Genetic Diversity via Mutation QP.pdf

Question 1

Q1.(a) A mutation can lead to the production of a non-functional enzyme. Explain how. (6)

Answer 1

1. Change / mutation in base / nucleotide sequence (of DNA / gene);
2. Change in amino acid sequence / primary structure (of enzyme);
3. Change in hydrogen / ionic / disulfide bonds;
4. Change in the tertiary structure / shape;
5. Change in active site;
6. Substrate not complementary / cannot bind (to enzyme / active site) / no
   enzyme-substrate complexes form.
   Accept: no E S complexes form

Question 2

Scientists investigated the effect of a specific antibiotic on two strains of the same
species of bacterium.
• One strain, SR, shows a stringent response in the presence of this antibiotic.
Part of this response involves stopping cell division. This gives this strain a
greater resistance to the effects of this antibiotic.
• The other strain, non-SR, cannot carry out a stringent response.
The scientists grew cultures of the SR strain and the non-SR strain containing the
same number of bacterial cells. They then stopped each strain from dividing and
exposed them to different concentrations of the antibiotic. After a fixed time, the
scientists estimated the number of living bacteria remaining in the cultures. Figure 1 shows their results.

b) Describe differences in the effect of increasing the concentration of antibiotic on the
SR strain and the non-SR strain. (2)

Answer 2

1. Non-SR strain falls more / SR strain falls less / up to 10(μg / cm−3);
   Must include 10 but only required once in either MP1 or MP2
   Ignore: units or absence of
   This must be a comparative statement
2. Above 10(μg / cm−3), SR strain levels out / off and non-SR strain
   continues to decrease;
3. Greater difference between strains with increasing concentration of
   antibiotic.

Question 3

(c) One way in which the stringent response gives resistance to this antibiotic is by
stopping cell division.
The scientists concluded that stopping cell division is not the only way in which the
stringent response gives resistance to this antibiotic.
Explain how Figure 1 supports this conclusion. (2)

Answer 3

1. Division stopped (of both strains by scientist);
   Reject: references to mitosis stopping
2. SR strain still more resistant / fewer die / none die (at higher
   concentrations of antibiotic).
   Accept: SR strain and non-SR strain would be similar if

Question 4

(d) The stringent response involves a number of enzyme-catalysed reactions.
Explain how scientists could use this knowledge to design drugs that make the
treatment of infections caused by the SR strain more successful. (2)

Answer 4

1. Make a competitive / non-competitive inhibitor;
   Mark in pairs
   either MP1 and MP2 OR MP3 and MP4
2. Competitive competes with / blocks active site / non-competitive inhibitor
   affects / changes active site;
   Do not mix and match
   OR
3. (Make a drug) that inhibits / denatures / destroys enzyme / stringent
   response;
   Accept: drug that ‘knocks out’ / destroys enzyme
4. Give at the same time as / before an antibiotic.

Question 5

The antibiotic damages the bacterium by causing the production of substances

called free radicals.
The scientists exposed the SR strain and the non-SR strain to the antibiotic. They
then measured the amounts of free radicals and an enzyme called catalase in both
strains.

(e) Use the information provided and Figure 2 to suggest an explanation for the greater
resistance of the SR strain to this antibiotic. (3)

Answer 5

1. Fewer free radicals (than non-SR);
   Note: has to be comparative statement
2. Produces more catalase (than non-SR);
   Accept converse statements for non-SR.
3. Catalase (might be) linked to production of fewer free radicals / breaking
   down / removing free radicals.
   Accept: hydrolysis of radicals by catalase.

Question 6

Q2.The figure below summarises the process of meiosis. The circles represent cells and the
structures within each cell represent chromosomes.

(a) Describe and explain the appearance of one of the chromosomes in cell X. (3)

Answer 6

1. Chromosome is formed of two chromatids;
2. (Because) DNA replication (has occurred);
3. (Sister) chromatids held together by centromere.

Question 7

(b) Describe what has happened during division 1 in the figure above.
(2)

Answer 7

1. Chromosomes in homologous pair;

2. One of each into daughter cells / haploid number.

Question 8

(c) Identify one event that occurred during division 2 but not during division 1.
(1)

Answer 8

(c) Separation of (sister) chromatids / division of centromere.

Question 9

(d) Name two ways in which meiosis produces genetic variation.
(2)

Answer 9

1. Independent segregation (of homologous chromosomes);
   Accept random assortment
2. Crossing over / formation of chiasmata.

Question 10

Q5.To reduce the damage caused by insect pests, some farmers spray their fields of crop
plants with pesticide. Many of these pesticides have been shown to cause environmental
damage.
Bt plants have been genetically modified to produce a toxin that kills insect pests. The use
of Bt crop plants has led to a reduction in the use of pesticides.
Scientists have found that some species of insect pest have become resistant to the toxin
produced by the Bt crop plants.
The figure below shows information about the use of Bt crops and the number of species
of insect pest resistant to the Bt toxin in one country.

(a) Can you conclude that the insect pest resistant to Bt toxin found in the years 2002
to 2005 was the same insect species? Explain your answer (1)

Answer 10

) (No – no mark)

Graph / bar chart only shows number of species, not the name of the species.

Question 11

(b) One farmer stated that the increase in the use of Bt crop plants had caused a
mutation in one of the insect species and that this mutation had spread to other
species of insect. Was he correct? Explain your answer.
(4)

Answer 11

) (No – no mark)
1. Mutations are spontaneous / random;
2. Only the rate of mutation is affected by environment;
3. Different species do not interbreed / do not produce fertile offspring;
4. So mutation / gene / allele cannot be passed from one species to
another.

