Chapter 18 Cell Biology Review

You have discovered a new protein that regulates microtubule dynamics. First, you isolated proteins from a cellular extract that bound to a tubulin affinity column. You then separated the proteins from each other by loading the mixture of proteins on an ion-exchange column, eluting the column with increasing salt concentration and collecting small "fractions" of protein as they dripped from the column. To test whether each fraction contained microtubule regulators, you mixed it with fluorescent tubulin and purified centrosomes, and then analyzed the reaction microscopically to measure the size of the aster microtubules formed. You found that fractions 8, 9, and 10 promoted the formation of unusually long aster microtubules. Because electrophoretic separation of the fractions on a gel revealed a plentiful protein with an apparent molecular weight of 98 kD, you named the protein p98. A. Does p98 behave like a MAP or a catastrophin? B. Propose two ways in which p98 might change the dynamic behavior of microtubules to account for the observed change in microtubule length. Hint: There are four simple possible mechanisms. C. Video microscopy of fluorescent tubulin in reactions with purified centrosomes allowed you to follow the behavior of individual microtubules over time. You graphed the changes in microtubule length in the absence (Figure Q18-51A) and presence (Figure Q18-51B) of p98. Five representative microtubules are shown for each condition. Does p98 alter the rate of microtubule growth or shrinkage? Does p98 alter the frequency of catastrophes (a sudden and rapid decline in microtubule length) or rescues (when a microtubule switches from shrinking to growing)? Explain your answers. D. After demonstrating the consequences of p98 on microtubule dynamics in vitro with the use of purified components, you want to determine whether the protein has the same effects in a complex cellular extract that naturally contains p98. Briefly describe an experiment that will allow you to determine what p98 normally does in mitotic extracts.

Answer A. P98 will behaves like MAP. Answer B. P98 might change the dynamic behavior of microtubules which account for the observed change in microtubule length either by Assembly and Diassembly of microtubule which mainly depends on 1. Growth and Shrinkage of microtubule and 2. Also depends on nucleotide-binding site on tubulin (either by GTP or GDP). Answer C. Data not sufficient Answer D. To determine whether the protein has the same effects in a complex cellular extract that naturally contains p98, what we can exactly do is to do microinjection of Fluorescent labeled tubulin into fibroblasts and allow fluorescent labeled tubulin to incorporate into cytoplasmic microtubule. And to observe consequences of p98, view the cultured human fibroblast cells in fluorescene microscope.

Which of the following statements about apoptosis is true?

Apoptosis is promoted by the release of cytochrome c into the cytosol from mitochondria.

Match each of the main classes of spindle microtubules from list 1 with their functions and features from list 2.

A—1 B—2, 5 C—3, 4

Match the following labels to the numbered label lines on Figure Q18-15. Figure Q18-15 A. G1phase B. mitotic cyclin C. mitotic Cdk D. S phase E. S-phase cyclin F. S-phase Cdk G. MPF H. G2 phase

A. 5 B. 6 C. 3 D. 2 E. 4 F. 1 G. 7 H. 8

Name the stage of M phase in which the following events occur. Place the numbers 1-8 next to the letter headings to indicate the normal order of events. A. alignment of the chromosomes at the spindle equator B. attachment of spindle microtubules to chromosomes C. breakdown of nuclear envelope D. pinching of cell in two E. separation of two centrosomes and initiation of mitotic spindle assembly F. re-formation of the nuclear envelope G. condensation of the chromosomes H. separation of sister chromatids

A. 5, metaphase B. 4, prometaphase C. 3, prometaphase D. 8, cytokinesis E. 2, prophase F. 7, telophase G. 1, prophase H. 6, anaphase

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

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?

Components of an interphase nucleus are insufficient to cause mitotic spindle formation

Which of the following statements about the cell cycle is false?

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

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?

Programmed cell death, so that cells fit into the normal size and amount that they need to

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

Which of the following statements is false?

The activating phosphatase (Cdc25) removes all phosphates from mitotic Cdk so that M-Cdk will be active.

Which of the following is not good direct evidence that the cell-cycle control system is conserved through billions of years of divergent evolution?

Yeast cells have only one Cdk, whereas humans have many Cdks. The Cell-Cycle Control System

When introduced into mitotic cells, which of the following is expected to impair anaphase B but not anaphase A?

an antibody against the motor proteins that move from the minus end of microtubules toward the plus end

Condensins

assemble into complexes on the DNA when phosphorylated by M-Cdk

Which of the following precede the re-formation of the nuclear envelope during M phase in animal cells?

assembly of the contractile ring

Programmed cell death occurs

by means of an intracellular suicide program

Apoptosis differs from necrosis in that necrosis

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

The principal microtubule organizing center in animal cells is the

centrosome

At the end of DNA replication, the sister chromatids are held together by the

cohesins

Levels of Cdk activity change during the cell cycle, in part, because...

cyclin levels change during the cycle

A cell with nuclear lamins that cannot be phosphorylated in M phase will be unable to

disassemble its nuclear lamina at prometaphase

The concentration of mitotic cyclin (M cyclin)

falls toward the end of M phase as a result of ubiquitylation and degradation

Which of the following does not occur during M phase in animal cells?

growth of the cell

A mutant yeast strain stops proliferating when shifted from 25°C to 37°C. When these cells are analyzed at the two different temperatures, using a machine that sorts cells according to the amount of DNA they contain, the graphs in Figure Q18-3 are obtained. Which of the following would not explain the results with the mutant?

