Biology 9700 · AS & A Level · Gene control

Gene control — practice question

In prokaryotes, the lac operon is made up of a set of structural genes that are controlled by the same regulatory system and transcribed as a single unit. A further operon present in prokaryotes is the trp operon. Fig. 6.1 gives a summary of the organisation and control of the trp operon.
(a(i))[3]

Describe how the lac operon and the trp operon differ in both structure and control.

(a(ii))[1]

Explain why the structural genes in operons are transcribed together.

(a(iii))[2]

$trpA$ is a structural gene example and $trpR$ is a regulatory gene example. Describe how the roles of structural genes and regulatory genes are different.

(a(iv))[2]

$trpA$ codes for the enzyme tryptophan synthase. Tryptophan synthase catalyses the production of the amino acid tryptophan. Explain why tryptophan synthase is classed as a repressible enzyme.

(b(i))[4]

Regulation of gene expression in eukaryotes is more intricate than in prokaryotes. In plants, control of gene expression can involve plant hormones such as gibberellin, and proteins called JAZ and MYC. Describe how gibberellin activates genes in plant cells.

(b(ii))[1]

Transcription of some plant genes is blocked when JAZ proteins bind to other proteins known as MYC. When JAZ proteins are degraded, MYC proteins can bind to DNA. This allows transcription to start. State the term used to describe proteins such as MYC proteins.

Worked solution & mark scheme

This 13-mark question has a full step-by-step worked solution and mark scheme. One marking point: $trp$ operon specifies an inactive repressor; $lac$ operon specifies an active repressor

  • Full mark scheme, point by point
  • Step-by-step worked solution
  • Write your answer & get it marked instantly by AI