Controls Over Genes
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Transcript of Controls Over Genes
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Controls Over Genes
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More on TranscriptionMore on Transcription
Promoters are regions on DNA that show where RNA Polymerase must bind to begin the Transcription of RNA
Called the TATA boxTranscription factors are also involved
(proteins that mediate the binding of RNA polymerase)
Specific base sequences act as signals to stopCalled the termination signal
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mRNA ProcessingmRNA Processing
After the DNA is transcribed into RNA, editing must be done to the nucleotide chain to make the RNA functional
Introns, non-functional segments of DNA are snipped out of the chain (RNA splicing)
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mRNA EditingmRNA Editing
Exons, segments of DNA that code for proteins, are then rejoined by the enzyme ligaseA guanine triphosphate cap is added to the 5” end of the newly copied mRNAA poly A tail is added to the 3’ end of the RNAThe newly processed mRNA can then leave the nucleus
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CAP
TailNew Transcript
Result of TranscriptionResult of Transcription
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mRNA Transcript
•mRNA leaves the nucleus through its pores and goes to the ribosomes
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Why Control Gene Expression?
Some genes are “on” (being transcribed) almost all the time Called housekeeping genes Examples: ribosome components,
enzyme for basic metabolic pathwaysMany genes are only turned on when
they are needed
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Why Control?
Transcribing genes that are not needed is a waste of energy and may interfere with the status of the cell
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Regulation
Respond to a range of stimuli Prokaryotes respond to external stimuli
(food, enzymes turned on) Eukaryotes also respond to internal
stimuli (hormones, growth factors)
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Regulation
Developmentally regulated Multicellular organisms progress
through developmental stages Different genes expressed at different
times during developmentCell specialization
Different genes expressed in different cells
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The strategy behind regulation..
Gene control is control over amount of gene produced (RNA or protein) in cell
Multiple ways to control the amount of gene product in a cell
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Controlling gene product amount
1. Rate of transcription – rate mRNA is produced; faster produced = more product
2. mRNA degradation – rate mRNA is broken down; faster broken down = less product
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Controlling gene product amount
3. mRNA processing – capping, splicing; slower processing = less product
4. Translation – rate of translation or # of ribosomes translating; fast/more = more product
Although control probably involves all of these, the most understood are changes in the rate of transcription
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Gene Control – lac operon
Lac operon is a gene in bacteriaBacteria have 3 genes in a row
(operon) that involve breaking down lactose for energy
In order to be efficient, these genes should not be expressed unless lactose is present
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Lac Operon - vocab
Regulatory protein – control transcription, translation, and gene products by interacting with DNA, RNA, or proteins
Repressor – protein that binds with an operator on prokaryotic DNA to prevent transcription
Operator – short base sequence between a promoter and genes; a binding site for repressors
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Lac operon – vocab.
Promoter – piece of DNA where RNA polymerase can bind and start transcription
Negative control – regulatory protein that slows down gene activity
Positive control – regulatory protein that enhances gene activity
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Lac operon vocab.
Operon – a promoter and a pair of operators that control a bacterial gene
Activator – protein that exerts positive control over an operon
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gene 1 gene 2 gene 3
lactose operon
regulatory gene
transcription,translation
operator
operator
promoter
repressor protein
Figure 15.3aPage 241
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Lac operon
Goal 1 – transcription low when lactose is absent
Lac I (gene upstream from operon) produces a repressor which binds to promoter region
Binding of repressor prevents RNA polymerase from binding and transcribing genes
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Lac operon
Goal 2 – increase transcription when lactose is present
Allolactose will bind to the repressor, changing its conformation and causing it to fall off the promoter site
Promoter site now available for RNA polymerase to bind; transcription of lac genes begins
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Lac operon
Goal 3 – turn off transcription when lactose is used up
Allolactose metabolizes, freeing up the repressor
The free repressor is available to bind the promoter site and stop transcription
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Control of lac operon
Negative control – glucose present - repressor inactivates the lac operon
Positive control – lactose present – activator protein (called CAP) makes promoter more favorable for RNA polymerase to bind and begin transcription
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Low Lactose
Repressor binds to operator
Binding blocks promoter
Transcription is blocked
Figure 15.3bPage 241
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High Lactose
gene 1operator operatorpromoter
mRNA RNA polymerase
lactose
allolactose
Figure 15.3cPage 241
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Most Genes Are Turned Off
Cells of a multicelled organism rarely use more than 5-10 percent of their genes at any given time
The remaining genes are selectively expressed
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Homeotic Genes
Occur in all eukaryotesMaster genes that control
development of body partsEncode homeodomains (regulatory
proteins)Homeobox sequence can bind to
promoters and enhancers
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X Chromosome Inactivation
In female mammals, in all cells one of the
two X chromosomes is completely
inactivated
Inactivation is random
Inactivated chromosome can be observed in the interphase nucleus as Barr body
Genes on the inactivated chromosome are not expressed