Biology Final Study 1-18 oof
Before chromosomes segregate in M phase, they and the segregation machinery must be appropriately prepared. Indicate whether the following statements are true or false Cohesins are required to make the chromosomes more compact and thus to prevent tangling between different chromosomes.
False. Condensins are required to make the chromosomes more compact and thus to prevent tangling between different chromosomes.
Before chromosomes segregate in M phase, they and the segregation machinery must be appropriately prepared. Indicate whether the following statements are true or false Sister chromatids are held together by condensins from the time they arise by DNA replication until the time they separate at anaphase.
False. Sister chromatids are held together by cohesins from the time they arise by DNA replication until the time they separate at anaphase.
Are the statements below true or false? Explain your answer. Statement 2: Therefore, the cell-cycle control system operates primarily by a timing mechanism, in which the entry into one phase starts a timer set for sufficient time to complete the required tasks. After a given amount of time has elapsed, a molecular "alarm" triggers movement to the next phase.
False. The cell-cycle control system does use a timing mechanism of sorts, but it also employs various surveillance and feedback mechanisms (checkpoints). Cells will not embark on later events or phases until the earlier events or phases have been completed successfully. In response to a defect or a delay in a cell-cycle event, cells engage molecular brakes to arrest the progression of the cell cycle at various checkpoints, to allow time for completion or repair.
Before chromosomes segregate in M phase, they and the segregation machinery must be appropriately prepared. Indicate whether the following statements are true or false The centromere nucleates a radial array of microtubules called an aster, and its duplication is triggered by S-Cdk.
False. The centrosome nucleates a radial array of microtubules called an aster, and its duplication is triggered by S-Cdk.
Before chromosomes segregate in M phase, they and the segregation machinery must be appropriately prepared. Indicate whether the following statements are true or false. The mitotic spindle is composed of actin filaments and myosin filaments
False. The contractile ring is composed of actin filaments and myosin filaments.
Before chromosomes segregate in M phase, they and the segregation machinery must be appropriately prepared. Indicate whether the following statements are true or false Each centrosome contains a pair of centrioles and hundreds of γ-tubulin rings that nucleate the growth of microtubules.
True
The principal microtubule-organizing center in animal cells is the ____________. (a) centrosome. (b) centromere. (c) kinetochore. (d) cell cortex.
a
Consider an animal cell that has eight chromosomes (four pairs of homologous chromosomes) in G1 phase. How many of each of the following structures will the cell have at mitotic prophase? A. Centromeres
16
Consider an animal cell that has eight chromosomes (four pairs of homologous chromosomes) in G1 phase. How many of each of the following structures will the cell have at mitotic prophase? A. Kinetochores
16
Consider an animal cell that has eight chromosomes (four pairs of homologous chromosomes) in G1 phase. How many of each of the following structures will the cell have at mitotic prophase? A. sister chromatids
16
Consider an animal cell that has eight chromosomes (four pairs of homologous chromosomes) in G1 phase. How many of each of the following structures will the cell have at mitotic prophase? A. Centrosomes
2
Rank the following cytoskeletal filaments from smallest to largest in diameter (1 = smallest in diameter, 4 = largest) ______ intermediate filaments ______ microtubules ______ actin filament ______ myofibril
3 4 2 1
Consider an animal cell that has eight chromosomes (four pairs of homologous chromosomes) in G1 phase. How many of each of the following structures will the cell have at mitotic prophase? A. Centrioles
4
A friend declares that chromosomes are held at the metaphase plate by microtubules that push on each chromosome from opposite sides. Which of the following observations does not support your belief that the microtubules are pulling on the chromosomes? (a) the jiggling movement of chromosomes at the metaphase plate (b) the way in which chromosomes behave when the attachment between sister chromatids is severed (c) the way in which chromosomes behave when the attachment to one kinetochore is severed (d) the shape of chromosomes as they move toward the spindle poles at anaphase
A
Cells in the G0 state ________________. (a) do not divide. (b) cannot re-enter the cell cycle. (c) have entered this arrest state from either G1 or G2. (d) have duplicated their DNA.
A
Cytokinesis in animal cells ________________. (a) requires ATP. (b) leaves a small circular "scar" of actin filaments on the inner surface of the plasma membrane. (c) is often followed by phosphorylation of integrins in the plasma membrane. (d) is assisted by motor proteins that pull on microtubules attached to the cell cortex.
A
Mitogens are _____. (a) extracellular signals that stimulate cell division. (b) transcription factors important for cyclin production. (c) kinases that cause cells to grow in size. (d) produced by mitotic cells to keep nearby neighboring cells from dividing.
A
Programmed cell death occurs ________________. (a) by means of an intracellular suicide program. (b) rarely and selectively only during animal development. (c) only in unhealthy or abnormal cells. (d) only during embryonic development.
