Regulating Gene Expression (College Board AP® Biology) : Study Guide

Introduction to regulating gene expression

• The genes expressed by a cell determine a cell's structure and function; this means that it is essential for cells to regulate their gene expression

• There are several ways in which gene expression can be regulated, including:

  • regulatory proteins and regulatory sequences

    • Regulatory proteins: proteins that bind to specific base sequences in DNA in order to switch gene expression on or off, e.g. transcription factors

    • Regulatory sequences: sequences of DNA to which regulatory proteins bind, e.g. promoters

  • epigenetic changes: reversible modifications to DNA or histones, e.g.:

    • methylation of DNA: methyl groups can be added to DNA, reducing transcription rates

    • acetylation of histones: acetyl groups are added to histones, increasing transcription rates

  • small interfering RNA (siRNA): single-stranded molecules of RNA that bind to complementary regions of mRNA, preventing them from being translated

Coordinate regulation in prokaryotes

• Some genes are coordinately regulated, meaning that groups of genes are switched on or off at the same time in response to a single signal

• Coordinately regulated genes in prokaryotes are grouped in clusters known as operons

  • Operons may contain several genes that code for proteins that work together to perform a single function

Inducible system example: the lac operon

• The lac operon in bacteria controls the expression of genes that code for the enzymes in lactose digestion

  • Expression is switched on and off in response to the presence/absence of lactose

• The lac operon is an example of an inducible system, meaning that its expression is switched off unless it is induced by the presence of an inducer molecule; in this case lactose

• The lac operon contains several components

  • Structural genes code for proteins that are required for cellular function; in this case for the enzymes that digest lactose

  • Regulatory genes code for regulatory proteins that switch the expression of structural genes on and off

  • Regulatory sequences to which regulatory proteins can bind

    • Promoters are regions of DNA to which transcription enzymes bind

    • The operator is a region of DNA to which regulatory proteins bind, influencing transcription

In the absence of lactose:

• When lactose is absent the regulatory gene is transcribed and translated to produce a lac repressor protein

• The lac repressor protein binds to the operator region upstream of lacZ

• RNA polymerase is unable to bind to the promoter

• Transcription of the structural genes does not take place and no enzymes are synthesized

In the presence of lactose:

• When lactose is present it binds to the repressor protein, preventing it from binding to the operator

• RNA polymerase binds to the promoter and transcription is initiated

• Lactose digestion enzymes are produced and lactose can be broken down

Repressible system example: the trp operon

• The trp operon regulates the expression of the enzymes needed for the production of the amino acid tryptophan (trp)

  • Expression is switched on and off in response to the presence/absence of tryptophan

• It is an example of a repressible system; it is switched on unless the product for which it codes is already present at high concentrations

In the absence of tryptophan:

• When tryptophan is absent the trp repressor protein is unable to bind to the operator

• RNA polymerase binds to the promoter and initiates transcription of the enzymes needed for tryptophan production

• Tryptophan is synthesized

In the presence of tryptophan:

• Tryptophan binds to the trp repressor protein, changing its shape and enabling it to bind to the operator

• This prevents RNA polymerase from binding to the promoter and expression of the enzymes needed to produce tryptophan is switched off

  • This means that energy is not wasted producing tryptophan when it is already available

Transcription factors

• Transcription factors are proteins that bind to promoter regions on DNA to affect gene expression

  • The action of transcription factors affects the phenotype of organisms

• The binding of a transcription factor to the promoter can either aid or inhibit the binding of RNA polymerase to the start of a gene, therefore influencing whether or not transcription is initiated

  • Transcription factors that aid the binding of RNA polymerase are known as activators

  • Transcription factors that prevent the binding of RNA polymerase are known as repressors

Coordinate regulation in eukaryotes

• In eukaryotes, groups of genes may be influenced by transcription factors to regulate expression in a coordinated manner

  • In contrast to the operons of prokaryotes, these genes may be located on different chromosomes