Gene expression

Question 1

Explain why the scientists used the same restriction endonuclease enzymes on each DNA sample.

Answer 1

Cut (DNA) at same (base) sequence / (recognition) sequence;

(So) get (fragments with gene) R / required gene.

Question 2

Explain why primer A3 and primer A4 only bind to specific DNA fragments.

Answer 2

Each has / they have a specific base sequence; That is complementary (to allele r or R).

Question 3

The scientists wanted to know on which chromosome the gene with alleles R and r was located. From the flies with genotype RR, they obtained cells that were in mitosis and added a labelled DNA probe specific for allele R. They then looked at the cells under an optical microscope.
Explain why they used cells that were in mitosis.

Answer 3

(Cells in mitosis) chromosomes visible;

(So) can see which chromosome DNA probe attached to.

Question 4

Explain why this DNA probe will only detect allele A.

Answer 4

Probe (base sequence) complementary (to DNA of allele A / where A is (and) binds by forming base pairs / hydrogen bonds;

So (only) this DNA labelled / has green dye / gives out (green) light;

Question 5

The scientists used tumour area as an indicator of tumour size.
Explain why tumour area may not be the best indicator of tumour size and suggest a more reliable measurement.

Answer 5

Tumours may be different depths / area does not take depth into account / tumours are 3-D / are not 2-D;

(Measure) tumour volume / mass / weight.

Question 6

A mutation of a tumour suppressor gene can result in the formation of a tumour. Explain how.

Answer 6

(Tumour suppressor) gene inactivated / not able to control / slow down cell division;
Rate of cell division too fast / out of control.

Question 7

Not all mutations result in a change to the amino acid sequence of the encoded polypeptide.
Explain why.

Answer 7

(Genetic) code degenerate;

Mutation in intron.

Question 8

Explain how the methylation of tumour suppressor genes can lead to cancer.

Answer 8

Methylation prevents transcription of gene;
Protein not produced that prevents cell division / causes cell death / apoptosis;
No control of mitosis.

Question 9

MM is caused by a faulty receptor protein in cell-surface membranes. Cells in MM tumours can be destroyed by the immune system.
Suggest why they can be destroyed by the immune system.

Answer 9

Faulty protein recognised as an antigen / as a ‘foreign’ protein; T cells will bind to faulty protein / to (this) ‘foreign’ protein; (Sensitised) T cells will stimulate clonal selection of B cells; (Resulting in) release of antibodies against faulty protein.

Question 10

Explain how the structure of DNA is related to its functions.

Answer 10

Sugar-phosphate (backbone) / double stranded / helix so provides strength / stability / protects bases / protects hydrogen bonds;

Long / large molecule so can store lots of information; Helix / coiled so compact;

Base sequence allows information to be stored / base sequence codes for amino acids / protein;

Double stranded so replication can occur semi-conservatively / strands can act as templates / complementary base pairing / A-T and G-C so accurate replication / identical copies can be made;

(Weak) hydrogen bonds for replication / unzipping / strand separation / many hydrogen bonds so stable / strong;

Question 11

More cancer cells could be destroyed if the drug was given more frequently. Suggest why the drug was not given more frequently

Answer 11

1. More / too many healthy cells killed;
2. (So) will take time to replace / increase in number;
   Neutral: will take time to ‘repair’
3. Person may die / have side effects

Question 12

The faulty allele that causes Ellis-van Creveld syndrome is the result of a mutation of a gene called EVC. This mutation leads to the production of a protein that has one amino acid missing.
(1)
(i) Suggest how a mutation can lead to the production of a protein that has one amino acid missing.

Answer 12

Loss of 3 bases / triplet = 2 marks;;

Question 13

Suggest how the production of a protein with one amino acid missing may lead to a genetic disorder such as Ellis-van Creveld syndrome.

Answer 13

Change in tertiary structure / active site;

(So) faulty / non-functional protein / enzyme;

Question 14

A mutation in the gene coding for enzyme B could lead to the production of a non-functional enzyme. Explain how.

Answer 14

Change in base sequence (of DNA / gene) leading to change in amino acid sequence / primary structure (of enzyme);

Change in hydrogen / ionic / disulphide bonds leading to change in the tertiary structure / active site (of enzyme);

Substrate not complementary / cannot bind (to enzyme / active site) / no enzyme-substrate complexes form;

Question 15

Explain how this mutation leads to the formation of non-functioning PAH.

Answer 15

Change in amino acid / (sequence of) amino acids / primary structure;

2. Change in hydrogen / ionic / disulphide bonds alters tertiary structure / active site (of enzyme);
3. Substrate not complementary / cannot bind (to enzyme / active site) / no enzyme- substrate complexes form;

Question 16

One mutation causing phenylketonuria was originally only found in one population in central Asia. It is now found in many different populations across Asia. Suggest how the spread of this mutation may have occurred.

