A-LevelBiologyNucleic acids and protein synthesis9700/42 · Oct/Nov 2020 · Q511 Marks

Tyrosinase is an enzyme found in mammals. It is involved in the synthesis of melanin pigment. Mutations in the tyrosinase gene affect a mammal's hair colour. Table 5.1 compares DNA sequences for codons 974–985 of: • the normal tyrosinase gene of humans (human) • the normal tyrosinase gene of cats that have pigmented hair (normal cat) • the tyrosinase gene of cats that show an albino phenotype (albino cat). The corresponding amino acid sequences of each tyrosinase are shown in the shaded rows. [Table 5.1]

(a)(i)A silent mutation involves a base substitution that does not result in an amino acid change. Use Table 5.1 to identify, with reasons, a silent mutation distinguishing humans from normal cats with pigmented hair.[2]
(a)(ii)State the changes that resulted in the premature STOP codon in the albino cat DNA sequence.[2]
(a)(iii)Explain why albino cats, homozygous for the mutation that resulted in the premature STOP codon, do not produce melanin.[2]
(a)(iv)Bioinformatics was used to compare the whole sequence of the tyrosinase genes of humans and cats. Explain why bioinformatics was used to compare these gene sequences and suggest a conclusion that could be made from the percentage similarity data obtained.[2]
(b)Siamese cats have a temperature-sensitive tyrosinase that only functions in the cooler areas of the skin. This means they only produce a small quantity of melanin pigment. Melanin is mainly on their ears, face, paws and tail. Fig. 5.1 shows a Siamese cat. [Figure 5.1] The Siamese allele of tyrosinase (ts) is recessive to the normal allele that causes full pigmentation all over the body (T) but is dominant to the albino allele (ta). Draw a genetic diagram to show how a cross between an albino cat and a fully pigmented cat can result in offspring that include kittens with Siamese colouring. parent phenotypes: parent genotypes: gametes: F1 genotypes: F1 phenotypes: ratio: albino x fully pigmented[3]

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Mark scheme

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

[2] any two from:
[2] 1 (codon) 976 where, T is replaced by C / TCT becomes TCC or (codon) 977 where, A is replaced by T / TCA becomes TCT ;
[2] 2 ser(ine) in, both / cats and humans ;
[2] 3 degenerate code;
[2] any two from:
[2] 1 base / C, deleted at, second codon / (codon) 975 ;
[2] 2 causes frameshift ;
[2] 3 next 9 codons changed / all codons afterwards are changed ;
[2] any two from:
[2] 1 ref. to shortened polypeptide ;
[2] 2 no tyrosinase made ;
[2] 3 change in, 3D / tertiary, shape / structure or change in active site ;
[2] 4 (so) tyrosinase, non-functional / cannot bind to substrate / inactive ;
[2] 5 tyrosine not converted to, DOPA / dopaquinone ;
[2] any one from:
[2] why
[2] 1 large amount of, data / DNA sequences;
[2] 2 fast / accurate / efficient; A description of fast / accurate / efficient / detects each small difference = accurate
[2] any one from:
[2] conclusions
[2] 3 more similarity the more recent a common ancestor / AW ;
[2] 4 molecular clock;
[3] parent phenotypes: (albino x fully pigmented)
[3] parent genotypes: tªtª x Ttˢ ;
[3] gametes: (tª Tots)
[3] F1 genotypes: Ttª tˢtª ;
[3] F1 phenotypes: fully pigmented Siamese and
[3] ratio: 1 : 1 ;
[3] F1 phenotypes must match genotypes
[3] ecf for mp2 and mp3 if parent genotype incorrect

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