CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
Exam-style questions and sample answers have been written by the authors. In examinations, the way marks are awarded
may be different.
Coursebook answers
Chapter 5
b It suggests that any change in the primary
structure would threaten survival – in
other words, the precise tertiary structure
of the histones is vital for survival. It
highlights the critical importance of
histones. Only organisms with conserved /
unchanged molecules are likely to survive.
Before you start
Structural features of the eukaryotic nucleus:
•
•
•
•
surrounded by two membranes, the nuclear
envelope
function: separates the biochemistry of
the nucleus from that of the cytoplasm /
continuous with the ER
the nuclear envelope is perforated by nuclear
pores
function: helps to control what enters and
leaves the nucleus, e.g. mRNA, ribosomes
contains the chromosomes/chromatin/
euchromatin and heterochromatin/DNA
(and histones)
function: chromatin contains the DNA
and histones; DNA is the genetic material;
histones provide a scaffolding for DNA to
allow it to be packaged without tangling
contains the nucleolus
function: the nucleolus makes ribosomes.
c
87 000 µm of DNA is packed into 10 µm
of chromosome
therefore packing ratio = 87 000 ÷ 10 =
8700
d total length of chromosomes = 46 × 6 µm =
276 µm
1.8 m = 1800 mm = 1 800 000 µm
1 800 000 µm of DNA is packed into
276 µm of chromosomes
therefore packing ratio = 1 800 000 ÷ 276 =
6522
e Histones provide a scaffolding around
which DNA molecules can coil. A coiled
strand can be packed into a smaller
volume than a straight strand.
Science in Context
Some possible issues are listed below:
•
•
•
•
•
Is it an advantage for the human race that its
individual members grow old and die?
Should the treatment be universally available?
If not, who should benefit?
If you had a choice, would you choose to
extend your natural life? What if you did and
your partner didn’t?
How long would you like to live for, assuming
you would be healthy for most of your life?
Should you be entitled to a particular
number of years of healthy life before the
treatment was withdrawn? If so, would this
create a black market for any drugs or other
treatments involved?
Self-assessment questions
1
1
8.7 cm = 87 mm = 87 000 µm
2
Microtubules can be shortened by removal of
tubulin subunits (depolymerisation).
3
Asexual reproduction involves production of
new individuals that are genetically identical
to the parent. The nuclei in the cells of these
individuals must be genetically identical.
Mitosis is the type of nuclear division
required to produce genetically identical
nuclei.
4
a
92 chromatids
b92 DNA molecules (each chromatid
contains one DNA molecule)
c
92 kinetochores
d
46 chromatids
e
92 chromatids
a The primary structure of a protein is the
amino acid sequence.
Cambridge International AS & A Level Biology – Jones, Fosbery, Taylor & Gregory © Cambridge University Press 2020
CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
5
Exam-style questions
a
The mark schemes, suggested answers and
comments that appear here were written by the
author(s). In examinations, the way marks are
awarded to answers like these may be different.
Notes about mark schemes
A or accept indicates an alternative acceptable
answer.
metaphase
R = reject. This indicates a possible answer that
should be rejected.
; The bold semicolon indicates the award of
1 mark.
b
/ This indicates an alternative answer for the same
mark. The alternatives may be separated from the
rest of the answer by commas.
( ) Text in brackets is not required for the mark.
Underlining This is used to indicate essential
word(s) that must be used to get the mark.
anaphase
6
Their function is to hold chromatids together
and to attach chromosomes to the spindle.
7
Of 75 000 cells, 9 were undergoing mitosis.
Mitosis lasts one hour. Therefore, cell cycle is
75 000 / 9 hours long
= 8333 hours
= 8333 / 24 days
= 347 days.
(Cell cycles vary in length in adult animals
from less than eight hours to more than one
year.)
8
9
2
It suggests that specialisation is not due to
different genes being present in different cells,
but that different genes are switched on in
different cells. In a liver cell, for example, only
the genes that control the activities of a liver
cell are switched on, even though liver cells
contain all the information to make any cell.
Cancer cells are immortal. They achieve this
by renewing their telomeres after each division.
To do this, telomerase is required. (A normal
cell would eventually use up its telomeres and
die if it divided repeatedly like a cancer cell.)
AW means ‘alternative wording’. It is used to
indicate that a different wording is acceptable
provided the essential meaning is the same, and is
used where students’ responses are likely to vary
more than usual.
AVP means ‘additional valid point’. This means
accept any additional points given by the student
that are not in the mark scheme, provided they
are relevant. But accept only as many additional
points as indicated by the bold semicolons, e.g.
AVP ; ; means award a maximum of 2 extra marks.
ORA means ‘or reverse argument’ and is used
when the same idea could be expressed in the
reverse way. For example: ‘activity increases
between pH2 and pH5 ORA’ means accept
‘activity decreases between pH5 and pH2’.
max. This indicates the maximum number of
marks that can be given.
1
D ;
[1]
2
B ;
[1]
3
D ; explanation: as an example, human
cells contain 46 chromosomes. At S phase
in the cell cycle, each chromosome makes
a copy of itself, forming two chromatids.
