Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
c
Diagram 3 shows chromosomes in the nucleus of an animal cell.
3
Draw a diagram to show what the nucleus would look like in anaphase of mitosis. [3]
[Total: 6]
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7 In Chapter 1 it was noted that microtubules are tiny tubes made of protein subunits which join together
and the protein is called tubulin. Colchicine is a natural chemical which binds to tubulin molecules, thus
preventing the formation of microtubules.
a Why should the binding of colchicine to tubulin molecules interfere with the formation of microtubules? [2]
b What structure or structures involved in mitosis are made of microtubules? [2]
c When cells treated with colchicine are observed, the dividing cells are all seen to be in the same stage
of mitosis. Suggest with reasons the identity of this stage. [3]
[Total: 7]
8 If a 10 cm long piece of string was packed into a 5 cm long tube, the packing ratio would be 2
(2 cm of string per cm of tube). The packing ratio is a useful measure of the degree of
compactness achieved.
The same idea can be applied to the problem of packing DNA into chromosomes.
a A chromosome 10 μm long was found to contain 8.7 cm of DNA. What is the packing ratio of
DNA in this chromosome? Show your working. [3]
b There are 46 chromosomes in an adult human cell. Their average length is about 6 μm. The total
length of DNA in the 46 chromosomes is 1.8 m. What is the overall packing ratio for DNA in human
chromosomes? Show your working. [4]
c A nucleosome packages 50 nm of DNA into a diameter of 11 nm. What is the packing ratio of
DNA in nucleosomes? [1]
d The linker DNA between nucleosomes is 53 base pairs long. Each base pair occupies a 0.34 nm
length of the DNA double helix. Nucleosomes plus their linker DNA form a 10 nm wide fibre
(the ‘beads-on-a string’ form of chromatin). What is the packing ratio of DNA in this form of
chromatin? Show your working. [5]
e Nucleosomes and their linker DNA (the beads-on-a-string form of chromatin) are coiled into a
helix to produce a 30 nm wide fibre called a solenoid. This has about six nucleosomes per turn of
the helix. 40000 nm of DNA are packed into a 1 μm length of this solenoid. What is the packing
ratio of 30 nm fibres? [1]
f Each nucleosome is made of eight histone protein molecules. These molecules are highly conserved
during evolution, meaning there are extremely few changes in their amino acid sequences over time
(far fewer than is usual). What does this suggest about their functioning? [2]