5 The cell cycle Answers to Practice questions

1 a) B, D, A, C [1]

b) C, B, E, A, D [1]

c) i) 0.37 [1]

ii) 666 minutes [2]

iii) 96 minutes [2]

2 a) i) After 10 minutes, because movement stopped. [2]

ii) Anaphase starts at 100 minutes, at a distance of 20 μm. After a further 100 minutes,

the distance has decreased to 4 μm. So the mean rate of movement is 16/100 which is

0.16 μm min–1 [1]

iii) Extend the curve beyond 200 minutes until it intercepts with the axis. It intercepts at

250 minutes. A tangent drawn to the curve between 200 and 250 minutes back to the

distance axis gives a distance moved (AB) of between 12 and 14 μm. Divided by a total

time of 250 minutes (BC) the rate of movement is AB/BC which is between 0.05 and

0.06 μm min-1 [2]

b) i) Vincristine would inhibit anaphase, because the spindle fibres are used to move the

chromatids to the poles of the cell. [2]

ii) Hair growth and blood-cell replacement both depend on cell division. Treatment with

vincristine prevents mitosis in tissues other than the tumour being treated. [2]

3 a) DNA replication [1]

b) S phase [1]

c) Samples 3 and 4 [1]

d) The daughter cells resulting from budding are smaller. Binary fission gives two equally sized

cells. [1]

e) In mitosis, the nuclear envelope breaks down and spindle fibres form. There are no plasmids

to replicate. In binary fission, both the circular DNA molecule and any plasmids in the cell

undergo DNA replication. Unlike mitosis, there is no nuclear envelope to break down and

there are no spindle fibres. [3]

4 a) i) Your diagram should be the same as the third tube but with the top band thicker. [2]

ii) After one division of the bacteria on 14N medium, their DNA is a mixture of 14N and 15N

because the band of extracted DNA settles at a higher point in the tube since it is less

dense. However, it is not as high as DNA that contains just 14N. Its density is

intermediate between all 14N DNA and all 15N DNA which suggests that after just one

division, half the DNA still contains 15N.

5 The cell cycle Answers to Practice questions

b) i) Two bands, top and bottom positions, same thickness [3]

ii) Two bands, top and bottom positions, both thicker [2]