin head from the actin filament B. moves the myosin head one step forward on the actin filament C. attaches the myosin head to the actin filament D. moves the actin filament in a force-generating "power stroke"

Answer 75

A. releases the myosin head from the actin filament

Question 76

Which of the following is an important function of intermediate filaments? A. moving fesicles from location to location in the cell B. forming attachments for cells to move along a substrate C. providing tensile strength to the cell and the nucleus D. separating chromosomes during mitosis

Answer 76

C. providing tensile strength to the cell and the nucleus

Question 77

The enzyme katanin, named after Japanese samurai swords, is activated during the onset of mitosis and chops microtubules into short pieces. What is the fate of the microtubule fragments created by katanin? A. The cleaved microtubule fragments are stable since they can be used as precursors to synthesize the new mitotic spindle microtubules. B. The cleaved microtubule fragments are quickly ubiquitinated and degraded by the proteosome. C. The cleaved microtubule fragments are recognized as a substrate by M-Cdk, which phosphorylates them to target them for destruction. D. The cleaved microtubule fragments largely lack a GTP-cap so they rapidly depolymerize.

Answer 77

D. The cleaved microtubule fragments largely lack a GTP-cap so they rapidly depolymerize.

Question 78

Which of the following is a TRUE statement? A. The plus ends of microtubules grow faster because they have a larger GTP cap. B. GTP is hydrolyzed by tubulin to cause the bending of flagella. C. Kinesin moves endoplasmic reticulum (ER) membranes along microtubules so that the network of ER tubules becomes stretched throughout the cell. D. Cells having an intermediate filament network that cannot be depolymerized would die.

Answer 78

C. Kinesin moves endoplasmic reticulum (ER) membranes along microtubules so that the network of ER tubules becomes stretched throughout the cell.

Question 79

All of the following is true about cyclins EXCEPT: A. chaperones guide cyclins to the proteosome for degradation B. cyclins bind Cdk's to help activate them C. cyclin concentration varies during the cell cycle D. cyclins have no intrinsic enzymatic activity

Answer 79

A. chaperones guide cyclins to the proteosome for degradation

Question 80

Investigators incubate myosin with an ATP analog that can bind to myosin but cannot be hydrolyzed. What effect will this treatment have on the activity of myosin? A. The myosin will bind to an actin filament and initiate a power stroke but will not be subsequently released. B. The myosin will bind to an actin filament but will not be released; its movement along actin will be inhibited. C. The myosin will be unable to bind to an actin filament. D. No effect-the myosin will behave normally.

Answer 80

C. The myosin will be unable to bind to an actin filament.

Question 81

Motor neuron degeneration occurs in several diseases and leads to loss of muscle control. One form of motor neuron degeneration was found to have defects in retrograde transport (backward transport to cell body, in blue below) that were caused by mutations in a gene that codes for a particular protein. What protein, when mutated, would inhibit backward, but not outward, transport along axon microtubules? A. tubulin B. kinesin C. myosin D. dynein

Answer 81

D. dynein

Question 82

Figure 17-38 shows an electron micrograph of a skeletal muscle fiber, where various points along a fiber and various regions have been labeled. Which of the following statements is TRUE about muscle contraction? A .Point A will move closer to point B. B. Region E will shrink in size. C. Point B will move closer to point C. D. Region D will become smaller.

Answer 82

C. Point B will move closer to point C.

Question 83

An actin-binding protein called cofilin binds preferentially to ADP-containing actin filaments rather than ATP-containing actin filaments. Based on this preference, which is true? A. Cofilin binds to the plus ends of actin filaments B. Cofilin binds to older actin filaments. C. Cofilin competes with profilin for binding to actin. D. Cofilin binds to the plus ends of treadmilling actin filaments.

Answer 83

B. Cofilin binds to older actin filaments.

Question 84

What would be the most obvious outcome of repeated cell cycles consisting of S phase and M phase only? A. The cells would not be able to replicate their DNA B. The cells would get larger and larger. C. The cells produced would get smaller and smaller. D. The mitotic spindle could not assemble

Answer 84

C. The cells produced would get smaller and smaller.

Question 85

A cell that is terminally differentiated will A. replicate its DNA. B. dismantle the cell-cycle control system. C. commit apoptosis. D. reenter the cell cycle only once a year.

