Controls Over Genes

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

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)

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

CAP

TailNew Transcript

Result of TranscriptionResult of Transcription

mRNA Transcript

•mRNA leaves the nucleus through its pores and goes to the ribosomes

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

Why Control?

Transcribing genes that are not needed is a waste of energy and may interfere with the status of the cell

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)

Regulation

Developmentally regulated Multicellular organisms progress

through developmental stages Different genes expressed at different

times during developmentCell specialization

Different genes expressed in different cells

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

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

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

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

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

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

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

gene 1 gene 2 gene 3

lactose operon

regulatory gene

transcription,translation

operator

operator

promoter

repressor protein

Figure 15.3aPage 241

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

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

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

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

Low Lactose

Repressor binds to operator

Binding blocks promoter

Transcription is blocked

Figure 15.3bPage 241

High Lactose

gene 1operator operatorpromoter

mRNA RNA polymerase

lactose

allolactose

Figure 15.3cPage 241

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

Homeotic Genes

Occur in all eukaryotesMaster genes that control

development of body partsEncode homeodomains (regulatory

proteins)Homeobox sequence can bind to

promoters and enhancers

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