RI Nov 17 H2 P3 ans

Raffles Institution Nov 2017 (H2 Biology) Paper 3 2023 Nov 2017 H2 Bio Paper 3 N17P3Q1 (a) (i) State what is meant by a genetic disease and explain how genetic diseases are caused. [4] 1. Disease is a health / physiological impairment as a result of; 2. Mutation* in the DNA; 3. which could have occurred in the individual with the disease , or inherited from parents; 4. The mutation can be in the form of a nucleotide substitution, insertion or deletion; 5. which would cause a change in the mRNA codon and subsequently, a change in the amino acid in the protein resulting in a loss-of-function; 6. Or could have resulted in a premature stop codon, producing a truncated protein; 7. Mutations could also be at the chromosomal level, where there is a change in the number of chromosomes or chromosomal translocation, duplication, deleti on or inversion; (ii) Justify the claim that the PKU phenotype is the result of genotype and the environment acting together. [2] 1. The PKU phenotype would only be observed in individuals that are homozygous recessive for the disease allele (genotype); 2. and when phenylalanine is present in the diet of the individual (environment); (iii) Describe two ways in which these molecules are similar in structure and explain why these features are important for their function. [4] Both molecules have 1. an amino group (NH2/NH3+); 2. a carboxyl group (COOH/COO-) Importance to function: 3. to participate in condensation reaction; 4. to form peptide bonds in the production of proteins; (b) (i) Suggest, in outline, a procedure that could determine whether or not a PKU allele is present. [4] 1. Extract genomic DNA from cells obtained in the blood / from a mouth swab; 2. Carry out polymerase chain reaction (PCR)* using primers* complementary to regions flanking the PKU allele; 3. Cut the amplified fragment using restriction enzyme*; 4. Carry out gel electrophoresis* to separate the fragments according to size, 5. Carry out Southern blot* and use radioactive probes complementary to the PKU allele to detect specific fragments 6. Carry out RFLP analysis: compare with restriction patterns for known genotypes to determine unknown genotype; A: after PCR – carry out DNA sequencing & compare with known DNA sequences of the normal allele and the PKU allele

Raffles Institution Nov 2017 (H2 Biology) Paper 3 2023 (ii) Predict and explain the effect of this treatment on the result of the Guthrie assay for a baby that is homozygous for the PKU allele. [3] 3. The results of the test would be a false negative , showing wrongly that the baby does not have PKU; 4. No colony / reduced colony diameter would be observed; 5. Despite high levels of