A-LevelBiologyGenetic technology9700/42 · May/June 2018 · Q514 Marks
A severe reduction of blood flow to the brain causes cells to die. This is called a stroke. The after-effects of a stroke can range from recovery to permanent brain damage and death. A new emergency gene therapy treatment for people who are at risk of brain damage from a stroke was tested in mice. • The human granulocyte colony-stimulating factor, hG-CSF, is a protein that stimulates the production of stem cells in bone marrow. • mRNA coding for hG-CSF was obtained and used to make cDNA. • This cDNA was inserted into an adeno-associated virus (AAV) vector and given in eye drops to mice just after they experienced a stroke.
- (a)(i)Explain what is meant by gene therapy.[2]
- (a)(ii)Describe the roles of reverse transcriptase and DNA polymerase in making cDNA for hG-CSF.[2]
- (a)(iii)The AAV vector used was unable to replicate itself within the target cells. Suggest why the researchers chose a vector that could not replicate.[2]
- (b)(i)A study was carried out to investigate the effect of the gene therapy described in (a). Four groups of mice were used. • Group A mice had a stroke. They received eye drops containing AAV vector carrying cDNA for hG-CSF once only. • Group B mice had a stroke. They received eye drops containing AAV vector carrying cDNA for hG-CSF four times. • Group C mice had a stroke. They received eye drops containing AAV vector carrying the GFP gene coding for green fluorescent protein, instead of the cDNA for hG-CSF, once only. • Group D mice did not have a stroke. They were not given any eye drops. (i) Explain why the mice in group C were used in the study.[3]
- (b)(ii)Explain why the mice in group D were used in the study.[1]
- (c)Table 5.1 summarises some results from the study and shows: • the percentage of mice surviving • the percentage of brain volume occupied by fluid-filled space • the score on a behavioural test in which normal mice score 0.5 and brain-damaged mice score nearer to 1.0. Table 5.1 mouse treatment group percentage of mice surviving percentage of brain occupied by fluid-filled space behavioural test score / arbitrary units A 63 3.6 0.67 B 100 3.0 0.67 C 25 5.2 0.90 D 100 3.0 0.50 Use the results in Table 5.1 to evaluate the benefits of gene therapy treatment, with AAV vector carrying the gene for hG-CSF, for people who have a stroke.[4]
Mark scheme
Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.
- [2] any 2 of: 1 to treat disease caused by, faulty / recessive, allele ; A cure 2 deliver, gene / allele / DNA (into target cells of individuals); 3 named example; e.g. SCID / cystic fibrosis
- [2] (reverse transcriptase) uses mRNA to make (ss)DNA; (DNA polymerase) makes DNA double-stranded / description ;
- [2] 1 to prevent virus spreading (throughout the body); 2 to limit, side effects / immune response / cancer / illness / infection / cell destruction;
- [3] any 3 of: 1 control; 2 idea of to see if gene therapy has worked or to compare the effect of no gene therapy with gene therapy or to compare the effect of no hG-CSF gene with presence of hG-CSF gene ; 3 to see where the gene (carried by the vector), goes (in, body / brain) or to see if the gene (carried by the vector), enters cells or to see where cells have been transformed; 4 to see the effects of the, vector / virus, alone;
- [1] any 1 of: 1 (control) to compare results (of mice without stroke) to mice with stroke ; 2 to establish baseline figures (in mice without a stroke) / act as a reference point ; 3 to make the study valid ;
- [4] any 4 of: survival 1 increases survival ; brain damage 2 reduces (percentage of brain occupied by) fluid filled space ; behaviour test 3 improves / lowers, behavioural test score ; A score closer to normal general 4 comparative data quote to support, mp1/mp2/mp3; A processed data mp1 25% v 63% 25% v 100% 63% v 100% mp2 5.2% v 3.6% 5.2% v 3.0% 3.6% v 3.0% mp3 0.90 v 0.67 5 eye drops four times is better than eye drops once ; ora 6 if effective with mice likely to be effective with humans ; 7 ref. to unknown differences between mice and humans / study is only on mice; 8 treatment is, non-invasive / quick ;
Write your answer and get it marked
On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.
Sign up free →