A
Which of the following does not occur during M phase in animal cells? (a) growth of the cell (b) condensation of chromosomes (c) breakdown of nuclear envelope (d) attachment of chromosomes to microtubules
A
Which of the following precede the re-formation of the nuclear envelope during M phase in animal cells? (a) assembly of the contractile ring (b) decondensation of chromosomes (c) reassembly of the nuclear lamina (d) transcription of nuclear genes
A
Which of the following statements about kinetochores is true? (a) Kinetochores assemble onto chromosomes during late prophase. (b) Kinetochores contain DNA-binding proteins that recognize sequences at the telomere of the chromosome. (c) Kinetochore proteins bind to the tubulin molecules at the minus end of microtubules. (d) Kinetochores assemble on chromosomes that lack centromeres.
A
Which of the following statements about the cell cycle is false? (a) Once a cell decides to enter the cell cycle, the time from start to finish is the same in all eukaryotic cells. (b) An unfavorable environment can cause cells to arrest in G1. (c) A cell has more DNA during G2 than it did in G1. (d) The cleavage divisions that occur in an early embryo have short G1 and G2 phases.
A
Progression through the cell cycle requires a cyclin to bind to a Cdk because _________. (a) the cyclins are the molecules with the enzymatic activity in the complex. (b) the binding of a cyclin to Cdk is required for Cdk enzymatic activity. (c) cyclin binding inhibits Cdk activity until the appropriate time in the cell cycle. (d) without cyclin binding, a cell-cycle checkpoint will be activated.
B
Which of the following events does not usually occur during interphase? (a) Cells grow in size. (b) The nuclear envelope breaks down. (c) DNA is replicated. (d) The centrosomes are duplicated.
B
Indicate which of the three major classes of cytoskeletal elements each statement below refers to. A. monomer that binds ATP B. includes keratin and neurofilaments C. important for formation of the contractile ring during cytokinesis D. supports and strengthens the nuclear envelope E. their stability involves a GTP cap F. used in the eucaryotic flagellum G. a component of the mitotic spindle H. can be connected through desmosomes I. directly involved in muscle contraction J. abundant in filopodia
A) actin filaments B) intermediate filaments C) actin filaments D) intermediate filaments E) microtubules F) microtubules G) microtubules H) intermediate filaments I) actin filaments J) actin filaments
Identify the cytoskeletal structures depicted in the epithelial cells shown in Figure Q17-1.
A) actin filaments B) microtubules C) intermediate filaments
The graph in Figure Q17-22 shows the time course of the polymerization of pure tubulin in vitro. You can assume that the starting concentration of free tubulin is much higher than it is in cells. A.Explain the reason for the initial lag in the rate of microtubule formation. B.Why does the curve level out after point C?
A. Before they can polymerize to form microtubules, tubulin molecules have to form small aggregates that act as nucleation centers. This aggregation step is slow because the molecules have to come together in the right configuration. Therefore there is a lag phase before the microtubules start to be formed. B. After point C an equilibrium point has been reached where the rates of polymerization and depolymerization are exactly balanced.
Which of the following statements is false? (a) DNA synthesis begins at origins of replication. (b) The loading of the origin recognition complexes (ORCs) is triggered by S-Cdk. (c) The phosphorylation and degradation of Cdc6 help to ensure that DNA is replicated only once in each cell cycle. (d) DNA synthesis can only begin after prereplicative complexes assemble on the ORCs.
B
Which organelle fragments during mitosis? (a) endoplasmic reticulum (b) Golgi apparatus (c) mitochondrion (d) chloroplast
B
The Retinoblastoma (Rb) protein blocks cells from entering the cell cycle by ______. (a) phosphorylating Cdk. (b) marking cyclins for destruction by proteolysis. (c) inhibiting cyclin transcription. (d) activating apoptosis.
C
You are curious about the dynamic instability of microtubules and decide to join a lab that works on microtubule polymerization. The people in the lab help you grow some microtubules in culture using conditions that allow you to watch individual microtubules under a microscope. You can see the microtubules growing and shrinking, as you expect. The professor who runs the lab gets in a new piece of equipment, a very fine laser beam that can be used to sever microtubules. She is very excited and wants to sever growing microtubules at their middle, using the laser beam. A. Do you predict that the newly exposed microtubule plus ends will grow or shrink? Explain your answer. B. What do you expect would happen to the newly exposed plus ends if you were to grow the microtubules in the presence of an analog of GTP that cannot be hydrolyzed, and you then severed the microtubules in the middle with a laser beam?
A. The newly exposed microtubule plus ends will most likely shrink if you sever the microtubules in the middle. This is because a microtubule grow by adding GTP carrying subunits to the plus end. The GTP is hydrolyzed over time, leaving only a cap of GTO-carrying subunits at the + end, with the remainder of the tubulin protofilament containing GDP- carrying subunits. Therefore, if you sever a growing microtubule in its middle, you will most likely create a plus end that contains GDP-carrying subunits. The GDP- carrying subunits are less tightly bound than the GTP-carrying subunits and will peel away from each other, causing depolymerization of the microtubule and shrinkage. B. If you were to polymerize the microtubules in the presence of a nonhydrolyzable analog of GTP and you then severed the microtubules with a laser, the newly exposed plus end would contain a GTP cap and so would probably continue to grow.
