A-LevelBiologyNucleic acids and protein synthesis9700/23 · Oct/Nov 2021 · Q19 Marks

Fig. 1.1 is a diagram showing a stage in protein synthesis. [Figure 1.1]

(a)(i)Name the stage of protein synthesis that is shown in Fig. 1.1.[1]
(a)(ii)Identify A, B and C in Fig. 1.1. A B C[3]
(a)(iii)State the base sequences at D and E. D E[1]
(b)Mutagenesis is a process that leads to a change in the amino acid sequences of proteins. Scientists carry out mutagenesis to investigate the importance of particular amino acids in protein structure and function. Outline how changing one amino acid in the β-globin polypeptide of haemoglobin may change the structure and function of a molecule of haemoglobin.[4]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

[1] translation;
[3] A - messenger RNA / mRNA ;
B - transfer RNA / tRNA ;
C - (poly)peptide (chain); I PPC/(primary structure of) protein / amino acid chain
[1] accept names of bases D – AAA E - GUG;
[4] any four from: structure to max three
ignore all refs to DNA and red blood cells
1 different amino acid will have different, R group / side chain; A glutamic acid / glu, to, valine / val
2 may prevent formation of bond(s) between R groups ;
3 may form, new / different, bond(s) between R groups;
4 change in, secondary / 2° / tertiary / 3° / quaternary / 4°, structure ; A change in, globular shape / 3D shape A different folding of, ẞ-globin / polypeptide / haemoglobin R quaternary if referring to, a single polypeptide / ẞ-globin
5 any further detail on, 2° / 3° / 4°, structure ; e.g. interactions between a-globin and ẞ-globin may change / AW
6 AVP; e.g. change from, polar / charged / hydrophilic, to, non-polar / non-charged / hydrophobic (R group / amino acid)
function
7 change in ability, to bind / AW, to oxygen / carbon dioxide ; A lower / higher for change A change in percentage saturation of haemoglobin with oxygen A change in affinity for, oxygen / carbon dioxide A changes the volume of, oxygen / carbon dioxide, carried A carry, more / less, oxygen / carbon dioxide R if haemoglobin is said to have an active site

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