A-LevelBiologyNucleic acids and protein synthesis9700/21 · May/June 2022 · Q613 Marks

Cotransporter proteins are membrane proteins found in companion cells of phloem tissue. Messenger RNA (mRNA) is the molecule in cells that carries genetic information in the DNA that codes for cotransporter proteins to the sites of protein synthesis in the cytoplasm.

(a)Complete Table 6.1 to compare the structure of a molecule of mRNA with the structure of a molecule of DNA. Table 6.1 feature mRNA DNA names of four bases name of pentose sugar present number of strands[3]
(b)(i)Name stage 1 shown in Fig. 6.1.[1]
(b)(ii)Describe what happens at stage 2, shown in Fig. 6.1, to shorten the length of the RNA molecule.[2]
(c)(i)Describe the role of the transfer RNA shown in Fig. 6.2 in the synthesis of a cotransporter protein.[4]
(c)(ii)Outline the role of cotransporter proteins in companion cells.[3]

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 (May/June 2022). The mark or mark type is shown in brackets.

[3] one mark each correct row
[3] names of four bases mRNA: adenine, cytosine, guanine, uracil DNA: adenine, cytosine, guanine, thymine ;
[3] name of pentose sugar present mRNA: ribose DNA: deoxyribose;
[3] number of strands mRNA: 1; I single DNA: 2; I double
[1] transcription ;
[2] can award either mp1 with mp2,or mp3 with mp4
[2] 1 removal of introns (from primary transcript / RNA) ;
[2] 2 exons joined together;
[2] or
[2] 3 splicing;
[2] 4 of, primary transcript / RNA;
[4] any four from:
[4] 1 ref. to amino acid activation / charging amino acids before attachment to tRNA / AW ; A aminoacylation
[4] 2 tRNA attached to, specific amino acid / glycine ; R 'picks up'
[4] 3 idea that anticodon / CCG, identifies specific amino acid;
[4] 4 tRNA transports / AW, amino acid to ribosome ;
[4] 5 ref. to (tRNA) entering the A (aminoacyl) site in ribosome ;
[4] 6 tRNA, anticodon / CCG, binds to mRNA, codon / GGC ; A correct ref. to, complementary pairing / base pairing / H bond formation as AW for binding
[4] 7 (specific) amino acid / glycine, in correct position in the polypeptide chain / AW ; A helps to form / AW, primary structure of cotransporter protein
[4] 8 AVP; e.g. tRNA, recycled / reused forms aminoacyl tRNA / amino acid-tRNA complex tRNA-ligase / aminoacyl-tRNA synthetase
[3] any three from:
[3] 1 movement of sucrose with, protons / H+ ions, into companion cells ; A correct ref. to (other named) assimilates
[3] 2 (sucrose moved) from, apoplast / cell wall / mesophyll cell ;
[3] 3 sucrose moves against its concentration gradient (into companion cell);
[3] 4 (needs cotransporter protein because) sucrose is polar so cannot pass through membrane ;
[3] 5 maintain concentration gradient for sucrose between companion cell and sieve tube (element) ;