In the cell, the concentration of actin monomer is higher than the concentration required for purified actin monomers to polymerize in vitro. Thymosin is a protein that can bind actin monomers. If you were to add a drug that inhibits the ability of thymosin to bind actin monomers, what effect would this have on actin polymerization? Explain your answer.
Addition of a drug that keeps thymosin from binding actin monomer will increase the rate of actin polymerization in the cell. The reason is that actin monomers do not spontaneously form filaments in the cell, despite their high concentrations, is that the monomers are normally bound by proteins (such as thymosin) and are thereby prevented from adding to the end of an actin filament.
Do you agree or disagree with the following statement? Explain your answer. Nucleotide hydrolysis has a similar role in actin polymerization and in tubulin polymerization.
Agree: ATP bound actin monomers are added onto actin filaments; GTP bound tubulins subunits are added to the growing end of a microtubule. Nucleotide hydrolysis occurs after the addition of the subunit onto the filament in both actin and microtubules. ATP hydrolysis in actin polymerization decreases binding strength of monomoers in actin filaments, GTP hydrolysis during tubulin polymerization redueces strength of binding between tubulin. Nucleotide hydrolysis decreases the strength of binding between subunits and enhances depolymerization in actin filaments and microtubules.
A cell with nuclear lamins that cannot be phosphorylated in M phase will be unable to ________________. (a) reassemble its nuclear envelope at telophase. (b) disassemble its nuclear lamina at prometaphase. (c) begin to assemble a mitotic spindle. (d) condense its chromosomes at prophase.
B
You create cells with a version of Cdc6 that cannot be phosphorylated and thus cannot be degraded. Which of the following statements describes the likely consequence of this change in Cdc6? (a) Cells will enter S phase prematurely. (b) Cells will be unable to complete DNA synthesis. (c) The origin recognition complex (ORC) will be unable to bind to DNA. (d) Cdc6 will be produced inappropriately during M phase.
B
You have isolated a strain of mutant yeast cells that divides normally at 30°C but cannot enter M phase at 37°C. You have isolated its mitotic cyclin and mitotic Cdk and find that both proteins are produced and can form a normal M-Cdk complex at both temperatures. Which of the following temperature-sensitive mutations could not be responsible for the behavior of this strain of yeast? (a) inactivation of a protein kinase that acts on the mitotic Cdk kinase (b) inactivation of an enzyme that ubiquitylates M cyclin (c) inactivation of a phosphatase that acts on the mitotic Cdk kinase (d) a decrease in the levels of a transcriptional regulator required for producing sufficient amounts of M cyclin
B
Intermediate filaments are made from elongated fibrous proteins that are assembled into a ropelike structure. Figure Q17-7 shows the structure of an intermediate filament subunit. You are interested in how intermediate filaments are formed, and you create an intermediate filament subunit whose α-helical region is twice as long as that of a normal intermediate filament by duplicating the normal α-helical region while keeping a globular head at the N-terminus and a globular tail at the C-terminus; you call this subunit IFαd. If you were to assemble intermediate filaments using IF2αd as the subunit, which of the following predictions below describes the most likely outcome? (a) Filaments assembled using IFαd will interact with different cytoskeletal components. (b) Filaments assembled using IFαd will form dimers that are twice as long as dimers assembled from normal intermediate filaments. (c) Sixteen tetramers assembled from IFαd will be needed for a ropelike structure to form. (d) Dimers of IFαd will form by interactions with the N-terminal globular head and the C-terminal globular tail.
B) filaments assembled using IFαd will form dimers that are twice as long as dimers assembled from normal intermediate filaments.
Of the following mutations, which are likely to cause cell-cycle arrest? A. a mutation in a gene encoding a cell-surface mitogen receptor that makes the receptor active even in the absence of the mitogen B. a mutation that destroyed the kinase activity of S-Cdk C. a mutation that allowed G1-Cdk to be active independently of its phosphorylation status D. a mutation that removed the phosphorylation sites on the Rb protein E. a mutation that inhibited the activity of Rb
B, D
Irradiated mammalian cells usually stop dividing and arrest at a G1 checkpoint. Place the following events in the order in which they occur. A. production of p21 B. DNA damage C. inhibition of cyclin-Cdk complexes D. accumulation and activation of p53
B, D, A, C
Apoptosis differs from necrosis in that necrosis ________________. (a) requires the reception of an extracellular signal. (b) causes DNA to fragment. (c) causes cells to swell and burst, whereas apoptotic cells shrink and condense. (d) involves a caspase cascade.
C
How does S-Cdk help guarantee that replication occurs only once during each cell cycle? (a) It blocks the rise of Cdc6 concentrations early in G1. (b) It phosphorylates and inactivates DNA helicase. (c) It phosphorylates the Cdc6 protein, marking it for destruction. (d) It promotes the assembly of a prereplicative complex.
C
The concentration of mitotic cyclin (M cyclin) ________________. (a) rises markedly during M phase. (b) is activated by phosphorylation. (c) falls toward the end of M phase as a result of ubiquitylation and degradation. (d) is highest in G1 phase.
