A-LevelBiologyGenetic technology9700/42 · Feb/Mar 2023 · Q410 Marks

Array comparative genome hybridisation (aCGH) is a technique involving the use of a microarray to analyse a genome or sections of a genome.

(a)Outline the steps required to prepare the genome of an individual so that the genome is ready for analysis using a microarray chip.[3]
(b)(i)DiGeorge syndrome is a dominant inherited disease in humans. DiGeorge syndrome is caused by deletion of a large number of nucleotides from chromosome 22. The number of nucleotides deleted varies between individuals in a range from 800000 to 3100000. The largest deletions can cause the removal of up to 46 protein-coding genes from the chromosome. Fig. 4.1 shows the results of aCGH using a microarray specific for the section of chromosome 22 within which the DiGeorge syndrome deletion occurs. The microarray analysed DNA from two individuals: • one with DiGeorge syndrome • one who did not have DiGeorge syndrome (control DNA for comparison). In the aCGH results shown in Fig. 4.1: • Each small circle represents the results from a single probe on the microarray. • The x-axis shows the position of each probe on chromosome 22. The position is shown as distance along the chromosome in millions of nucleotides. • A result close to 100% fluorescence on the y-axis means that the DNA from the individual with DiGeorge syndrome fluoresces at the same intensity as the control DNA for that probe. • A result close to 50% fluorescence on the y-axis means that the DNA from the individual with DiGeorge syndrome fluoresces half as much as the control DNA for that probe. [Figure 4.1] With reference to Fig. 4.1, estimate the number of nucleotides deleted from the affected chromosome 22 in the individual with DiGeorge syndrome. Give your answer to the nearest 100 000 nucleotides.[1]
(b)(ii)Explain how the microarray technique works to give the results shown in Fig. 4.1.[3]
(b)(iii)Suggest why the phenotypes of two individuals with DiGeorge syndrome can be different.[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2023). The mark or mark type is shown in brackets.

[3] (a) any three from:
[3] (a) 1 ref. to isolate / extract, DNA, from, cells / tissue / named ;
[3] (a) 2 ref. to cut DNA, into small fragments / using restriction enzymes ;
[3] (a) 3 ref. to denature / AW, into single-stranded DNA ;
[3] (a) 4 add fluorescent, tag / marker / dye ;
[1] (b)(i) 2 400 000 or 2 500 000 ;
[3] (b)(ii) any three from:
[3] (b)(ii) 1 DNA from DiGeorge syndrome and control DNA labelled with different colour fluorescent tags ;
[3] (b)(ii) 2 DNA hybridises with probes (on microarray); DiGeorge Syndrome
[3] (b)(ii) 3 less DNA binds to probes (than control DNA) ;
[3] (b)(ii) 4 so there is, less / 50%, fluorescence where nucleotides have been deleted ;
[3] (b)(ii) 5 fluorescence is, equal / 100%, where nucleotides have not been deleted;
[3] (b)(iii) any three from:
[3] (b)(iii) 1 different number of nucleotides may be deleted;
[3] (b)(iii) 2 different genes may be deleted;
[3] (b)(iii) 3 an individual only has one allele (of the deleted genes);
[3] (b)(iii) 4 different individuals may have different alleles of the same gene or in one individual remaining allele may be recessive but dominant in another individual;
[3] (b)(iii) 5 so different proteins made;
[3] (b)(iii) 6 ref. to individuals may have different environments ; A diet