Therefore there are 92 chromatids going into
mitosis. After mitosis, each daughter nucleus
contains 46 chromatids. These are now called
chromosomes. The term chromosome can
therefore be confusing. The confusion arises
because the term ‘chromosome’ was originally
applied to the structures which appear
Cambridge International AS & A Level Biology – Jones, Fosbery, Taylor & Gregory © Cambridge University Press 2020
CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
during mitosis, each of which consists of two
chromatids (duplicate chromosomes) and the
term chromosome is now also applied to the
46 structures found in the interphase nucleus
between cell divisions.
[1]
4
B ;
[1]
5
A ;
[1]
6
centrosome:
centromere leading for each chromatid ;[3]
9
so the presence of colchicine would
interfere with this ; AW
b
makes spindle during mitosis ;
c
located just outside nucleus ;
has nine triplets of microtubules ;
10 Award 1 mark for correct statement, True or
False. No explanation is required.
centromere:
A
region of a chromosome that holds two
chromatids together ;
True ;
Centrosomes replicate during interphase,
before M phase begins.
point of attachment for microtubules during
mitosis ;
B
point of attachment of chromatids to spindle ;
True ;
[max. 6 with a max. of 3 for centrosome]
Sister chromatids are formed by the
replication of DNA. Each contains one
daughter DNA molecule identical to the
parent molecule.
A anaphase ;
C
AVP ;
B prophase ;
C metaphase ;
D
Award 1 mark for each correct summary.[3]
i
metaphase ;
False ;
This occurs during M phase during
spindle manufacture (polymerisation)
and chromatid movement
(depolymerisation).
[Total: 6]
False ;
Microtubules extend from the kinetochore
to the nearest pole. The kinetochores
in sister chromatids are connected to
opposite poles.
[3]
b Information for this answer can be found in
Figure 5.9 in the Coursebook.
[1]
ii prophase drawing shows two single
chromosomes, each with a centromere
(not paired chromatids), ‘randomly’
distributed, surrounded by cell surface
membrane but with no spindle ;
[1]
3
[3]
[Total: 7]
part of centrosome ; award this mark only if
not mentioned under centrosome
a
(held up in) prophase ;
so, metaphase / subsequent phases,
cannot occur ; no function (during mitosis) ;
8
[2]
spindle cannot form (due to presence of
colchicine) ;
centriole:
a
[2]
spindle ;
centrioles ;
contains two centrioles ;
7
a microtubules are made out of tubulin
molecules ;
the tubulin molecules stick together in a
particular pattern to form the microtubules,
a microtubule organising centre ;
[Total: 6]
E
False ;
Kinetochores are found on chromatids.
F
False ;
b a long and a short chromatid, each with
a centromere, are shown inside each
new nucleus ;
[1]
Telomeres are the caps at the ends of
chromosomes. Microtubules are attached
at the centromeres (kinetochores).
c six chromatids about half-way between
equator and each pole (12 chromatids in
all) ; two long, two short, two hooked in
each direction ;
G
True ;
Chromatids separate at the start of
anaphase.
[Total: 7]
Cambridge International AS & A Level Biology – Jones, Fosbery, Taylor & Gregory © Cambridge University Press 2020
CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
valid point) in the marking scheme is
indicative of this.
11 The following table of data is based on
data obtained from the same source as the
figure used for the question (visit https://
ourworldindata.org). The figure is interactive
on the website. The following table of data
may help teachers when marking.
Year 1990
Age
group
millions
70+
6.52
has (roughly) doubled ;
that could just be a result of
population increase ;
Year 2016
%
34.2
Age
group
70+
millions
15.67
%
could be due to people living longer ;
could be due to the fact that exposure
to risk factors is increasing / could be
due to changes in lifestyle / could be
due to increasing pollution / could
be due to increased smoking (of
tobacco) ;
37.3
50 – 69
8.73
45.8
50 – 69
19.27
45.9
15 – 49
3.36
17.6
15 – 49
6.51
15.5
under
–15s
0.46
2.4
under
–15s
0.55
1.3
Total
19.07
100
Total
41.99
100
a
the total number of cancer sufferers
has increased ;
there is no / little change in the
number of cancer cases among the
under-15s (actually gone down from
2.4% to 1.2% of total) ;
cancer is caused by mutation ;
in, genes / a gene, controlling, cell division
/ mitosis ;
would expect an increase if there were
more under-15s in 2016 than 1990 ;
a mutated gene causing cancer is called an
oncogene ;
[max. 2]
could be due to better medical
treatment of cancer in under-15s ;
b a, substance / chemical, (or environmental
factor) that can cause cancer ;
[1]
c
i
50–69 ;
there is no / little change in the actual
number of cancer cases among the
under-15s, although the % has gone
down from 2.4% to 1.3% of total
(almost halved) ;
[1]
ii it contains the greatest number of
people ;
the people have had longer to
accumulate mutations than younger
age groups ;
therefore, no new factors / no change
in factors, causing cancer (?) ;
any comment suggesting that further
information is needed to fully
interpret the trends ;
the death rate in the 70+ age group is
higher, so fewer remaining sufferers
of cancer ;
[3]
iii Questions asking students to comment
on data can elicit a great variety of
answers, so some judgement is often
needed in deciding what is a suitable
marking point. The use of AVP (any
4
any calculation of percentages ;
AVP ; ; ;
[max. 5]
[Total: 12]
Cambridge International AS & A Level Biology – Jones, Fosbery, Taylor & Gregory © Cambridge University Press 2020