Answer 85

B. dismantle the cell-cycle control system.

Question 86

In terms of structure, if cytoplasmic intermediate filaments are described as ropes, nuclear lamins could be best described as: A. string B. cylinders C. vesicles D. mesh

Answer 86

D. mesh

Question 87

What is a reasonable explanation for why there are no known motor proteins that move on intermediate filaments? A. Intermediate filaments have no polarity so their ends are chemically indistinguishable. B. Intermediate filaments are rope-like and so would not be rigid enough, like microtubules are, to support a motor protein. C. Intermediate filaments are bundled; motor proteins can only walk on a single protofilament. D. Intermediate flaments are too dynamic so they would likely fall apart before a motor protein could get to its target.

Answer 87

A. Intermediate filaments have no polarity so their ends are chemically indistinguishable.

Question 88

Which of the following is a TRUE statement? A. The transverse tubules in a muscle cell are extensions of the endoplasmic reticulum. B. Activation of myosin movement on actin filaments is triggered by the phosphorylation of troponin in some situations and by Mg2+ binding to troponin in others. C. Without actin, cells can form a functional mitotic-spindle and pull their chromosomes apart but cannot divide. D. Dynein moves the lamellipodia and filopodia along the cell's substratum so they can act as "feelers" to find anchor points.

Answer 88

A. The transverse tubules in a muscle cell are extensions of the endoplasmic reticulum.

Question 89

Actin filaments and microtubules share all of the following EXCEPT: A. Activation of small G proteins help regulate their assembly pattern. B. Their building block subunits contain a nucleotide triphosphate. C. They have a plus end and a minus end D. They are dynamic: they can grow and shrink

Answer 89

A. Activation of small G proteins help regulate their assembly pattern.

Question 90

(Q016) At desmosomes, cadherin molecules are connected to A. actin filaments. B. microtubules. C. gap junctions. D. intermediate filaments.

Answer 90

D. intermediate filaments.

Question 91

(Q017) Hemidesmosomes are important for A. attaching epithelial cells to the extracellular matrix. B. tubulation of epithelial sheets. C. linkages to glycosaminoglycans. D. forming the basal lamina.

Answer 91

A. attaching epithelial cells to the extracellular matrix.

Question 92

(Q006) Which of the following statements about animal connective tissues is TRUE? A. Enzymes embedded in the plasma membrane synthesize the collagen in the extracellular matrix extracellularly. B. Cells can attach to a collagen matrix by using fibronectin, an integral membrane protein. C. In connective tissue, the intermediate filaments within the cells are important for carrying the mechanical load. D. Proteoglycans can resist compression in the extracellular matrix.

Answer 92

D. Proteoglycans can resist compression in the extracellular matrix.

Question 93

(Q019) Which of the following statements about gap junctions is FALSE? A. Because gap junctions only allow uncharged molecules to pass through, they are not used for electrically coupling cells. B. Gap junctions are made of connexons. C. Gap junctions can close in response to extracellular signals. D. Molecules up to 1000 daltons in molecular mass can move across gap junctions.

Answer 93

A. Because gap junctions only allow uncharged molecules to pass through, they are not used for electrically coupling cells.

Question 94

(Q018) A major distinction between the connective tissues in an animal and other main tissue types such as epithelium, nervous tissue, or muscle is the A. amount of extracellular matrix in connective tissues. B. numerous connections that connective-tissue cells make with each other. C. ability of connective tissues to withstand mechanical stresses. D. ability of connective-tissue cells such as fibroblasts to change shape.

Answer 94

A. amount of extracellular matrix in connective tissues.

Question 95

(Q038) Ras is a GTP-binding protein that is often defective in cancer cells. A common mutation found in cancer cells causes Ras to behave as though it were bound to GTP all the time, which will cause cells to divide inappropriately. From this description, the normal Ras gene is a/an A. tumor suppressor. B. oncogene. C. GEF D. proto-oncogene.

