2016 VJC H2 Biology Prelims Paper 3 Answers

1    VICTORIA JUNIOR COLLEGE BIOLOGY DEPARTMENT JC2 PRELIMINARY EXAMINATIONS 2017 HIGHER 2 9744/3 Answers Note: A: Accept; R: Reject 1 Huntington's disease (HD) is a rare neurodegenerative disease. Fig. 1.1 shows a pedigree of HD across three generations (I to III). Fig. 1.1 (a) With reference to Fig. 1.1, account for the mode of inheritance of the disease. [3] ● HD is inherited in an dominant manner; ● every generation has affected offspring as long as one parent is affected ( I1);; ● a single defective allele is sufficient for trait; ● Inherited in an autosomal manner; ● male and female offspring are similarly affected; ● an affected male parent ( II2) can produce an affected son (III1);; (b) (i) Explain the likely effect of the abnormal increase in CAG repeats on HTT protein structure and function. [3] ● Production of an abnormally long polypeptide / longer than the normal polypeptide (R! premature termination since length of CAG repeats is associated with disease); / ● Alters primary structure of polypeptide; ● disrupts the R group interactions such as hydrogen bonding, ionic, hydrophobic interactions and disulfide bridges;; ● essential for correct/ extensive folding into tertiary structure with specific 3D shape;;/ idea of 3D shape/conformation or tertiary structure is affected;; ● normal function of protein is lost/ abnormal protein is made; Max 3m (ii) Suggest possible reasons why individuals having number of repeats ranging from 21- 39 do not develop the disease. [2] ● Insertion mutation of multiples of 3 that code for chain of 20 glutamines (A! less than 40 glutamines) do not drastically affect 3D shape/ structure and thus function of the protein;; ● Slight effect on protein function but not drastic enough to develop disease;; ● A chain of more than 40 glutamines affect interactions between R groups that lead to folding into specific tertiary structure of the protein to affect normal function and cause HD;;

2    (c) (i) Explain why PCR can be used for the diagnosis of HD. [2] ● PCR makes use of specific primers that flank the region of the HTT gene/ exon 1 that contains the CAG repeats;; ● to amplify the different fragment lengths to allow for differentiating between normal and mutant allele;; (ii) Explain how gel electrophoresis was used to detect the band patterns of the offspring in Fig.1.1. [4] ● During electrophoresis, negatively charged DNA fragments migrate through a gel towards the positive electrode;; ● under an electric field; ● agarose gel acts as a molecular sieve; ● Larger fragments (i.e. has more CAG triplets) move slower compared to shorter fragments;; ● Gel is stained with methylene blue and observed under white light;; / ethidium bromide and observed under uv light;; (d) Based on this information, draw in the band patterns (in Fig. 1.2) for individuals #6, #10 and