C
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 M phase. (b) The cell would be unable to enter G2. (c) The cell would enter M phase under conditions when normal cells would not. (d) The cell would pass through M phase more slowly than normal cells
C
Which of the following statements about the anaphase-promoting complex (APC) is false? (a) It promotes the degradation of proteins that regulate M phase. (b) It inhibits M-Cdk activity. (c) It is continuously active throughout the cell cycle. (d) M-Cdk stimulates its activity.
C
You engineer yeast cells that express the M cyclin during S phase by replacing the promoter sequence of the M cyclin gene with that of S cyclin. Keeping in mind that yeast cells have one common Cdk that binds to all cyclins, which of the following outcomes is least likely during this experiment? (a) There will be both M cyclin-Cdk and S cyclin-Cdk complexes in the cell during S phase. (b) Some substrates that are normally phosphorylated in M phase will now be phosphorylated in S phase. (c) G1 cyclins will be expressed during S phase. (d) S-Cdk targets will be phosphorylated during S phase.
C
Match the type of intermediate filament with its appropriate location. lamins _________ A. nerve cells neurofilaments _________ B. epithelia vimentins _________ C. nucleus keratins _________ D. connective tissue
C A B D
Which of the following statements about the cytoskeleton is false? (a) The cytoskeleton is made up of three types of protein filament. (b) The bacterial cytoskeleton is important for cell division and DNA segregation. (c) Protein monomers that are held together with covalent bonds form cytoskeletal filaments. (d) The cytoskeleton of a cell can change in response to the environment.
C) protein monomers that are held together with covalent bonds form cytoskeletal filaments
Actin-binding proteins bind to actin and can modify its properties. You purify a protein, Cap1, that seems to bind and cap one end of an actin filament, although you do not know whether it binds the plus end or the minus end. To determine which end of the actin filament your protein binds to, you decide to examine the effect of Cap1 on actin polymerization by measuring the kinetics of actin filament formation in the presence and the absence of Cap1 protein. You obtain the following results (see Figure Q17-36). Do you think Cap1 binds the plus end or the minus end of actin? Explain your reasoning.
Cap1 binds the plus end of actin, because it seems to inhibit actin polymerization. Actin filaments grow through the addition of monomers to the plus end of the actin filament. A capping protein that binds the plus end of actin can block monomer addition to the actin filament. Thus, less actin polymerization will be seen in the presence of the Cap1 protein.
Disassembly of the nuclear envelope ________________. (a) causes the inner nuclear membrane to separate from the outer nuclear membrane. (b) results in the conversion of the nuclear envelope into protein-free membrane vesicles. (c) is triggered by the phosphorylation of integrins. (d) must occur for kinetochore microtubules to form in animal cells.
D
In which phase of the cell cycle do cells check to determine whether the DNA is fully and correctly replicated? (a) at the transition between G1 and S (b) when cells enter G0 (c) during M (d) at the end of G2
D
Levels of Cdk activity change during the cell cycle, in part because ________________. (a) the Cdks phosphorylate each other. (b) the Cdks activate the cyclins. (c) Cdk degradation precedes entry into the next phase of the cell cycle. (d) cyclin levels change during the cycle.
D
MPF activity was discovered when cytoplasm from a Xenopus M-phase cell was injected into Xenopus oocytes, inducing the oocytes to form a mitotic spindle. In a control experiment, Xenopus interphase cytoplasm was injected into oocytes and shown not to induce the formation of a mitotic spindle. Which of the following statements is not a legitimate conclusion from the control experiment? (a) The piercing of the oocyte membrane by a needle is insufficient to cause mitotic spindle formation. (b) An increased volume of cytoplasm is insufficient to cause mitotic spindle formation. (c) Injection of extra RNA molecules is insufficient to cause mitotic spindle formation. (d) Components of an interphase nucleus are insufficient to cause mitotic spindle formation.
D
The G1 DNA damage checkpoint ________________. (a) causes cells to proceed through S phase more quickly. (b) involves the degradation of p53. (c) is activated by errors caused during DNA replication. (d) involves the inhibition of cyclin-Cdk complexes by p21.
D
What would be the most obvious outcome of repeated cell cycles consisting of S phase and M phase only? (a) Cells would not be able to replicate their DNA. (b) The mitotic spindle could not assemble. (c) Cells would get larger and larger. (d) The cells produced would get smaller and smaller.
D
When introduced into mitotic cells, which of the following is expected to impair anaphase B but not anaphase A? (a) an antibody against myosin (b) ATPγS, a nonhydrolyzable ATP analog that binds to and inhibits ATPases (c) an antibody against the motor proteins that move from the plus end of microtubules to the minus end (d) an antibody against the motor proteins that move from the minus end of microtubules toward the plus end
D
Which of the following is not good direct evidence that the cell-cycle control system is conserved through billions of years of divergent evolution? (a) A yeast cell lacking a Cdk function can use the human Cdk to substitute for its missing Cdk during the cell cycle. (b) The amino acid sequences of cyclins in plants are similar to the amino acid sequences of cyclins in humans. (c) The Cdk proteins in humans share conserved phosphorylation sites with the Cdk proteins in yeast. (d) Yeast cells have only one Cdk, whereas humans have many Cdks.
