Cell Cycle and Regulation Questions and Answers

Answer: C

The order is G1, S, G2, M.

After division a cell enters G1, the first gap phase, in which it grows and prepares to copy its DNA. In the S (synthesis) phase the DNA is replicated. G2, the second gap, follows, as the cell checks the replicated DNA and prepares for division, and then M, mitosis and cytokinesis, completes the cycle.

Every other option breaks the logic of the cycle: DNA cannot be synthesised before the first gap (S, G1, ...), the two gap phases are separated by S rather than adjacent (M, G1, G2, S), and mitosis cannot come before replication (G2, M, S, G1).

Answer: B

The APC regulates separation of sister chromatids and exit from mitosis.

The anaphase-promoting complex (cyclosome) is a ubiquitin ligase. Activated by Cdc20, it tags securin for destruction, freeing separase to cleave cohesin so that sister chromatids separate at anaphase. It also tags mitotic cyclins (cyclin B) for destruction, and the resulting fall in Cdk1 activity lets the cell leave mitosis and divide.

'Sister chromatid separation only' leaves out the second role. Polar microtubule formation depends on spindle assembly, and the actin contractile ring of cytokinesis is organised by RhoA signalling, not directly by the APC.

Answer: D

Carcinoma is a malignant growth of epithelial tissues.

Cancers are named by their tissue of origin. Carcinomas arise from epithelia, the cells lining surfaces and glands, and are the commonest cancers (skin, lung, breast, colon).

Malignant tumours of bone and muscle are sarcomas, which arise from connective and supporting tissues, and cancers of bone marrow blood-forming cells are leukaemias.

Answer: A

The Z ring is important for septum formation.

FtsZ, a tubulin-like GTPase, polymerises into a ring at the future division site on the inner face of the cytoplasmic membrane. This Z ring is the scaffold of the divisome: it recruits the other division proteins, including those that make the septal peptidoglycan, and its constriction draws in the membrane and wall to form the septum that divides the cell in two.

Pili are made by secretion systems at the cell surface, attachment uses pili and adhesins, and movement uses flagella. None depends on the Z ring.

Answer: A

The kinase that inhibits CDK at the G2 checkpoint is Wee 1.

Wee1 phosphorylates Cdk1 (Cdc2) on tyrosine 15, which keeps the cyclin B-Cdk1 complex inactive and stops the cell entering mitosis. When DNA replication is incomplete or DNA is damaged, the checkpoint keeps Wee1 active; Wee1 mutants of fission yeast enter mitosis too early and are small, which is how the gene got its name.

Cdc25 is the opposite: a phosphatase that removes the inhibitory phosphate and activates CDK. Cdc20 activates the anaphase-promoting complex at the spindle checkpoint. RB acts at the G1/S transition by binding E2F, and it is a substrate of CDKs rather than a kinase that phosphorylates them.

Answer: A

The APC is activated by CDC20.

At the metaphase to anaphase transition, once every kinetochore is attached and the spindle checkpoint is satisfied, Cdc20 binds and activates the anaphase-promoting complex. APC/C-Cdc20 ubiquitinates securin and cyclin B, freeing separase to cut cohesin so that sister chromatids separate.

Cdc25 is the phosphatase that activates CDK1 at the G2 to M transition. Cdc18 (the fission yeast relative of Cdc6) helps load the replication machinery for S phase. Cdc13 is the B-type cyclin of fission yeast. None of them activates the APC.

Answer: A

The two gap phases separate DNA synthesis from division and give the cell time to grow and to check its work. Interphase covers the first three, with mitosis occupying only a small part of the cycle. A cell spends most of its life growing rather than dividing.

Answer: B

The S stands for synthesis, and by the end of it every chromosome consists of two identical sister chromatids. The gap phases on either side are periods of growth and preparation instead. Each chromosome now has two chromatids joined at the centromere.

Answer: C

Most cells in an adult body are in this state, some able to re-enter the cycle when signalled and others permanently so. Neurons and cardiac muscle cells are the classic examples of the permanent case. Damage to those tissues is therefore repaired poorly if at all.

Answer: D

Many cells in G0, such as liver cells, re-enter the cycle when the tissue is injured or a growth signal arrives. Only some, notably neurons and cardiac muscle, are in a genuinely permanent state. Liver regeneration after injury is the standard demonstration of re-entry.