inability to begin M phase

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?

inactivation of an enzyme that ubiquitylates M cyclin

The G1DNA damage checkpoint

involves the degradation of p53

Disassembly of the nuclear envelope

must occur for kinetochore microtubules to form in animal cells

Cytokinesis in animal cells

requires ATP

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?

the jiggling movement of chromosomes at the metaphase plate

Of the following mutations, which are likely to cause cell-cycle arrest? If you predict a cell-cycle arrest, indicate whether the cell will arrest in early G1, late G1, or G2. Explain your answers. 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

A. A mutation in a gene encoding a cell-surface mitogen receptor that makes the receptor active even in the absence of the mitogen, in this condition cell cycle arrest do not takes place. This is because the cell becomes cancerous and undergoes uncontrolled divisions. B.A mutation that destroyed the kinase activity of S-Cdk, which results cell cycle arrest at G2 phase this is because the S-Cdk are used in the synthesis phase. If the S phase would not takes place then G1 would be inhibited. C. A mutation that allowed G1-Cdk to be active independently of its phosphorylation status, it results cell cycle arrest at early G1 phase D. A mutation that removed the phosphorylation sites on the Rb protein, cell cycle arrest at G2 phase.

Before chromosomes segregate in M phase, they and the segregation machinery must be appropriately prepared. Indicate whether the following statements are true or false. If false, change a single noun to make the statement true. A. Sister chromatids are held together by condensins from the time they arise by DNA replication until the time they separate at anaphase. B. Cohesins are required to make the chromosomes more compact and thus to prevent tangling between different chromosomes. C. The mitotic spindle is composed of actin filaments and myosin filaments. D. Microtubule-dependent motor proteins and microtubule polymerization and depolymerization are mainly responsible for the organized movements of chromosomes during mitosis. E. The centromere nucleates a radial array of microtubules called an aster, and its duplication is triggered by S-Cdk. F. Each centrosome contains a pair of centrioles and hundreds of -tubulin γ rings that nucleate the growth of microtubules.

A. False. Sister chromatids are held together by cohesins from the time they arise by DNA replication until the time they separate at anaphase. B. False. Condensins are required to make the chromosomes more compact and thus to prevent tangling between different chromosomes. C. False. The contractile ring is composed of actin filaments and myosin filaments. D. True. E. False. The centrosome nucleates a radial array of microtubules called an aster, and its duplication is triggered by S-Cdk F. True

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 B. centromeres C. kinetochores E. centrioles

The number of cells in an adult tissue or animal depends on cell proliferation. What else does it depend on?

Both cell lines would undergo apoptosis if cytochrome c was injected into the cytosol of both wild-type cells and cells that were doubly defective for Bax and Bak. Bax and Bak are both Bcl-2 family proteins that promote apoptosis. The injected cytochrome c binds to an adapter protein which activates a procaspase. When it binds, the initiation of the caspase cascade which leads to apoptosis occurs.

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.

Motor proteins walking along the cytoskeleton are important for the contractile ring that guides formation of the new cell wall.

Which organelle fragments during mitosis?

Golgi apparatus

Which of the following statements is true?

In anaphase B, microtubules associated with the cell cortex shorten.

Before mitosis, the number of centrosomes must [increase/decrease]. At the beginning of [anaphase/prophase] in animal cells, the centrosomes separate in a process driven partly by interactions between the [plus/minus] ends of microtubules arising from the two centrosomes. Centrosome separation initiates the assembly of the bipolar mitotic spindle and is associated with a sudden [increase/decrease] in the dynamic instability of microtubules. In comparison with an interphase microtubule array, a mitotic aster contains a [smaller/larger] number of [longer/shorter] microtubules. Extracts from M-phase cells exhibit [increased/decreased/unchanged] rates of microtubule polymerization and increased frequencies of microtubule [shrinkage/growth]. The changes in microtubule dynamics are largely due to [enhanced/reduced] activity of microtubule- associated proteins and [increased/decreased] activity of catastrophins. The new balance between polymerization and depolymerization of microtubules is necessary for the mitotic spindle to move the [replicated chromosomes/daughter chromosomes] to the metaphase plate.

Increase Prophase Plus Increase Larger Shorter Unchanged Reduced Increased

Which of the following statements about the anaphase-promoting complex (APC) is false?

It is continuously active throughout the cell cycle.

Which of the following statements about kinetochores is true?

Kinetochores assemble onto chromosomes during late prophase.

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?

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

What would be the most obvious outcome of repeated cell cycles consisting of S phase and M phase only?

The cells produced would get smaller and smaller.

Which of the following statements is false?

The cleavage furrow always forms in the middle of the cell.

The four phases of the cell cycle, in order, are G, _______, ______________, and _______________. A cell contains the most DNA after _______________ phase of the cell cycle. A cell is smallest in size after ______________ phase of the cell cycle. Growth occurs in _______________, ________________, and _______________ phases of the cell cycle. A cell does not enter mitosis until it has completed _____________ synthesis. DNA M protein G1 nucleotide S G2 organelle

The four phases of the cell cycle, in order, are G, ___S____, ______G2________, and ________M_______. 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_______ synthesis.

Which of the following statements is false?

The loading of the origin recognition complexes (ORCs) is triggered by S-Cdk.j

Which of the following statements is true?

The mitotic spindle helps segregate the chromosomes to the two daughter cells.

Which of the following events does not usually occur during interphase?

The nuclear envelope breaks down.