A-LevelBiologyBiological molecules9700/21 · Oct/Nov 2022 · Q312 Marks

Carbonic anhydrase is a globular protein found in red blood cells.

(a)(i)Explain why this protein is described as globular.[2]
(a)(ii)State the function of carbonic anhydrase in red blood cells.[2]
(b)(i)Identify the base X and state the evidence in Fig. 3.1 that supports this identification. base X evidence from Fig. 3.1[3]
(b)(ii)The section of the DNA molecule in Fig. 3.1 is part of a gene coding for a polypeptide. Base X, shown in Fig. 3.1, is located in an exon on the strand of DNA that is transcribed during protein synthesis. A mutation that results in the deletion of base X will affect the polypeptide produced. Explain how this deletion may affect the polypeptide produced during protein synthesis.[4]
(b)(iii)Gene mutations can occur in either introns or exons. Suggest the effect of a gene mutation in an intron.[1]

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 (Oct/Nov 2022). The mark or mark type is shown in brackets.

[2] (a)(i) any two from: spherical / ball shaped;
(water-) soluble ;
amino acids with hydrophilic R groups / hydrophilic amino acids, on outside of molecule ; A amino acids with hydrophobic R groups in centre of molecule
physiological / metabolic, role ;
carbonic anhydrase is an enzyme, (enzymes are globular proteins);
[2] (a)(ii) any two from: catalyses / AW, reaction between water and carbon dioxide ;
(to form) carbonic acid;
catalyses formation of carbon dioxide and water from carbonic acid in lungs ;
[3] (b)(i) guanine ;
any two from: double ring (base) / purine (base);
forms three hydrogen bonds with complementary base ;
AVP; e.g. detail of difference between adenine and guanine
[4] (b)(ii) any four from: three bases / triplet of bases / codon, code(s) for one amino acid;
frameshift mutation;
change in, triplet of bases / codon;
ref. to every, triplet / codon, after deletion will be different ;
may introduce a stop codon ;
primary structure / sequence of amino acids / AW, will be altered ;
(stop codon results in) formation of, shortened / truncated, polypeptide ;
AVP; e.g. ref. to errors in RNA splicing ref. to affects post-translational modification bonds that form between R groups, will / may, be altered tertiary structure of protein will be altered non-functional protein formed altered function of the protein
[1] (b)(iii) any one valid suggestion e.g.: no effect on protein structure (because introns are non-coding);
non-functional / reduced function of, protein (as intron not spliced after transcription) ;
may affect regulation of gene expression; A example e.g. reduction in the rate of transcription RNA polymerase does not bind
ref. to errors in RNA splicing;
ref. to formation of an oncogene ;