D
Which of the following statements about apoptosis is true? (a) Cells that constitutively express Bcl2 will be more prone to undergo apoptosis. (b) The prodomain of procaspases contains the catalytic activity necessary for procaspase activation. (c) Bax and Bak promote apoptosis by binding to procaspases in the apoptosome. (d) Apoptosis is promoted by the release of cytochrome c into the cytosol from mitochondria.
D
Which of the following statements is false? (a) Cytokinesis in plant cells is mediated by the microtubule cytoskeleton. (b) Small membrane vesicles derived from the Golgi apparatus deliver new cell-wall material for the new wall of the dividing cell. (c) The phragmoplast forms from the remains of interpolar microtubules of the mitotic spindle. (d) Motor proteins walking along the cytoskeleton are important for the contractile ring that guides formation of the new cell wall.
D
Which of the following statements is false? (a) Mitotic Cdk must be phosphorylated by an activating kinase (Cak) before it is active. (b) Phosphorylation of mitotic Cdk by the inhibitory kinase (Wee1) makes the Cdk inactive, even if it is phosphorylated by the activating kinase. (c) Active M-Cdk phosphorylates the activating phosphatase (Cdc25) in a positive feedback loop. (d) The activating phosphatase (Cdc25) removes all phosphates from mitotic Cdk so that M-Cdk will be active.
D
Which of the following statements is false? (a) The cleavage furrow is a puckering of the plasma membrane caused by the constriction of a ring of filaments attached to the plasma membrane. (b) The cleavage furrow will not begin to form in the absence of a mitotic spindle. (c) The cleavage furrow always forms perpendicular to the interpolar microtubules. (d) The cleavage furrow always forms in the middle of the cell.
D
Which of the following statements is true? (a) Anaphase A must be completed before anaphase B can take place. (b) In cells in which anaphase B predominates, the spindle will elongate much less than in cells in which anaphase A dominates. (c) In anaphase A, both kinetochore and interpolar microtubules shorten. (d) In anaphase B, microtubules associated with the cell cortex shorten.
D
Which of the following statements about the cytoskeleton is true? (a) All eucaryotic cells have actin, microtubules, and intermediate filaments in their cytoplasm. (b) The cytoskeleton provides a rigid and unchangeable structure important for the shape of the cell. (c) The three cytoskeletal filaments perform distinct tasks in the cell and act completely independently of one another. (d) Actin filaments and microtubules have an inherent polarity, with a plus end that grows more quickly than the minus end.
D) actin filaments and microtubules have an inherent polarity, with a plus end that grows more quickly than the minus end
Which of the statements below about intermediate filaments is false? (a) They can stay intact in cells treated with concentrated salt solutions. (b) They can be found in the cytoplasm and the nucleus. (c) They can be anchored to the plasma membrane at cell-cell junction. (d) Each filament is about 10 μm in diameter.
D) each filament is about 10 μm in diameter
Do you agree or disagree with this statement? Explain your answer. Minus end-directed microtubule motors (like dyneins) deliver their cargo to the periphery of the cell, whereas plus end-directed microtubule motors (like kinesins) deliver their cargo to the interior of the cell.
Disagree. Plus end directed motors transport cargo to the cell periphery and minus end directed motors can be used for cargo delivery to the center of the cell. The centrosome established the principal array of microtubules in most animal cells, it nucleates microtubule growth at the minus end. Thus, the plus ends of the microtubules are near the PM and the minus ends are buried in the centrosome.
What is the cause of the massive amount of programmed cell death of nerve cells (neurons) that occurs in the developing vertebrate nervous system, and what purpose does it serve?
Immature neurons are produced in excess of the number that will eventually be required. They compete for the limited amount of survival factors secreted by the target cells they contact. Those cells that fail to get enough survival factor undergo programmed cell death. Up to half or more of the original nerve cells die in this way. This competitive mechanism helps match the number of developing nerve cells to the number of target cells they contact.
What is the main molecular difference between cells in a G0 state and cells that have simply paused in G1?
In G0, the cell-cycle control system is partly dismantled, so that some of the Cdks and cyclins are not present. Cells paused in G1, by contrast, still contain all the components of the cell-cycle control system. Whereas the latter cells can rapidly progress through the cycle when conditions are right, G0 cells need to synthesize the missing cell-cycle control proteins so as to re-enter the cycle, which usually takes 8 hours or more.
The number of cells in an adult tissue or animal depends on cell proliferation. What else does it depend on?
Programmed cell death also influences cell numbers. Most animal cells require survival signals from other cells to avoid programmed cell death, so that the levels of such signals can help determine how many cells live and how many die.
Why should it be that drugs such as colchicine, which inhibit microtubule polymerization, and drugs such as Taxol®, which stabilize microtubules, both inhibit mitosis?
Mitosis requires that the spindle microtubules behave dynamically—continuously polymerizing and depolymerizing—to probe the cell cortex, to seek attachments to kinetochores, and to segregate the chromosomes. Static microtubules are unable to do any of these things.
