Gene Control

Turning off and on

Gene Control

• Molecular mechanisms that govern when and how fast genes will be transcribed and translated. Not all genes are working all the time in all cells

Controls are triggered by…

• Programmed schedules of development (think of a fetus developing in the womb)

• In response to chemical conditions

• Receiving a signal, such as a hormone

Examples of controls

• Programmed schedule: the changes that take place in puberty

• Chemical conditions: Methylation of DNA often inactivates a gene; demethylation might turn it on later

• Acetylation; acetyl group attaches to a histone, loosens the wrap and makes it easier to transcribe

• Receiving a signal: a hormone triggers a response from a cell

What does the controlling?

• Mostly regulatory proteins

• Negative control: slows or stops gene activity (methylation)

• Positive control: turns on or enhances gene activity (acetylation)

Gene Control

• Is different in prokaryotes versus eukaryotes

• Less complicated in prokaryotes

• Often controlled by operons: one promoter and a set of 2 operators. Operator is a binding site for an oppressor.

Prokaryote Example

• Lac operon: controls the production of enzymes that digest lactose. Only turned on when lactose and present and glucose is absent.

• Glucose is the preferred sugar, only make provisions for lactose if it’s the only energy source available

Eukaryote examples

• All cells contain all genes. Many are basic cell function genes that operate regularly.

• Cells also differentiate by activating only certain genes

• For example, a neuron does not do the same thing as a cell in the pancreas

When do the controls happen?

• Page 242-243• Can occur during

transcription, in mRNA processing, in translation, and following translation

Types of Controls in Eukaryotes

• Homeotic genes: interact with others to control development

• X chromosome inactivation makes sure only one X is working in both sexes

• This is dosage compensation (females are mosaics)

More Examples

• Signaling: hormones in animals

• Protein hormones attach to a receptor outside the cell membrane and stimulate a chain of events known as the 2nd messenger system

More Examples

• Steroid hormones pass through the cell membrane to find its receptor inside

• An activator protein winds up next to the promoter for the targeted gene.

• A hormone always triggers a response

Loss of control

• Can mean cancer• Mutations in the

checkpoint genes of the cell cycle and/or repressor genes or enhancer genes lead to too much mitosis