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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