Question 1

What are the biochemical activities of Wee1 and Cdc25 proteins? How do these activities regulate MPF?

Accepted Answer

Wee1 is a kinase that phosphorylates Cdc2 on Tyr-15 and inhibits its activity.Cdc25 is a phosphatase that removes this phosphate and re-establishes activity in the presence of a phosphate group on Thr-161 in Cdc2.

Question 2

Exit from mitosis depends upon the degradation of:

Accepted Answer

Exit from mitosis is triggered by the degradation of cyclin B, which is accomplished by the anaphase-promoting complex (APC). This degradation activates a protein called separase, which ultimately cleaves cohesion and allows the two sister chromatids to separate. Condensin helps to condense chromosomes during mitosis, but its degradation is not necessary for mitotic exit. Securin inhibits separase, so its degradation actually promotes entry into anaphase rather than exit from mitosis.

Question 3

Cyclin D/Cdk4 functions during:

Accepted Answer

Cyclin D/Cdk4 functions during G₁ phase of the cell cycle by promoting the transition from G₁ to S phase.

Question 4

Injection of an immature Xenopus oocyte with MPF induces:

Accepted Answer

MPF or maturation promoting factor is a key regulator of meiosis and oocyte maturation in Xenopus oocytes. Injection of an immature oocyte with MPF triggers both meiosis and oocyte maturation. Therefore, option D is the correct answer. Synthesis of progesterone is not directly induced by MPF.

Question 5

Investigators found that the cyclin B concentration rises and falls in synchrony with mitotic events and MPF activity.However,correlation is not causality.How did they show that cyclin B was responsible for these cell-cycle events?

Accepted Answer

The answer of Investigators found that the cyclin B concentration...

Question 6

Which of the following is a target of the APC?

Accepted Answer

Cyclin B is targeted for degradation by the Anaphase Promoting Complex (APC), which is crucial for the exit from mitosis and the initiation of anaphase by allowing sister chromatid separation.

Question 7

During which stage of the cell cycle is the chromosome content of a mammalian liver cell 1n?

Accepted Answer

The chromosome content of a mammalian liver cell is 1n (haploid) at none of the stages listed because liver cells are somatic cells with a chromosome content of 2n (diploid) throughout the cell cycle.

Question 8

Site-directed mutagenesis of Tyr-15 to Phe in cdc2:

Accepted Answer

The substitution of tyrosine with phenylalanine at position 15 in cdc2 confers a wee phenotype, characterized by early entry into mitosis, shortened G2 phase, and small cell size. This mutation prevents tyrosine dephosphorylation of cdc2, which is necessary for its activation by cyclin-dependent kinase-activating kinase (CAK). Therefore, cdc2 remains active throughout the cell cycle, driving cells into mitosis prematurely. This phenotype is similar to that caused by mutations in cdc25, a phosphatase that dephosphorylates cdc2 on tyrosine 15.

Question 9

Mitotic cyclin levels in early Xenopus embryos are regulated by:

Accepted Answer

Mitotic cyclin levels in early Xenopus embryos are regulated by degradation through the Anaphase Promoting Complex (APC), which targets cyclins for ubiquitination and subsequent proteasomal degradation.

Question 10

What is START? How does nutritional status determine whether S.cerevisiae cells will pass START?

Accepted Answer

The answer of What is START? How does nutritional status...

Question 11

Human genes/proteins that regulate the cell cycle are most easily isolated by:

Accepted Answer

Complementation of yeast mutants is the most effective way to isolate human genes/proteins that regulate the cell cycle. This is because yeast is a simple eukaryotic organism and their cell cycle mechanisms are highly conserved with humans. Therefore, when a human gene/protein replaces a mutated yeast gene/protein, it can restore the normal cell cycle function, providing a powerful tool for gene/protein identification.

Question 12

Which of the following S.pombe mutants is(are)larger than normal?

Accepted Answer

Cdc25− mutants are larger than normal because the loss of cdc25 function prevents the activation of cdc2, delaying the cell cycle and causing cells to grow larger before division.

Question 13

During G₁,prereplication complexes:

Accepted Answer

During G1 phase, prereplication complexes bind to the origin recognition complex (ORC), which is responsible for initiating DNA replication. Phosphorylation of the complex by B-type cyclin-CDKs and DDK occurs later in the cell cycle during S phase, after the formation of the ORC has already occurred. Therefore, option C is the best choice.

Question 14

What is the difference between a kinetochore and a centromere?

Accepted Answer

The answer of What is the difference between a kinetochore...

Question 15

Which of the following is targeted by the SCF complex?

Accepted Answer

The SCF complex targets Sic1 for ubiquitination and subsequent degradation, allowing the cell cycle to progress from G1 to S phase.

Question 16

Which of the following is a substrate of Cln3-Cdc28?

Accepted Answer

Cln3-Cdc28 is a cyclin-dependent kinase complex responsible for initiating the G1 to S phase transition in the cell cycle. SBF (Swi4-Swi6) is a transcription factor that regulates genes involved in DNA replication and cell division. It is a substrate of Cln3-Cdc28 and is phosphorylated by the complex to activate the expression of genes needed for the G1 to S phase transition. SPF, MPF, and MBF are not substrates of Cln3-Cdc28. SPF stands for the Spindle Pole Body function, MPF (Maturation Promoting Factor) is a complex involved in the G2 to M phase transition, and MBF (MluI cell cycle Box Binding Factor) is a transcription factor that also regulates genes involved in DNA replication and cell division, but is regulated by other cyclin-dependent kinase complexes.

Question 17

All the following statements about destruction of cyclins are true,except:

Accepted Answer

Ubiquitin targets cyclins for destruction, not Cdks.

Question 18

What is a cdc mutant? What protein is encoded by cdc2 in fission yeast?

Accepted Answer

The answer of What is a cdc mutant? What protein...

Question 19

The S.cerevisiae homolog of Xenopus MPF is:

Accepted Answer

The Xenopus MPF is a complex of cyclin B and Cdc2. In S. cerevisiae, the homologs of cyclin B are the Clb (cyclin of budding yeast) proteins and the homolog of Cdc2 is Cdc28. Therefore, the S. cerevisiae homolog of Xenopus MPF is Clb1-Cdc28 (Choice C).

Question 20

Name three activities of G₁ cyclin-CDKs that ensure progression through the cell cycle.

Accepted Answer

The answer of Name three activities of G₁ cyclin-CDKs that...

Quiz 18: Regulating the Eukaryotic Cell Cycle