Answer 16

Founder effect / colonies split off / migration / interbreeding;

Question 17

The scientists analysed the DNA on the Y chromosome and the DNA in the mitochondria of the Swedish wolves. They concluded that the Swedish wolf population descended from one male wolf from Finland and one female wolf from Russia.

Explain why DNA on the Y chromosome helped them to reach this conclusion.

Answer 17

Y chromosome inherited / comes from male parents / only found in males;

Question 18

Suggest why DNA in the mitochondria helped them to reach this conclusion.

Answer 18

Mitochondria in egg / female gamete / no mitochondria come from sperm / male gamete;

Question 19

Explain why the labelled DNA probe could be used to find out whether the haplotypes were the same.

Answer 19

Complimentary base sequence / complementary DNA; binds to both (haplotypes);
Label would show up in both;

Question 20

For what purpose did they use
restriction endonuclease
electrophoresis?

Answer 20

To cut the DNA;

To separate the (pieces of) DNA;

Question 21

Suggest why the plasmids were injected into the eggs of silkworms, rather than into the silkworms.

Answer 21

(If injected into egg), gene gets into all / most of cells of silkworm;
So gets into cells that make silk.

Question 22

Suggest why the scientists used a marker gene and why they used the EGFP gene.

Answer 22

Not all eggs will successfully take up the plasmid; Silkworms that have taken up gene will glow.

Question 23

Suggest two reasons why it was important that the spider gene was expressed only in the silk glands of the silkworms.

Answer 23

1. So that protein can be harvested;

2. Fibres in other cells might cause harm

Question 24

The jellyfish gene attached to the human Factor IX gene (Stage 2) codes for a protein that glows green under fluorescent light. Explain the purpose of attaching this gene.

Answer 24

(Acts as a) marker gene to show that the (human) gene has been taken up / expressed;
1. Accept: gene marker
(Only) implant cells / embryos that show fluorescence / contain the jellyfish gene;

Question 25

The promoter DNA from sheep (Stage 3) causes transcription of genes coding for proteins found in sheep milk.
Suggest the advantage of using this promoter DNA.

Answer 25

Factor IX present in / extracted from milk;
Gene only expressed in mammary glands / udder / gene not expressed elsewhere;
Do not need to kill sheep (to obtain Factor IX);

Question 26

Many attempts to produce transgenic animals have failed. Very few live births result from the many embryos that are implanted.

Suggest one reason why very few live births result from the many embryos that are implanted.

Answer 26

Embryo / antigens foreign;

Embryo is rejected / attacked by immune system;

Question 27

It is important that scientists still report the results from failed attempts to produce transgenic animals. Explain why.

Answer 27

Saves time / money for others;

Question 28

What would the scientists have inserted into the plasmid along with the spider gene to ensure that the spider gene was only expressed in the silk glands of the silkworms?

Answer 28

Promoter (region / gene).

Question 29

Oestrogen is a hormone that affects transcription. It forms a complex with a receptor in the cytoplasm of target cells. Explain how an activated oestrogen receptor affects the target cell.

Answer 29

(Receptor / transcription factor) binds to promoter which stimulates RNA polymerase / enzyme X;
Transcribes gene / increase transcription;

Question 30

Oestrogen only affects target cells. Explain why oestrogen does not affect other cells in the body.

Answer 30

Other cells do not have the / oestrogen / ERα receptors;

Question 31

similar structure of both

Use Figure 2 to suggest how endoxifen reduces the growth rate of these breast tumours.

Answer 31

Similar shape to oestrogen;
Binds receptor / prevents oestrogen binding;
Receptor not activated / will not attach to promoter / no transcription;

Question 32

The scientists found that the promoter DNA of the p16 gene had become methylated. The promoter is the sequence of bases where the enzyme RNA-polymerase binds to a DNA molecule.
Explain how methylation of the promoter sequence of the p16 gene could cause the changes shown in Figure 3.

Answer 32

RNA-polymerase could not bind (to DNA / to promoter);mRNA of p16 could not be
made / no transcription of p16 gene;

Question 33

Give two characteristic features of stem cells.

Answer 33

Will replace themselves / keep dividing / replicate;

Undifferentiated

Question 34

The other viral enzyme is called integrase. Integrase inserts the DNA copy anywhere in the DNA of the host cell. It may even insert the DNA copy in one of the host cell’s genes.
(c) (i) The insertion of the DNA copy in one of the host cell’s genes may cause the cell to make a non-functional protein. Explain how.

Answer 34

Alters base / nucleotide sequence / causes frame shift;

Different sequence of amino acids in polypeptide / protein / primary structure alters the tertiary structure;

Question 35

Some of the children in the trial developed cancer. How might the insertion of the DNA have caused cancer?

Answer 35

Affects tumour suppressor gene;
Inactivates (tumour suppressor) gene;
Rate of cell division increased / tumour cells continue to divide;