Exam 17: The Cell Cycle

Three models for contractile-ring positioning in animal cells are presented in the schematic diagrams below. Answer the following question(s) according to these models. -In experiments on the flattened eggs of the sea urchin Tripneustes gratilla, cytokinesis was analyzed after placing a small physical barrier (e.g. a glass needle or an oil droplet) between the spindle and various areas of the cell cortex. Blocks that were located over the equatorial plane prevented furrow formation in the proximal membrane, whereas blocks located in other areas did not affect furrowing. Which model (1 to 3) is NOT supported by these results? Write down 1, 2, or 3 as your answer.

How is centrosome duplication similar to DNA replication?

In the following schematic diagram of a typical eukaryotic cell cycle, choose two major time points (among A to E) at which the cell-cycle control system normally arrests the cycle if DNA damage is detected. Your answer would be a two-letter string composed of letters A to E only in alphabetical order, e.g. CE.

Indicate whether each of the following descriptions better applies to mitogens (M), growth factors (G), or survival factors (S). Your answer would be a four-letter string composed of letters M, G, or S only, e.g. GMSG. ( ) They unblock cell-cycle progression ( ) They suppress apoptosis ( ) They trigger a wave of G₁/S-Cdk activity ( ) They inhibit the degradation of cellular macromolecules

Sort the following schematic diagrams (A to F) to reflect the order of events in a typical eukaryotic M phase. An interphase cell in G₂phase is drawn on the left for comparison. Your answer would be a six-letter string composed of letters A to F only, e.g. BCEADF.

Which stage is usually the longest in meiosis?

You have been studying the effect of loss-of-function mutations in the Cdk inhibitor protein (CKI) p21. You add the drug fucoxanthin to cell cultures harboring either wild-type or mutant versions of the p21 gene. Fucoxanthin is known to induce cell-cycle arrest in G1. After a day, you add the thymidine analog BrdU to the culture media, collect the cells after an hour, treat them with anti-BrdU antibody and the fluorescent DNA stain DAPI, and finally subject them to fluorescence-activated cell sorting (FACS). The FACS data can be viewed as a two-dimensional dot plot composed of thousands of dots, in which each cell is represented by one dot at coordinates that correspond to the intensities of the DAPI fluorescence signal (X axis) and BrdU fluorescence signal (Y axis) for that cell. Answer the following question(s) according to the simplified dot plot below, generated from your experiment. -Which one of the FACS plots (a or b) would you expect to correspond to the loss-of-function p21 mutants? Write down a or b as your answer.

Consider two mammalian cells, one in G₁ and the other in G₀ (stationary) phase. If they are stimulated to pass the restriction point by the addition of an extracellular proliferation signal, but the signal is then immediately removed, what would you expect to happen?

Loss of the gene encoding shugoshin in many multicellular organisms leads to sterility, suggesting defects in meiosis. What would you expect to occur in meiotic cells lacking shugoshin?

You have synchronized a culture of HeLa cells so that the cells are all at the same stage in the cell cycle. You then treat the cells with serum containing nutrients and growth factors in the presence or absence of the drug rapamycin. The distribution of cell sizes in the two cell populations is shown below. Based on these results, do you think rapamycin is an activator or inhibitor of the TOR complex? Is the S6 kinase expected to be up-regulated or down-regulated in the presence of rapamycin?

Which one of the following chromosomes has formed stable attachment(s) to the spindle microtubules? A) B) C) D) E)

Which of the following events occurs at the onset of S phase in a mitogen-stimulated vertebrate cell?

The following schematic diagram shows the activation of M-Cdk in mitosis. Indicate which proteins below correspond to those indicated as A to D in the diagram. Your answer would be a four-letter string composed of letters A to D only, e.g. BACD. ( ) Wee1 ( ) CAK ( ) Cdc25 ( ) M-cyclin

In the following schematic diagram of an early Drosophila embryo, which step (A to C) corresponds to cellularization? Write down A, B, or C as your answer.

In the fission yeast Schizosaccharomyces pombe, the proteins Cut1 and Cut2 form a complex that is catalytically inactive. At the onset of anaphase, Cut2 is polyubiquitylated by a large E3 complex containing Cut4, Cut9, Cut23, and other proteins, and is subsequently destroyed. Cut1 then cleaves Rad21, a non-SMC component of a complex that also contains two SMC proteins, thus allowing sister-chromatid separation. Mutations in the cut genes lead to the cut phenotype, in which the cell attempts cytokinesis without chromosome segregation. Indicate which of the following proteins or protein complexes corresponds to or contains the product of the genes cut1 (A), cut2 (B), cut4 (C), and rad21 (D). Your answer would be a four-letter string composed of letters A to D only, e.g. DCAB. ( ) Securin ( ) Cohesin ( ) Anaphase-promoting complex ( ) Separase

Indicate true (T) and false (F) statements below regarding cytokinesis in eukaryotic cells. Your answer would be a four-letter string composed of letters T and F only, e.g. TFFF. ( ) The preprophase band, composed of microtubules and actin filaments, marks the site of cytokinesis in plant cells. ( ) In some cell types, the site of contractile-ring formation is determined before mitosis. ( ) The early cell plate in dividing plant cells appears after phragmoplast formation. ( ) The membrane required for the newly formed cell plate in plant cells is provided by endocytosis from the equatorial plasma membrane.

Imagine a prometaphase chromosome pair that is attached to one spindle pole. Which of the following would happen if both arms of the chromosome are severed with a strong laser beam?

Indicate whether each of the following, when active, tends to activate (A) or inactivate (I) M-Cdks. Your answer would be a four-letter string composed of letters A and I only, e.g. IIAI. ( ) M-Cdk ( ) Cdc20-APC/C ( ) Cdh1-APC/C ( ) Sic1

Which of the following events contributes to driving the mammalian cell past the restriction point of the cell cycle?

Which of the following motor proteins are more directly involved in anaphase B?