Question 12

(c) There was a time lag between the introduction of Bt crops and the appearance of
the first insect species that was resistant to the Bt toxin.
Explain why there was a time lag. (3)

Answer 12

1. Initially one / few insects with favourable mutation / allele;
2. Individuals with (favourable) mutation / allele will have more offspring;
3. Takes many generations for (favourable) mutation / allele to become the
   most common allele (of this gene).

Question 13

Q1 Drosophilia (fruit flies ) only have 4 chromosomes
The diagraph summarises meoisis in them
a) complete a diagram to show how chromosomes in the 4 daughter cells

Answer 13

I I
II

II

II

Question 14

Q1 Drosophilia (fruit flies ) only have 4 chromosomes 
The diagraph summarises meoisis in them 
b) crossing over does not occur very frequently in males 
Explain what crossing over is and how it leads to genetic variation (4 marks)

Answer 14

1. During meiosis, homologous pairs of chromosomes pair together
2. The chromatids twist around each other and sections swap over
3. Chromatids had a different combination of alleles
4. so each of the 4 daughter cells has chromatids with different combinations of alleles

Question 15

Q1 Drosophilia (fruit flies ) only have 4 chromosomes 
The diagraph summarises meoisis in them

explain how independent segregation leads to genetic variation (2 marks)

Answer 15

1. Homologous chromosome pairs can split up in any way

2. So daughter cells produced can contain any combination of maternal and paternal chromosomes with different alleles

Question 16

Suggest and explain how chromosome disconjunction causes turner syndrome (3 marks )

Answer 16

1. Sex chromosomes fail to separate properly during meiosis
2. One of the daughter cells / gametes ends up without a copy of the x chromosomes/ while some may get two
3. If the gamete without the x chromosome is fertilized then the resulting zygote will be missing an X chromosome = turner syndrome

Question 17

A zeedonk is an offspring produced from breeding a mountain zebra with a donkey. The body cells of a mountain zebra contain 32 chromosomes and the body cells of a donkey contain 62 chromosomes. Use this information to suggest why zeedonks are usually infertile. (2 marks)

Answer 17

Zeedonk has an uneven number of chromosomes

Meiosis cannot occur

Haploid cells are not produced

Question 18

Use Figure 2 to explain what is meant by Non-Disjunction. (2 marks)

Answer 18

Chromatids failed to separate

During Anaphase

So both chromatids go to one pole of the spindle

Question 19

When large amounts of nutrients are present, the fungus reproduces asexually. Explain one advantage of this. (2 marks)

Answer 19

Many spores released nearby

So more of nutrient source more rapidly used

Question 20

Explain why the arrow labeled M2 represents the 2nd division of Meiosis. (2 marks)

Answer 20

The previous stage’s chromosome number halved (2n to n) so must be 2st stage of Meiosis

In 2nd stage, separation of chromatids occurs so chromosome number remains constant

4 haploid cells produced

Question 21

The spores vary genetically. Describe the 2 ways by which this variation is produced. (2 marks)

Answer 21

Crossing Over

Independent Segregation

Question 22

Figure 1 shows one pair of homologous chromosomes. Name what is in the middle, joining them. (1 mark)

Answer 22

Centromere

Question 23

Describe the role of Centromere in Mitosis. (2 marks)

Answer 23

Holds chromatids together

attaches chromatids to spindle

so allows chromatids to be separated

Question 24

Homologous Chromosomes carry the same genes but they are not genetically identical. Explain why. (1 mark)

Answer 24

Homologous chromosomes carry different alleles

Question 25

0 1 Figure 1 shows all the chromosomes present in one human cell during mitosis. A
scientist stained and photographed the chromosomes. In Figure 2, the scientist has

arranged the images of these chromosomes in homologous pairs.
Figure 1 Figure 2
0 1 . 1 Give two pieces of evidence from Figure 1 that this cell was undergoing mitosis.
Explain your answers.
[2 marks]

Answer 25

1. The (individual) chromosomes are
   visible because they have condensed;

2. (Each) chromosome is made up of two
chromatids because DNA has
replicated;
3. The chromosomes are not arranged in
homologous pairs, which they would be
if it was meiosis;

Question 26

1 . 5 In Figure 2 the chromosomes are arranged in homologous pairs.
What is a homologous pair of chromosomes?

[1 mark]

Answer 26

(Two chromosomes that) carry the same
genes;

In question 01.5, only 36.9% of students could define the term ‘homologous chromosome’. It is
likely that more students could have written about independent segregation or crossing over of
homologous chromosomes, but this question revealed that they did not fully understand this
biological term. Many students only referred to the origin of the chromosomes as paternal and
maternal.

Question 27

Figure 2 shows 3 pairs of homologous chromosomes in a cell at the end of cell division. The appearance of each chromosome in Figure 2 is different from those shown in Figure 1. Explain why. (1 mark)

Answer 27

Chromatids have separated

during anaphase

Question 28

The diagram shows a cell undergoing cell division. Identify the type and stage of cell division shown. Give evidence from the diagram to support your answer. (3 marks)

Answer 28

Meiosis (Anaphase I)

Chromosomes are moving apart

Chromosomes are still double structures

Question 29

Describe how crossing over occurs during Meiosis I. (2 marks)

Answer 29

Chromosomes in each homologous pair twist around each other

Chromatids break & rejoin to chromatid on sister chromosome

Question 30

Apart from increasing genetic variation, explain why meiosis is important in organisms that reproduce sexually. (2 marks)

Answer 30

Haploid cells produced

during the fusion of gametes

Diploid number is restored

Question 31

What is an allele? (1 mark)

Answer 31

Different versions of the same gene

Question 32

Explain the appearance of one of the Chromosomes in Figure 1. (2 marks)

Answer 32

Due to DNA Replication

2 sister chromatids

are joined by a centromere