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Gene Regulation
Lac Operon
Eukaryotes use four primary ways to control the expression of genes.
• Transcriptional control – selection of genes to be transcribed and the rate of
transcription • Posttranscriptional control
– processing of mRNA and the rate at which mRNA leaves the nucleus.
• Translational control – how long mRNA remains in the cytoplasm
• Posttranslational control – additional processing that proteins require to be
functional or through feedback inhibition.
Oncogenes
• Genes that are responsible for signaling cells to divide - called proto-oncogenes
• When damaged cause uncontrolled cell division
• Carcinogen– Agents that cause cancer
(mutations)– include radiation, cigarette
smoke, and viruses
Normal Cells:Controlled growthContact inhibitionOne organized layerDifferentiated cells
Cancer Cells:Uncontrolled growthNo contact inhibitionDisorganized, multilayeredNondifferentiated cellsAbnormal nuclei
Mutagens• Spontaneous
– Alkylation– Oxidation– Etc
• Radiation– UV– Gamma, Beta, Alpha– Etc
• Chemical– Base analogue– Intercalating agents– Etc
Mutations
Gene Regulation in Prokaryotes
• In prokaryotes, genes must also be turned on and off.
• If genes are always expressed, then much energy is wasted. The organism will quickly become lethargic and very tired.
• An Operon consists of:– structural genes– promoter – Operator– Regulatory genes– Repressor and activator proteins
Repressible Operon - trp• repressible operon is ON by default. • regulatory genes produce inactive repressors. • end product of the pathway, tryptophan, interacts with the
inactive repressor (acting as a co-repressor), and causes a conformational change in the repressor.
• The active repressor is now able to bond to the operator and switch off transcription.
• Typical of anabolic pathways• http://bcs.whfreeman.com/thelifewire/content/chp13/13020
02.html• http://www.as.wvu.edu/~dray/219files/TrpAttenuation.mov
Inducible Operon - lac
Negative Regulation• Regulatory gene produces repressor• Repressor is bound to the Operator• Allolactose will bond to repressor protein• Repressor will be removed Operator• RNA polymerase can transcribe• http://bcs.whfreeman.com/thelifewire/content/chp
13/1302001.html• http://www.as.wvu.edu/~dray/219files/LacOperon.
mov
Lactose metabolism in E. coli
1. Transport lactose into the cell2. Break down lactose into glucose and
galactose
galactose glucose
lactoseGalactoside permease
E. Coli cell
Lactose metabolism in E. coli
• The enzyme Beta-galactosidase breaks down lactose into glucose and galactose
+Beta-galactosidase
Lac Operon Genes
• Operon: a group of genes that are commonly regulated and transcribed into mRNA
Name of the Gene Protein Product of the Gene
Lac I Gene Produces a Repressor Protein
Lac Z Gene Produces Beta-Galactosidase
Lac Y Gene Produces Galactoside Permease
Lac A Gene Produces Transacetylase
Role of lactose
• Lactose is called an inducer molecule because it signals the production of:
(a)Beta-galactosidase: the enzyme that cleaves lactose into glucose and galactose
(b)Galactoside permease: the transport protein that allows lactose to enter a cell
(c)Transacetylase: don’t worry about this one
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