What would happen to the progeny of a cell that proceeded to mitosis and cell division after entering S phase but had not completed S phase? Keep in mind that highly condensed chromatin, including the centromere region, is replicated late in S phase. Explain your answer.
The daughter cells would probably die. Those chromosomes that had not completed replication in S phase would have only one centromere, because the centromere is the last part of the chromosome to be replicated; the chromosome would therefore be segregated to only one of the two daughter cells at random. At least one, and probably both, of the daughter cells would thus receive an incomplete set of chromosomes and would be unlikely to be viable. Even if one daughter cell, by chance, received a full set of chromosomes, some of these chromosomes would be incompletely replicated and the cell would probably still not be viable.
For each of the following sentences, fill in the blanks with the best word or phrase
The four phases of the cell cycle, in order, are G1, S, M, and G2. A cell contains the most DNA after S phase of the cell cycle. A cell is smallest in size after M phase of the cell cycle. Growth occurs in G1, S, and G2 phases of the cell cycle. A cell does not enter mitosis until it has completed DNA synthesis.
Some lower vertebrates such as fish and amphibians can control their color by regulating specialized pigment cells called melanophores. These cells contain small, pigmented organelles, termed melanosomes, that can be dispersed throughout the cell, making the cell darker, or aggregated in the center of the cell to make the cell lighter. You purify the melanosomes from melanophores that have either aggregated or dispersed melanosomes and find that: 1. aggregated melanosomes co-purify with dynein; 2. dispersed melanosomes co-purify with kinesin. Given this set of data, propose a mechanism for how the aggregation and dispersal of melanosomes occur.
The melanosomes are transported in the cell on microtubules. When it is advantageous for the animal cell to become lighter, a signal is sent to the pigment cell that causes the melanosomes to associate with dynein. Because dynein is a minus-end directed motor, it will transport the melanaosmes toward the center of the cell, causing the melanosomes to aggregate in the center and the cell to take on a lighter appearance. When the animal wants to become darker, a signal is sent to the pigment cell that causes the melanosomes to associate with kinesin. Kinesin is usually a plus-end directed motor and will move the melanosomes away from the center of the cell so that they are more dispersed, making the cell look darker.
You have isolated a mutant in which a fraction of the new cells die soon after cell division and a fraction of the living cells have an extra copy of one or more chromosomes. When you grow the cells under conditions in which they transit the cell cycle more slowly, the defect disappears, suggesting that the mitotic spindle and segregation machinery are normal. Propose a basis for the defect.
This mutant may be lacking the checkpoint mechanism that delays the onset of anaphase and chromosome segregation until all chromosomes have attached properly to the mitotic spindle. If cells attempt chromosome segregation before all chromosomes have attached properly, some of the daughter cells will receive too few chromosomes (and thus will probably die) and other cells will receive additional chromosomes. Normally, cells use such a surveillance system to monitor the spindle attachment of each chromosome and engage molecular brakes until all chromosomes have attached properly. The molecular brakes will be dispensable in some dividing cells if all of the chromosomes rapidly become properly attached to the spindle. In other dividing cells, it will take longer for some of the chromosomes to find their appropriate attachments, and thus the molecular brakes will be essential to ensure faithful segregation of the duplicated copies of each chromosome to the two daughter cells. Slowing the cycle will allow more cells to segregate their chromosomes properly even in the absence of this "spindle attachment" checkpoint.
Before chromosomes segregate in M phase, they and the segregation machinery must be appropriately prepared. Indicate whether the following statements are true or false Microtubule-dependent motor proteins and microtubule polymerization and depolymerization are mainly responsible for the organized movements of chromosomes during mitosis.
True
Is the following statement true or false? After the nuclear envelope breaks down, microtubules gain access to the chromosomes and, every so often, a randomly probing microtubule captures a chromosome and ultimately connects to the kinetochore to become a kinetochore microtubule of the spindle.
True
Are the statements below true or false? Explain your answer. Statement 1: Generally, in a given organism, the S, G2, and M phases of the cell cycle take a defined and stereotyped amount of time in most cells.
True. In nearly all cells in an organism, the S, G2, and M phases of the cell cycle take the same amount of time. The different timing of cell division in different cell types is due to variable lengths of the G1 phase or to withdrawal into the G0 state.
Match the following labels to the numbered lines on Figure Q17-27. A. minus end of microtubule B. tail of motor protein C. cargo of motor protein D. head of motor protein Which of the two motors in Figure Q17-27 is most probably a kinesin? Explain your answer.
a) 4 b) 2 c) 1 d) 3 The top one is most probably a kinesin because most kinesins walk towards the positive end of a microtubule.
