Biology
Duration of the external examination: 2 hours.
Number of questions: 10.
Maximum mark for the external examination: 100.
All questions are open type questions.
Sample questions of the external examination on Biology:
Sample Question 1
Table 1.1 shows the relative amounts of the bases Adenine, Thymine, Guanine and Cytosine in DNA
from different organisms.
Source
Bacterium
Maize
Fruit fly
Chicken
Human
Adenine
23.8
26.8
30.7
28.0
29.3
Thymine
23.1
27.2
29.5
28.4
30.0
Table 1.1
Guanine
26.8
22.8
19.6
22.0
20.7
Cytosine
26.3
23.2
20.2
21.6
20.0
(a) Explain the importance of the ratios of A to T and G to C to the structure of DNA.
[3]
(b) In the space below, draw and label fully, a short length of a DNA molecule.
[4]
The bacteriophage virus Φ X-174 has single stranded DNA with the four bases present in the
following relative amounts.
Adenine
Thymine
Guanine
Cytosine
24.0
31.2
23.3
21.5
c) Suggest why the ratios of A to T and G to C for the virus do not correspond to the ratios found in
living organisms.
[1]
[Total: 8]
Question 1
(a)
(b)
Answer
1:1 / equal amounts of A and T and G and C ;
complementary / (base) pairing / A with T and G with C;
occupy central position in molecule ;
ref. copying of strands / replication ;
ref. H bonding maintains particular pairing / holds two strands together ;
ref. to mutation when pairing ratio is lost ;
nucleotides ; }
deoxyribose ; }
-1 if only one strand shown
phosphate ;
}
phosphodiester link / bond ;
ref. to strands antiparallel / indicate on diagram as
3';
Marks
3 max
4 max
H bonds between bases labelled ;
(c)
Ref. to no base pairing;
1
[Total: 8]
Sample Question 2
Fig. 2.1 outlines the way in which the gene for human insulin is incorporated into plasmid DNA and
inserted into a bacterium.
Fig 2.1
(a) Describe how the plasmid DNA is cut.
[3]
(b) Explain how the human insulin gene is joined to the plasmid DNA.
[3]
(c) List two advantages of treating diabetics with human insulin produced by genetic engineering.
1.
2.
[2]
[Total: 8]
Question 2
(a)
(b)
(c)
Answer
restriction endonuclease ;
specific site / sequence of bases ;
palindrome ;
hydrolysis of phosphodiester bond ;
sticky ends ;
DNA ligase ;
sticky ends ;
complementary base pairing ;
condensation forming phosphodiester bond ;
more reliable / continuous supply ;
less risk of contamination ;
Marks
3 max
3 max
2
[Total: 8]
Sample Question 3
Fig.3.1 shows the relationship between time, temperature and amount of product formed in an
enzyme-catalysed reaction.
In the experiment, the samples were incubated at different temperatures for periods of 1, 2 and 5
hours.
The quantities of products formed were then determined.
Fig 3.1
(a) Describe and explain the effect of increasing the incubation temperature on the quantity of product
formed, as shown by the curve for 5 hours in Fig 3.1.
[5]
(b) Explain why the optimum temperature is higher if the quantity of product formed is measured after 1
hour rather than after 5 hours.
[1]
[Total: 6]
Question 3
(a)
(b)
Answer
initial increase with increase in temp. ;
increase doubles for each 10 oC rise / Q10 of 2 ;
due to increase kinetic energy / collisions ;
enzymes are proteins ;
above optimum 35 oC falls ;
ref. denaturation ;
higher the temperature the more rapid the denaturation ;
so the more rapid the fall ;
enzyme has less time to be denatured / less enzyme is denatured;
Marks
5 max
1
[Total: 6]
Sample Question 4
Citrullus colocynthis, which is related to the water melon, grows under semi-desert conditions.
Fig.4.1 shows the variation in air temperature during the day together with variation in the temperature of
an intact leaf (A) and a leaf (B) cut off from the plant at 12:30.
No further transpiration is possible once the leaf (B) has been cut off from the plant.
Leaf temperatures above 46 oC cause heat injury.
(a) Describe and explain the extent and significance of the cooling effect of transpiration on leaves using
the data shown in Fig.4.1.
[4]
(b) Suggest two factors which enable Citrullus to survive when air temperatures are high.
1
2
[2]
[Total: 6]
Question 4
(a)
(b)
Answer
cooled to lower than air temperature ;
5 to 13 oC below ;
if unable to transpire leaf temperature rise ;
and heat injury / death of leaf could occur ;
8 to 10 oC above air temperature / 20 oC above intact leaf
high transpiration rate / evaporative cooling / water vapour loss via stomata ;
sufficient water available to maintain such rates ;
tolerance of temperatures above 35 oC / up to 45 oC ;
Marks
4 max
2 max
[Total: 6]