Cell Cycle and Regulation Questions and Answers

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.

Answer: A

Twenty-three of the twenty-four hours are interphase, which is a little over 95 per cent. The impression that cells are usually dividing comes from mitosis being so much more visible than interphase. A dividing cell is conspicuous, while an interphase nucleus looks quiet.

Answer: B

Past this point the cell no longer depends on external growth signals and will finish the cycle. It is the principal decision point, and its control is lost in most cancers. Yeast geneticists called the equivalent point START when they first found it.

Answer: C

The kinase is present at a steady level but inactive on its own, so activity follows the rise and fall of its partner cyclin. This is how a constant enzyme produces a cyclical output. Regulating the partner rather than the enzyme is a recurring trick in biology.

Answer: D

It is the cyclins that oscillate, being synthesised and then destroyed, and their partner kinases follow. Naming the kinases after their dependence on cyclin records exactly this relationship. Hartwell, Hunt and Nurse shared a Nobel Prize for working it out.

Answer: A

Both are true and the reason gives the mechanism: the oscillation is imposed by the regulatory subunit rather than by the catalytic one, which is an economical way to produce a repeating cycle. Only the cyclin has to be made and destroyed each time round.

Answer: B

Ubiquitin is attached to mark them for destruction, which makes the fall in concentration abrupt and irreversible. Irreversibility is what stops the cycle running backwards. Destroying the regulator rather than merely inhibiting it makes each step final.