A-LevelBiologyGenetic technology9700/41 · Oct/Nov 2022 · Q413 Marks

Adenosine deaminase (ADA) deficiency is an immune system disorder caused by a recessive autosomal mutation. Severe combined immunodeficiency caused by a lack of ADA is called ADA-SCID.

(a)Genetic engineering is used to make a recombinant human protein to treat people with ADA-SCID. Outline the principles of genetic engineering.[4]
(b)In 2016 gene therapy to cure ADA-SCID was approved in Europe. The gene therapy involves three main steps. • Blood (haematopoietic) stem cells are taken from the bone marrow of the person with ADA-SCID. • The functional gene and its promoter are inserted into the blood stem cells. • A single infusion (injection) of the gene-corrected cells is given to the patient.
(b)(i)Explain why a single infusion of gene-corrected stem cells is enough to cure the disease.[2]
(b)(ii)Explain why a promoter has to be transferred as well as the desired gene.[2]
(b)(iii)A modified retrovirus is used to insert the new gene into the DNA of the blood stem cells. State two ethical considerations of using a retrovirus for gene therapy.[2]
(c)The gene therapy technique used to cure ADA-SCID is not suitable for treating the genetic disease called Huntington's disease. A newer technique called gene editing could potentially be used instead to cure Huntington's disease. Explain why gene editing is more suitable as a potential cure for Huntington's disease.[3]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

[4] (a) any four from: 1 add DNA to give a new (named), characteristic / protein; 2 restriction, enzyme / endonuclease, to obtain, gene / allele / section of DNA or restriction, enzyme / endonuclease, to cut plasmid ; 3 combine gene with / insert gene into, vector / plasmid; 4 introduce, (recombinant) vector / plasmid, to, bacterium / (named host) cell ; 5 clone / multiply, (named, recombinant / GM) organism ; 6 AVP;
[2] (b)(i) 1 stem cells, proliferate / divide / multiply / make new cells ; 2 (correct) gene is, inherited by / passed to / present in, new cells ;
[2] (b)(ii) any two from: 1 to switch gene on / so gene is expressed / allow transcription; 2 at right time / all the time / in sufficient quantities ; 3 to allow binding of, RNA polymerase / transcription factors ;
[2] (b)(iii) any two from: 1 retrovirus / new gene, could / must not, insert in wrong place / disrupt other genes ; 2 could / must not, cause cancer ; 3 could / must not, cause, infection / disease ; 4 could / must not, cause, immune / allergic, response ; 5 AVP;
[3] (c) any three from: 1 HD is caused by a dominant allele ; 2 adding a 'normal' recessive allele would not work; 3 gene editing can / we need to, delete / remove / inactivate, HD / mutant, allele / DNA ; 4 ref. (surplus CAG) repeats / triplets / stutter ;

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