Microtubules are formed from the tubulin heterodimer, which is composed of the nucleotide-binding ____a______________ protein and the _____b_____________ protein. Tubulin dimers are stacked together into protofilaments; _________c_________ parallel protofilaments form the tubelike structure of a microtubule. _______d___________ rings are important for microtubule nucleation and are found in the ________e__________ , which is usually found near the cell's nucleus in cells that are not undergoing mitosis. A microtubule that is quickly growing will have a ________f__________ cap that helps prevent the loss of subunits from its growing end. Stable microtubules are used in cilia and flagella; these microtubules are nucleated from a _______g___________ and involve a "______h____________ plus two" array of microtubules. The motor protein _________i_________ generates the bending motion in cilia; the lack of this protein can cause Kartagener's syndrome in humans.
a) a-tubulin b) B-tubulin c) thirteen d) y-tubulin e) centrosome f) GTP g) basal body h) nine i) dynein
Which of the situations below will enhance microtubule shrinkage? (a) addition of a drug that inhibits GTP exchange on free tubulin dimers (b) addition of a drug that inhibits GTP hydrolysis of tubulin dimers (c) addition of a drug that increases the affinity of tubulin molecules carrying GDP for other tubulin molecules (d) addition of a drug that blocks the ability of a tubulin dimer to bind to γ-tubulin
a) addition of a drug that inhibits GTP exchange on free tubular dimers
Indicate whether each of the following statements refers to a ciliary microtubule, a microtubule of the mitotic spindle, both types of microtubule, or neither type of microtubule. A. The basal body is the organizing center. B. The monomer is sequestered by profilin. C. It is arranged in a "9 + 2" array. D. It is nucleated at the centrosome. E. It uses dynein motors. F. It is involved in sperm motility. G. It is involved in moving fluid over the surface of cells.
a) ciliary b) neither c) ciliary d) mitotic spindle e) both f) flagella d) cilliary
Intermediate filaments are found mainly in cells that are subject to mechanical stress. Mutations in genes that disrupt intermediate filaments cause some rare human diseases. For example, the skin of people with epidermolysis bullosa simplex is very susceptible to mechanical injury; people with this disorder have mutations in their _______a___________ genes, the intermediate filament found in epithelial cells. These filaments are usually connected from cell to cell through junctions called ________b__________s. The main filaments found in muscle cells belong to the ________c__________ family; people with disruptions in these intermediate filaments can have muscular dystrophy. In the nervous system, ________d__________s help strengthen the extremely long extensions often present in nerve cell axons; disruptions in these intermediate filaments can lead to neurodegeneration. People who carry mutations in the gene for _______e___________, an important protein for cross-linking intermediate filaments, have a disease that combines symptoms of epidermolysis bullosa simplex, muscular dystrophy, and neurodegeneration.
a) keratin b) desmosomes c) vimentin d) neurofilaments e) plectin
Which of the following statements about microtubules is true? (a) Motor proteins move in a directional fashion along microtubules by using the inherent structural polarity of a protofilament. (b) The centromere nucleates the microtubules of the mitotic spindle. (c) Because microtubules are subject to dynamic instability, they are used only for transient structures in a cell. (d) ATP hydrolysis by a tubulin heterodimer is important for controlling the growth of a microtubule.
a) motor proteins move in a directional fashion along microtubules by using the inherent structural polarity of a protofilament
Which of the following items below are not important for flagellar movement? (a) sarcoplasmic reticulum (b) ATP (c) dynein (d) microtubules
a) sarcoplasmic reticulum
For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; use each word or phrase only once. There are many actin-binding proteins in cells that can bind to actin and modify its activity. Some proteins such as __________A________ bind to actin monomers, sequestering them until needed for filament formation. ___B__________ proteins bind to the end of actin filaments. The ______C_________ proteins are important for nucleation of the branched actin structures commonly found in the _D____ of moving cells, whereas ____E__ proteins are important for the formation of unbranched actin filaments commonly found in ____F____. Proteins belonging to the ___G___ family of GTPases regulate changes in the actin cytoskeleton in response to extracellular signals.
a) thymosin b) capping c) ARP d) lamellipodia e) formin f) filopodia g) Rho
Condensins ________________. (a) are degraded when cells enter M phase. (b) assemble into complexes on the DNA when phosphorylated by M-Cdk. (c) are involved in holding sister chromatids together. (d) bind to DNA before DNA replication begins.
b
Which of the following statements is false? (a) Cytochalasins prevent actin polymerization. (b) Actin filaments are usually excluded from the cell cortex. (c) Integrins are transmembrane proteins that can bind to the extracellular matrix. (d) ARPs can promote the formation of branched actin filaments.
b) actin filaments are usually excluded from the cell cortex
Which of the following statements about organellar movement in the cell is false? (a) Organelles undergo saltatory movement in the cell. (b) Only the microtubule cytoskeleton is involved in organellar movement. (c) Motor proteins involved in organellar movement use ATP hydrolysis for energy. (d) Organelles are attached to the tail domain of motor proteins.
b) only the microtubule cytoskeleton is involved in organellar movement
At the end of DNA replication, the sister chromatids are held together by the ___________. (a) kinetochores. (b) securins. (c) cohesins. (d) histones.
c
Figure Q17-31A shows how the movement of dynein causes the flagellum to bend. If instead of the normal situation, the polarity of the adjacent doublet of microtubules were to be reversed (see Figure Q17-31B) what do you predict would happen? (a) No bending would occur. (b) Bending would occur exactly as diagrammed in Figure Q17-31A. (c) Bending would occur, except that the right microtubule doublet would move down relative to the left one. (d) The two microtubule doublets would slide away from each other.
