4.2- Genetic Diversity via Mutation QP

Question 1

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);
   Q.
   Ignore: references to changing base-pairing
   Accept: affect for change, if in correct context
   Accept: changes triplets / codons
2. Change in amino acid sequence / primary structure (of enzyme);
   Accept: different amino acid(s) coded for
   Q Reject: different amino acids produced / formed / made
3. Change in hydrogen / ionic / disulphide bonds;
   Accept: references to sulphur bonds
4. Change in the tertiary structure / shape;
   Neutral: alters 3D structure / 3D 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

) 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.
   This must be a comparative statement

Question 3

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
   resistance is due to only stopping division
   Need some comparison with non-SR

Question 4

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

(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

(SR strain)
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

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

.(a) 1. Chromosome is formed of two chromatids;

2. (Because) DNA replication (has occurred);
3. (Sister) chromatids held together by centromere.

Question 7

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

Question 8

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

Answer 8

Separation of (sister) chromatids / division of centromere.

Question 9

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

Scientists investigated the effect of hunting on the genetic diversity of otters. Otters
are animals that were killed in very large numbers for their fur in the past.
The scientists obtained DNA from otters alive today and otters that were alive before
hunting started.
For each sample of DNA, they recorded the number of base pairs in alleles of the
same gene. Mutations change the numbers of base pairs over time.
The figure below shows the scientists’ results.

The scientists obtained DNA from otters that were alive before hunting started.
Suggest one source of this DNA. (1)

Answer 10

Bone / skin / preserved remains / museums.

Question 11

What can you conclude about the effect of hunting on genetic diversity in otters?
Use data from the figure above to support your answer. (2)

Answer 11

1. (Hunting) reduced population size(s), so (much) only few alleles left;
   Accept bottleneck
2. Otters today from one / few surviving population(s);
   Accept founder effect
3. Inbreeding.

Question 12

Some populations of animals that have never been hunted show very low levels of
genetic diversity.
Other than hunting, suggest two reasons why populations might show very low
levels of genetic diversity. (2)

Answer 12

1. Population might have been very small / genetic bottleneck;
2. Population might have started with small number of individuals / by one
   pregnant female / founder effect;
3. Inbreeding.
   Allow any two

Question 13

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 13

(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.
Ignore references to correlation does not prove causation

Question 14

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 14

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)