Severe combined immunodeficiency (SCID) is a group of life-threatening diseases. SCID is caused by mutations that prevent the normal function of the immune system. Infants born with SCID are at very high risk of infectious diseases. One feature of SCID is that T-lymphocytes do not develop normally. In the development of normal T-lymphocytes, the production of circular pieces of DNA called T-lymphocyte receptor excision circles (TRECs) is an important event. It is possible to use the polymerase chain reaction (PCR) to detect TRECs in DNA extracted from a sample of blood. The results of this reaction can be used to identify children with SCID.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2020). The mark or mark type is shown in brackets.

1. [3] 2(a)(i) any three from: 1 primer DNA anneals to, single-stranded DNA / denatured DNA ; 2 using complementary base pairing; 3 formation of hydrogen bonds (between primer DNA and TREC DNA); 4 allows, Taq polymerase / DNA polymerase, to bind or starting point for, Taq polymerase / DNA polymerase, to attach; 5 ref. to specificity; e.g. only binds to TREC DNA
2. [2] 2(a)(ii) any two from: 1 PCR product means TRECs present/ora ; 2 if PCR detects TRECS, then T-lymphocytes are developing normally / ora; 3 TRECs, do not form / are in small numbers, in children with SCID / ora; 4 (detection of) PCR product means child does not have SCID ;
3. [3] 2(b)(i) any three from: 1 contribution (to the identification of children with SCID) by screening programme increased (from 2010 to 2016); 2 contribution (to the identification of children with SCID) by family history decreased and infection decreased (from 2010 to 2016) / AW; 3 between 2012 and 2013 the screening programme became the method with the highest percentage contribution; 4 by 2016 nearly all cases of SCID were diagnosed by the screening programme ; 5 comparative figures to support any of marking points 1 to 4 ; 6 AVP; e.g. some children diagnosed with SCID as a result of infection (despite screening) in 2010 the biggest contribution to diagnosing SCID was through infections
4. [2] 2(b)(ii) any two from: 1 early, diagnosis / treatment / AW ; 2 does not rely on family knowing about family history of SCID ; 3 prevents child with SCID from developing several infections before diagnosis ; 4 removes worry if not present ;
5. [3] 2(c) any three from: 1 isolate / obtain, functional / normal, ADA allele ; A gene throughout 2 insert allele into virus (vector) ; I ref. to plasmid 3 remove, stem cells / T-lymphocytes / target cells ; 4 insert, allele / gene / virus, into, stem cells / T-lymphocytes / target cells ; 5 return, stem cells / T-lymphocytes / target cells, to body; 6 AVP; e.g. ref. to retrovirus / lentivirus ref. to difficulty in finding suitable donor for bone marrow transplant
6. [3] 2(d) any three from: 1 (retrovirus / lentivirus) can insert, viral DNA / healthy allele, randomly into (host) DNA ; 2 may cause, cancer / side effects / allergic response ; 3 inserted, allele / DNA, may be inactivated or inserted, allele / DNA, may inactivate another (host) gene ; 4 virus may not enter, target cells / T-lymphocytes or virus may enter non-target cells; 5 AVP; e.g. ref. to safe / clean, conditions required to produce virus ineffective immune response against virus