AP Biology · Free-response skill

Read a model: an operon that switches itself off

The move. Read a gene-regulation model, describe how it works, and predict and justify the effect of a mutation in one of its parts.

How to do it

  1. Read the model part by part. For each part, note what binds to it or what it makes, and under what condition.
  2. To describe what the model does, write it as a chain of cause and effect: the signal, the protein it changes, where that protein binds, and what happens to transcription.
  3. To predict the effect of a mutation, change only the part that is mutated and run the chain again. State the result first: genes on, off, higher or lower.
  4. To justify, name the part of the model that no longer works and say which link in the chain it breaks. Then add why the normal system is useful to the cell, if the question asks.

Practice prompt Try it

A fictional soil bacterium makes a small molecule called nutrin, which it needs to grow. Three enzymes, made from genes nutA, nutB and nutC, build nutrin from simpler molecules. The table models how these genes are controlled.

Model of the nut operon in a fictional soil bacterium

PartWhat it isWhat it does
PPromoter of the nut operonRNA polymerase binds here to transcribe nutA, nutB and nutC into one mRNA
OOperator, between P and nutAThe active repressor binds here and blocks RNA polymerase
nutA, nutB, nutCGenes of the operonCode for the three enzymes that build nutrin
nutRA separate gene with its own promoter, always transcribedCodes for the repressor, which is inactive unless nutrin is bound to it

Using the model, respond to each part.

  1. Nutrin is plentiful in the cell. Using the table, walk through each step that leads to nutA, nutB and nutC being switched off.
  2. In mutant 1, the operator is changed so the repressor cannot attach to it. Will nutA, nutB and nutC be transcribed in mutant 1 when nutrin is plentiful? Say why, tracing the steps in the table.
  3. In mutant 2, the repressor is changed so nutrin cannot bind to it. Will nutA, nutB and nutC be transcribed in mutant 2 when nutrin is plentiful? Say why, tracing the steps in the table.
  4. Why is it useful to the normal bacterium to stop transcribing nutA, nutB and nutC when nutrin is plentiful?

Write each part, then check yourself against the checklist. Nothing you type here is saved or graded.

Check yourself and see a model answer

Checklist

Our own checklist of what a full answer does, one line per point. Tick each one your response earns.

0 of 6 points

Model answer, annotated

(a) When nutrin is plentiful, nutrin binds the repressor made by nutR and activates it. The active repressor binds the operator, O, which sits between the promoter and nutA. Bound there, it blocks RNA polymerase from moving from the promoter into the genes, so nutA, nutB and nutC are not transcribed.

  • Point 1 Gives every link: nutrin activates the repressor, the repressor binds O, RNA polymerase is blocked.

(b) In mutant 1, nutA, nutB and nutC are still transcribed when nutrin is plentiful. Nutrin still activates the repressor, but the changed operator gives the repressor nowhere to attach, so nothing blocks RNA polymerase, and it keeps transcribing the genes.

  • Point 2 States the prediction first: the genes stay on.
  • Point 3 Names the broken link: the active repressor cannot bind the changed operator.

(c) In mutant 2, nutA, nutB and nutC are also still transcribed when nutrin is plentiful. The repressor is inactive unless nutrin is bound to it, and nutrin cannot bind this repressor, so it stays inactive. An inactive repressor does not bind the operator, so RNA polymerase is not blocked.

  • Point 4 Prediction: the genes stay on in mutant 2 as well.
  • Point 5 Explains why: no nutrin binding, so the repressor stays inactive and leaves the operator free.

(d) When nutrin is already plentiful, the bacterium does not need to make more. Switching off the genes stops it from spending ATP and amino acids to build three enzymes, and spending materials to make nutrin, that it does not need, so it can use those resources for growth.

  • Point 6 Ties switching off to saving energy and materials when nutrin is not needed.