Answer 95

D. proto-oncogene.

Question 96

(Q005) Which of the following is NOT an example of a connective tissue? A. bone B. the layer of photoreceptors in the eye C. the jellylike interior of an eye D. cartilage

Answer 96

B. the layer of photoreceptors in the eye

Question 97

(Q037) Which of the following statements about tumor suppressor genes is FALSE? A. Gene amplification of a tumor suppressor gene is less dangerous than gene amplification of a proto-oncogene. B. An individual needs only one functional copy of a tumor suppressor gene to prevent cell over proliferation. C. Cells with one functional copy of a tumor suppressor gene will usually proliferate faster than normal cells. D. Inactivation of both alleles of a tumor suppressor gene leads to enhanced cell survival and proliferation.

Answer 97

C. Cells with one functional copy of a tumor suppressor gene will usually proliferate faster than normal cells.

Question 98

(Q014) Tight junctions A. are found in cells in connective tissues. B. are formed from claudins and occludins. C. interact with the intermediate filaments inside the cell. D. allow small, water-soluble molecules to pass from cell to cell.

Answer 98

B. are formed from claudins and occludins.

Question 99

(Q026) When a terminally differentiated cell in an adult body dies, it can typically be replaced in the body by a stock of A. cells more apically located than the terminally differentiated cells. B. Wnt proteins. C. induced pluripotent cells. D. proliferating precursor cells.

Answer 99

D. proliferating precursor cells.

Question 100

(Q011) Proteoglycans in the extracellular matrix of animal tissues A. are linked to microtubules through the plasma membrane. B. allow cartilage to resist compression. C. chiefly provide tensile strength. D. are polysaccharides composed of only glucose subunits.

Answer 100

B. allow cartilage to resist compression.

Question 101

(Q021) Which type of junction contributes the most to the polarization of epithelial cells? A. adherens junctions B. gap junctions C. desmosomes D. tight junctions

Answer 101

D. tight junctions

Question 102

(Q030) Induced pluripotent stem (iPS) cells A. require a supply of donor egg cells, as is the case for embryonic stem cells. B. can be created by the expression of a set of key genes in most somatic cell types, including cells derived from adult tissues. C. can differentiate into a greater variety of adult tissues than embryonic stem cells. D. have been used to create human clones.

Answer 102

B. can be created by the expression of a set of key genes in most somatic cell types, including cells derived from adult tissues.

Question 103

(Q028) A pluripotent cell A. can only give rise to stem cells. B. can only be produced in the laboratory. C. can give rise to all the tissues and cell types in the body. D. is considered to be terminally differentiated.

Answer 103

C. can give rise to all the tissues and cell types in the body.

Question 104

(Q009) A cell can crawl through a tissue because of the transmembrane __________ proteins that can bind to fibronectin outside of the cell. A. collagen B. claudin C. integrin D. gap junction

Answer 104

C. integrin

Question 105

(Q036) Which of the following genetic changes cannot convert a proto-oncogene into an oncogene? A. A mutation that introduces a stop codon immediately after the codon for the initiator methionine. B. A mutation within the coding sequence that makes the protein hyperactive. C. A mutation in the promoter of the proto-oncogene, causing the normal protein to be transcribed and translated at an abnormally high level. D. An amplification of the number of copies of the proto-oncogene, causing overproduction of the normal protein.

Answer 105

A. A mutation that introduces a stop codon immediately after the codon for the initiator methionine.

Question 106

(Q034) The Ras protein is a GTPase that functions in many growth factor–signaling pathways. In its active form, with GTP bound, it transmits a downstream signal that leads to cell proliferation; in its inactive form, with GDP bound, the signal is not transmitted. Mutations in the gene for Ras are found in many cancers. Of the choices below, which alteration of Ras activity is most likely to contribute to the uncontrolled growth of cancer cells? A. a change that decreases the affinity of Ras for GTP B. a change that prevents Ras from being made C. a change that decreases the rate of hydrolysis of GTP by Ras D. a change that increases the affinity of Ras for GDP

Answer 106

C. a change that decreases the rate of hydrolysis of GTP by Ras