c) bending would occur, except that the right microtubule doublet would move down relative to the left one
Which of the following statements is correct? Kinesins and dyneins ____________________. (a) have tails that bind to the filaments (b) move along both microtubules and actin filaments (c) often move in opposite directions to each other (d) derive their energy from GTP hydrolysis
c) often move in opposite directions to each other
The microtubules in a cell form a structural framework that can have all the following functions except which of the following? (a) holding internal organelles such as the Golgi apparatus in particular positions in the cell (b) creating long thin cytoplasmic extensions that protrude from one side of the cell (c) strengthening the plasma membrane (d) moving materials from one place to another inside a cell
c) strengthening the plasma membrane
Place the following in order of size, from the smallest to the largest. A. protofilament B. microtubule C. α-tubulin D. tubulin dimer E. mitotic spindle
c, d, a, b, e
Which of the following statements is true? (a) The mitotic spindle is largely made of intermediate filaments. (b) The contractile ring is made largely of microtubules and actin filaments. (c) The contractile ring divides the nucleus in two. (d) The mitotic spindle helps segregate the chromosomes to the two daughter cells.
d
You are interested in understanding the regulation of nuclear lamina assembly. To create an in vitro system for studying this process you start with partly purified nuclear lamina subunits to which you will add back purified cellular components to drive nuclear lamina assembly. Before you start doing experiments, your instructor suggests that you consider what type of conditions would be most amenable to the assembly of nuclear lamina from its individual subunits in vitro. Which of the following conditions do you predict would be most likely to enhance the assembly of the nuclear lamina? (a) addition of phosphatase inhibitors (b) addition of ATP (c) addition of a concentrated salt solution that is 10 times the concentration normally found in the nucleoplasm (d) addition of protein kinase inhibitors
d) addition of protein kinase inhibitors
The hydrolysis of GTP to GDP carried out by tubulin molecules ________________. (a) provides the energy needed for tubulin to polymerize (b) occurs because the pool of free GDP has run out (c) tips the balance in favor of microtubule assembly (d) allows the behavior of microtubules called dynamic instability
d) allows the behavior of microtubules called dynamic instability
Keratins, neurofilaments, and vimentins are all categories of intermediate filaments. Which of the following properties below is not true of these types of intermediate filaments? (a) They strengthen cells against mechanical stress. (b) Dimers associate by noncovalent bonding to form a tetramer. (c) They are found in the cytoplasm. (d) Phosphorylation causes disassembly during every mitotic cycle.
d) phosphorylation causes disassembly during every mitotic cycle
Cell movement involves the coordination of many events in the cell. Which of the following phenomena are not required for cell motility? (a) Myosin-mediated contraction at the rear of the moving cell. (b) Integrin association with the extracellular environment. (c) Nucleation of new actin filaments. (d) Release of Ca2+ from the sarcoplasmic reticulum.
d) release of Ca2+ from the sarcoplasmic reticulum
Microtubules are important for transporting cargo in nerve cell axons, as diagrammed in Figure Q17-24. Notice that the two types of cargo are traveling in opposite directions. Which of the following statements is likely to be false? (a) The gray cargo is attached to dynein. (b) The black cargo and the gray cargo require ATP hydrolysis for their motion. (c) The black cargo moving toward the axon terminal contains a domain that specifically interacts with the tail domain of a particular kind of motor. (d) The black cargo and the gray cargo are moving along microtubules of opposite polarity.
d) the black cargo and the gray cargo are moving along microtubules of opposite polarity
Which of the following statements about actin is false? (a) ATP hydrolysis decreases actin filament stability. (b) Actin at the cell cortex helps govern the shape of the plasma membrane. (c) Actin filaments are nucleated at the side of existing actin filaments in lamellipodia. (d) The dynamic instability of actin filaments is important for cell movement.
d) the dynamic instability of actin filaments is important for cell movement
Which of the following statements regarding dynamic instability is false? (a) Each microtubule filament grows and shrinks independently of its neighbors. (b) The GTP cap helps protect a growing microtubule from depolymerization. (c) GTP hydrolysis by the tubulin dimer promotes microtubule shrinking. (d) The newly freed tubulin dimers from a shrinking microtubule can be immediately captured by growing microtubules and added to their plus end.
d) the newly freed tubular dimers from a shrinking microtubule can be immediately captured by growing microtubules and added to their plus end
Phosphorylation of nuclear lamins regulates their assembly and disassembly during mitosis. You add a drug to cells undergoing mitosis that inhibits the activity of an enzyme that dephosphorylates nuclear lamins. What do you predict will happen to these cells? Why?
the cells would become arrested in mitosis. Lamins are typically phosphorylated during mitosis which causes the nuclear envelope to disassemble. Lamins can reassemble at the end of mitosis when they are dephosphorylated. Inhibiting this doesn't allow the laming to reassemble and therefore keeps the cells in mitosis.
