AP Biology · Lesson 3

Regulating gene expression

Know Know it

Every cell in your body has nearly the same DNA, yet a nerve cell and a liver cell make very different proteins. The difference is gene expression: which genes each cell transcribes and translates, and how much. Regulating expression saves energy and lets cells respond to change and take on special jobs.

In bacteria, genes for one pathway often sit together in an operon: a promoter, an operator and the genes, all copied into one mRNA. A repressor protein, made from a separate gene, can bind the operator and block RNA polymerase. In an inducible operon, the repressor is active on its own, so the genes are normally off; a small molecule called an inducer, often the food the enzymes break down, binds the repressor and releases it, so the genes turn on. In a repressible operon, the repressor is inactive on its own, so the genes are normally on; when the pathway's product builds up, it binds the repressor and activates it, so the genes turn off.

In eukaryotes, transcription factors bind DNA at the promoter and at more distant enhancers. A gene is transcribed strongly only when the right combination of factors is present, so the set of factors a cell has decides which genes it uses. Cells become different during development, called differentiation, largely by turning on different factors in response to signals.

The packing of DNA matters too. Histone acetylation loosens chromatin and usually raises transcription. DNA methylation usually silences genes. These epigenetic changes alter expression without changing the DNA sequence and can be passed to daughter cells. After transcription, alternative splicing, mRNA breakdown and small regulatory RNAs that block or destroy particular mRNAs also adjust how much protein is made.

Trap Ask what the small molecule does to the repressor

Do not decide on and off from the sugar or amino acid alone. Ask whether the molecule turns the repressor off (inducer: genes on) or turns it on (corepressor: genes off). Then follow the repressor: bound to the operator means transcription is blocked.

Worked example Easy

A skin cell and a muscle cell from the same person make different proteins. Which statement best explains this?

  1. The two cells contain different genes
  2. The two cells express different sets of the same genesAnswer
  3. Muscle cells lose the genes they do not use
  4. Skin cells have mutations in their muscle genes

How to solve it

  1. Body cells come from one zygote by mitosis, so they have the same DNA.
  2. What differs is which genes are transcribed and translated.
  3. So the difference is in gene expression, not in the genes themselves.

Why each choice is right or wrong

  • A. Incorrect. The two cells came from one fertilized egg by mitosis, so they hold the same genes, not different ones.
  • B. Correct. The cells share a genome but turn on different genes, so they make different proteins.
  • C. Incorrect. Muscle cells keep their unused genes; those genes are turned off, not lost from the DNA.
  • D. Incorrect. Skin cells do not need mutations to differ; their muscle genes are silent, not damaged.

Explain Explain it

Task verbs: predict, explain

To predict how an operon behaves, write a short chain: is the small molecule present, is the repressor active, is the operator blocked, are the genes transcribed. For a mutant, change one link in the chain and follow it to the end. Then explain by naming the link that changed.

Worked example Medium

A fictional bacterium has an inducible operon for breaking down the sugar ribitol. Ribitol binds the repressor and releases it from the operator. In a mutant, the operator is changed so the repressor cannot bind it. How is the operon expressed in the mutant?

  1. Off with or without ribitol
  2. On only when ribitol is present
  3. On only when ribitol is absent
  4. On with or without ribitolAnswer

How to solve it

  1. Normally, without ribitol, the active repressor binds the operator and blocks transcription.
  2. In the mutant, the repressor cannot bind the changed operator.
  3. So nothing blocks RNA polymerase, and the genes are on with or without ribitol.

Why each choice is right or wrong

  • A. Incorrect. Off always would fit a repressor stuck on the operator, but here the repressor cannot attach to it at all.
  • B. Incorrect. On only with ribitol is the normal pattern; in the mutant the repressor no longer blocks even without ribitol.
  • C. Incorrect. On only without ribitol is the pattern of a repressible operon, and ribitol cannot turn this one off.
  • D. Correct. The repressor can never sit on the operator, so RNA polymerase is never blocked and the genes are always transcribed.

Predict Your turn

A fictional bacterium has a repressible operon of genes for making the amino acid serine. Serine binds the repressor and activates it. Predict what happens to transcription of these genes when serine is plentiful in the cell, and explain how this happens.

Write it, then compare. Nothing you type here is saved or graded.

Compare with a model answer

A full answer does this

  • Predicts that transcription of the genes falls or stops when serine is plentiful
  • Explains that serine binds the repressor, which then binds the operator and blocks RNA polymerase from transcribing the genes

Model answer

When serine is plentiful, transcription of the genes falls and stops. Serine binds the repressor and activates it. The active repressor binds the operator, which sits between the promoter and the genes, and blocks RNA polymerase, so the genes for making serine are not transcribed. The cell stops spending energy making serine it already has.

Apply Apply it

Practice these topics. AP Practice questions on exactly what this lesson covers, with an explanation for every answer.

Practice these topics