Question 1
Paper 2Medium16 marksA gene in a population encodes part of a polypeptide. The table shows the coding-strand DNA sequence from a reference individual and three variants found in other individuals. The mRNA codons and their amino acids relevant to these sequences are also shown.
| Sequence | Coding-strand DNA, 5′→3′ |
|---|---|
| Reference | GAA CCT TGG |
| P | GAG CCT TGG |
| Q | GAA CAT TGG |
| R | GAA CCT TGC |
| mRNA codon | Amino acid |
|---|---|
| GAA | glutamic acid |
| GAG | glutamic acid |
| CCU | proline |
| CAU | histidine |
| UGG | tryptophan |
| UGC | cysteine |
(a)[4]
For P, Q and R, state the codon and nucleotide substitution. Explain why these differences are SNPs.
(b)[6]
For each variant, state the amino acid encoded at the changed codon and whether this amino acid differs from that in the reference sequence.
(c)[3]
Explain why the substitution in P does not change the amino acid.
(d)[3]
A student concludes that variants Q and R must produce a different phenotype from the reference individuals. Evaluate this conclusion using the information given.