Controlling Gene ExpressionPart A: In Prokaryotes

Control Mechanismsin Prokaryotes

• not all proteins are needed by all cells at all times, so gene regulation is important to an organism's survival

Prokaryotic Cells• use operons to control gene expression • an operon is:

– one promoter region – an operator – a cluster of genes that follows

EXAMPLE 1 LAC Operon • lactose is a disaccharide found in milk or milk

sugars • bacteria cells metabolize lactose (into glucose and

galactose) to generate energy • The genes that regulate the metabolism of lactose

are only switched on when lactose is present.

lac operon• the lac operon contains:

– a promoter – an operator – 3 genes – a repressor protein

• lactose is the signal molecule, in this case it is an inducer

lac operon animation

• Narrated animation: http://www.sumanasinc.com/webcontent/

animations/content/lacoperon.html

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Your Turn

» Trp Operon - read pages 333 & 334 » summarize how this operon works

trp operon• tryptophan is an amino acid needed for protein

synthesis and is constantly required. • intestinal bacteria cells can obtain tryptophan from a

mammalian diet, or they can synthesize it themselves

trp operon• the trp operon contains:

– a promoter – an operator – 5 genes (code for 3 enzymes needed for the

biosynthesis of tryptophan) • trp repressor protein • tryptophan is a signal molecule, in this case it is a

corepressor

trp operon animation

• Narrated animation: http://bcs.whfreeman.com/thelifewire/

content/chp13/1302002.html

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Create a VENN diagram comparing the lac and trp operon.

Controlling Gene ExpressionPart B: in Eukaryotes

Transcriptional Regulation• dissociating DNA and histones to

allow access to promoter by transcription factors – Activator molecules/regulatory

region remodeling histones – addition of acetyl groups

(exposes promoter sites)

• methylation

Agouti Mice

Translational

• changes in length of poly(A) tail can increase or decrease the length of time for translation