8 Gene Regulation
Learn how regulatory DNA, regulatory proteins, and chromatin control gene expression in prokaryotic and eukaryotic cells.
Why cells regulate genes
is the control of when, where, and how much a gene is expressed. It helps cells respond to environmental conditions, conserve resources, and produce proteins suited to particular cell types and functions.
A major control point is transcription: regulatory proteins interact with particular DNA sequences to increase or decrease RNA production. Eukaryotic cells can also regulate expression through structure and processes that occur after transcription.
DNA sequences and regulatory proteins
A is a DNA region where RNA polymerase and associated proteins assemble to begin transcription. Other regulatory sequences influence whether transcription proceeds and at what rate:
An operator is a DNA site commonly found in bacterial operons. A repressor bound to an operator can block transcription.
An binds activator proteins and increases transcription. In eukaryotes, DNA folding can bring a distant near the it regulates.
A silencer binds regulatory proteins that reduce transcription, for example by interfering with activators or transcription machinery.
Regulatory DNA sequences are : they affect genes on the same DNA molecule. Proteins such as activators and repressors are : they can be made elsewhere in the cell and diffuse to their binding sites. Gene expression depends on the combination of regulatory proteins and DNA sites.
Takeaway: DNA sequences provide binding sites, while regulatory proteins act on those sites to adjust transcription.
Prokaryotic : operons
In bacteria and archaea, genes with related functions are often grouped into an . A single controls transcription of several genes into one RNA molecule, allowing the genes to be regulated together. Repressors reduce transcription; activators help RNA polymerase bind or function effectively. Small molecules can alter these regulators in response to conditions inside the cell.
The is an inducible system containing genes needed to use lactose. When lactose is absent, the Lac repressor binds the operator and prevents substantial transcription. When lactose is available, some is converted to . binds the repressor, causing it to release the operator so transcription can occur.
The is expressed most strongly when lactose is available and glucose is scarce. Low glucose raises cAMP levels. cAMP binds CAP, an activator that helps RNA polymerase initiate transcription. Lactose therefore relieves repression, while low glucose promotes activation. This arrangement favors lactose use when the preferred fuel, glucose, is limited.
The provides a contrasting, repressible system. Its genes help the bacterium synthesize tryptophan. When tryptophan is scarce, the is transcribed. When tryptophan accumulates, it binds the repressor and enables the repressor to bind the operator and shut down transcription.
Takeaway: The is turned on by the presence of lactose and is promoted by low glucose; the is shut down when tryptophan is abundant.
Eukaryotic regulation and
Eukaryotic genes are generally regulated individually rather than grouped into bacterial-style operons. At a , general transcription factors help recruit RNA polymerase. Other, gene-specific transcription factors bind nearby regulatory sites or distant enhancers and silencers. Activators bound to enhancers can communicate with a through DNA looping and protein complexes, helping adjust transcription in response to signals or in particular cell types.
Gene expression also depends on how accessible DNA is within . DNA is wrapped around histone proteins, and changes to histones or DNA can alter access to genes and influence transcription. DNA methylation commonly correlates with reduced expression, although its effects depend on context.
Regulation can continue after RNA is made. RNA processing, RNA stability, and translation provide additional ways to control how much protein is produced.
Takeaway: Eukaryotic gene expression is shaped by transcription factors, DNA accessibility, and processes after transcription.