A-LevelBiologyNucleic acids and protein synthesis9700/21 · Oct/Nov 2023 · Q211 Marks

Fig. 2.1 is a transmission electron micrograph showing a section of a specialised epithelial cell found in the lining of the stomach. This cell produces extracellular proteins that are released into the bloodstream. [Fig. 2.1]

(a)Outline the role of structure X and structure Y, as shown in Fig. 2.1, in the production of extracellular proteins. structure X structure Y[2]
(b)(i)State the level of protein structure shown in Fig. 2.2.[1]
(b)(ii)State one similarity between the structure of a saturated fatty acid molecule and an amino acid molecule.[1]
(b)(iii)The addition of the fatty acid chain allows ghrelin to function as a cell signalling molecule. Suggest how the addition of this fatty acid chain allows a ghrelin molecule to act as a cell signalling molecule.[3]
(c)(i)Scientists have discovered that the gene coding for ghrelin contains 5000 base pairs. This is a much larger number of base pairs than is needed to code for the ghrelin molecule shown in Fig. 2.2. Calculate the percentage of base pairs found in the gene that codes for the ghrelin molecule shown in Fig. 2.2. Show your working and give your answer to one decimal place.[2]
(c)(ii)The primary transcript produced from the ghrelin gene is a longer molecule than the mRNA found in the cytoplasm. Explain how the primary transcript is modified before translation.[2]

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 2023). The mark or mark type is shown in brackets.

[2] X (nucleolus) synthesis / AW of, ribosomal subunits / ribosomes ;
A synthesis of organelles that are the sites of protein synthesis
Y (chromatin) contains gene(s), coding for the polypeptide(s) / AW ;
A site, of transcription / for production of mRNA
[1] primary ;
[1] any one from:
carboxylic acid / carboxyl, group ;
A both have C=O
contain carbon / hydrogen / oxygen ;
[3] any three from:
ref. to role in / forms part of, overall 3D shape;
A tertiary/quaternary structure
A forms part of hydrophobic region of ghrelin
allows ghrelin to bind to receptor ;
allows, ghrelin / part of ghrelin, to have a complementary shape / be complementary (to the receptor) ;
A description
suggested detail; e.g. interacts with hydrophobic region of the receptor molecule (for binding)
AVP; e.g. help interaction with / embeds in, hydrophobic portion of the cell surface membrane in context of bringing a complementary part of the molecule closer to receptor for binding
suggestion that fatty acid chain allowed, incorporation into / attachment to, a transport molecule in plasma to reach target cell
ref. to effect of binding triggering reactions inside the cell;
[2] 1.8%;
working; 30 × 3 × 100
5000
codons for 29 amino acids (including start met) and a stop codon
allow 1 mark if only 28 or 29 instead of 30, so 1.7% answer obtained
[2] any two from:
introns /non-coding regions, removed / AW ;
exons / coding regions, are joined together ;
gene splicing / RNA splicing; R DNA splicing
AVP; e.g. guanine (cap) added (to 5' end)
poly A tail / many As, added (to 3' end)