A-LevelBiologyNucleic acids and protein synthesis9700/42 · Oct/Nov 2022 · Q613 Marks

The lac operon of prokaryotes contains a group of structural genes that are under the same control and are transcribed together. Another operon found in prokaryotes is the trp operon. Fig. 6.1 summarises the structure and control of the trp operon. [Figure 6.1]

(a)(i)Describe the differences in structure and control between the lac operon and the trp operon.[3]
(a)(ii)Suggest why structural genes in operons are transcribed together.[1]
(a)(iii)trpA is an example of a structural gene and trpR is an example of a regulatory gene. Describe the differences between the functions of structural genes and regulatory genes.[2]
(a)(iv)trpA codes for the enzyme tryptophan synthase. Tryptophan synthase catalyses the formation of the amino acid tryptophan. Explain why tryptophan synthase is an example of a repressible enzyme.[2]
(b)(i)Control of gene expression in eukaryotes is more complex than in prokaryotes. In plants, the control of gene expression can involve plant hormones, such as gibberellin, and proteins known as JAZ and MYC. Describe how gibberellin activates genes in plant cells.[4]
(b)(ii)Transcription of some plant genes is prevented when JAZ proteins bind to other proteins known as MYC. When JAZ proteins are broken down, MYC proteins are free to bind to DNA. This allows transcription to begin. State the term that is used to describe proteins such as MYC proteins.[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.

[3] any three from:
1 trp operon regulatory gene / trpR, codes for an inactive repressor and lac operon regulatory gene / lacl, codes for active repressor ;
2 lac operon does not have an attenuator ; ora
3 lac operon has fewer structural genes or 3 vs 5 structural genes ;
4 lac operon uses, an inducer / (allo)lactose and trp operon uses, a repressor / tryptophan;
5 (allo)lactose, binds / inactivates, repressor, so repressor, leaves / cannot bind to, operator and tryptophan, binds / activates, repressor, so repressor can bind to operator ;
6 (allo)lactose causes genes to be, transcribed / switched on / expressed and tryptophan causes genes to be not, transcribed / switched on / expressed ;
[1] any one from:
1 (share) one promoter ;
2 all, enzymes / proteins / products, work together ;
[2]
structural genes: code for, enzymes / structural proteins / non-regulatory proteins / rRNA / tRNA ;
regulatory genes: code for, proteins / products, that control, gene expression / transcription ; A code for, transcription factors / repressor proteins
[2] any two from:
1 end-product inhibition / negative feedback / feedback inhibition ;
2 tryptophan, binds to / activates, repressor or tryptophan allows repressor to bind to operator ;
3 tryptophan, stops / reduces, trpA or gene transcription / trpA or gene expression / protein synthesis / tryptophan synthase being made ;
[4] any four from:
1 (gibberellin) binds to receptor ;
2 ref. enzyme ;
3 (enzyme causes) DELLA breakdown ;
4 DELLA no longer, binds to / inhibits, transcription factor / PIF;
5 (so) transcription factor / PIF / RNA polymerase, binds to, DNA / promoter ;
6 (growth) genes, switched on / expressed / transcribed ;
[1] mark first answer
transcription factor(s);