Unit 8 part 2

Question 1

Sickle cell disease (SCD) is a group of inherited disorders. People with SCD have sickle-shaped red blood cells. A single base substitution mutation can cause one type of SCD. This mutation causes a change in the structure of the beta polypeptide chains in haemoglobin.

(a)  Explain how a single base substitution causes a change in the structure of this polypeptide. (3)

Do not include details of transcription and translation in your answer.

Answer 1

1.   Change in (sequence of) amino acid(s)/primary structure;

Reject amino acids are formed.

Reject amino acids code.

2.   Change in hydrogen/ionic/disulfide bonds;

3.   Alters tertiary/30 structure;

Reject active site. Ignore quaternary.Ignore 3D.

Question 2

Mitochondrial DNA (mtDNA) is a small circular DNA molecule located in mitochondria. It is 16 569 nucleotides long and contains 37 genes and a control region.

Sports scientists investigated whether a mutation in the control region of mtDNA in human males was related to an ability to exercise for longer.

*The males in Group T had thymine at nucleotide position 16 519

• The males in Group C had a mutation resulting in cytosine at nucleotide position 16 519

(a)  The control regions of Group T and Group C were the same length. Name the type of gene mutation that is most likely to have occurred at nucleotide position 16 519

Answer 2

Substitution;

Question 3

Describe how alterations to tumour suppressor genes can lead to the development of tumours. (4)

Answer 3

1. (Increased) methylation (of tumour suppressor genes);

Accept abnormal methylation or hypermethylation

Ignore decreased acetylation of histones

2. Mutation (in tumour suppressor genes);
3. Tumour suppressor genes are not transcribed/expressed

OR Amino acid sequence/primary structure altered;

Accept mRNA for transcription/transcribed

Accept tertiary structure altered. Accept different amino acid

Ignore reference to protein not being formed

4. (Results in) rapid/uncontrollable cell division;

Accept cell division cannot be regulated

Ignore growth

Question 4

The mutation affecting NL3 in these mice was a substitution in the neuroligin-3 gene. What is a substitution mutation? (1)

Answer 4

1. Replacement of a base by a different base (in DNA);

Question 5

Suggest how gel electrophoresis could separate the proteins obtained from the synapses. (2)

Answer 5

1. (Depends on) size / mass (of protein);
2. (Depends on) charge (of protein);

Accept for 2 marks ‘Smaller / more highly charged move further’

Question 6

What is meant by a non-coding base sequence? (1)

Answer 6

Does not code for amino acid/tRNA/rRNA;

Accept ‘does not code for production of protein/polypeptide’

Reject ‘that produces/makes amino acid’

Question 7

Explain how the use of DNA probes could be used to identify a particular allele. (2)

Answer 7

1. Probes are single stranded / have a specific base sequence;
2. Complementary base sequence on (specific) spacer

Question 8

Suggest how a mutation can lead to the production of a protein that has one amino acid missing. (2)

Answer 8

Loss of 3 bases / triplet = 2 marks;;

‘Stop codon / code formed’ = 1 mark max unless related to the last amino acid

Loss of base(s) = 1 mark;

eg triplet for last amino acid is changed to a stop codon / code = 2 marks

3 bases / triplet forms an intron = 2 marks

Accept: descriptions for ‘intron’ eg non-coding DNA

‘Loss of codon’ = 2 marks

Question 9

Plants transport sucrose from leaves to other tissues for growth and storage. SUT1 is a sucrose co-transporter protein.

Scientists investigated whether the cells of tobacco plant leaves used SUT1 to transport sucrose to other tissues.

The scientists used a radioactively labelled DNA probe to show that the cells of tobacco plant leaves contained the SUT1 gene.

Describe how they would do this.

Do not include PCR in your answer. (4)

Answer 9

1. Extract DNA and add restriction endonucleases/restriction enzymes;
2. Separate fragments using electrophoresis;
3. (Treat DNA to) form single strands

OR (Treat DNA to) expose bases;

Ignore method used to separate strands

4. The probe will bind to/hybridise/base pair with the SUT1/gene;
5. Use autoradiography (to show the bound probe);

Accept use photographic or X ray film (to show the bound probe)

X rays alone is not sufficient