8.6 Gene Expression and Regulation KEY CONCEPT Gene expression is carefully regulated in both prokaryotic and eukaryotic cells.

8.6 Gene Expression and Regulation

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Gene expression

 - the process of a gene being transcribed and its product being synthesized Not all genes are expressed all the time  Genes are turned on/off by many factors  Varies in eukaryotes and prokaryotes •

Control in Prokaryotes

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What is a prokaryote

•Bacteria have some genes that run continuously and some that do not

8.6 Gene Expression and Regulation

•Jacob-Monod model   French biochemists that proposed hypothesis on bacterial gene regulation Genes on a chromosome are arranged in groups called operons 

Operons

- a group of genes w/ related functions   Contains genes that code for proteins- these genes are called

structural genes

Contains genes that promote the movement of RNA polymerase onto the structural genes called

promoter genes

 Contains a region of DNA that acts as an on/off switch for an operons called an

operator

  Operator can control movement of RNA polymerase from the promoter to the structural genes Protein called a

repressor protein

binds to the operator to turn it off

8.6 Gene Expression and Regulation



Regulator

is the part of the operons that continually makes a repressor protein • In the example below when tryptophan is present in high quantities the repressor is not activated • When tryptophan is scarce it no longer binds to the repressor and the repressor binds to the operons shutting it off

8.6 Gene Expression and Regulation

8.6 Gene Expression and Regulation

8.6 Gene Expression and Regulation

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Control in Eukaryotes

 Much more complex than on prokaryotes- more than one mechanism of control  Nearly every cell has full set of chromosomes   Because cells are differentiated only a few genes are actually functional- the rest are turned off Some may be on or off depending on stage of development or environmental factors •

Coiling of DNA

   When chromatin containing DNA is tightly coiled the DNA cannot be transcribed and the genes are therefore inactivated When chromatin uncoils RNA polymerase can get to the DNA and start matching base pairs Chemical such as growth factors and hormones may determine how tightly the DNA is coiled

8.6 Gene Expression and Regulation

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Methyl groups

 Methyl group is 1 carbon atom bonded to 3 hydrogen atoms   Tightly coiled chromatin has small clusters of atoms called methyl groups attached to DNA Methyl groups interfere with transcription of DNA •

Regulatory Proteins

 These proteins interact with operator sections of genes  Function to control gene expression  Different than in bacteria because structural proteins are not linked together in operons- they may be far apart or on different chromosomes •

Introns

 

Introns Exons

- intervening sequences of bases within genes that are not part of the code for a protein - the short sequences of DNA bases that actually code for the proteins  Are in between introns  Both are transcribed 

RNA splicing-

Before mRNA leaves nucleus the introns are removed leaving only the exons Different deletions may produce different proteins from the same gene

8.6 Gene Expression and Regulation Prokaryotic cells turn genes on and off by controlling transcription.

• A promotor is a DNA segment that allows a gene to be transcribed.

• An operator is a part of DNA that turns a gene “on” or ”off.” • An operon includes a promoter, an operator, and one or more structural genes that code for all the proteins needed to do a job.

– Operons are most common in prokaryotes.

– The

lac

operon was one of the first examples of gene regulation to be discovered.

– The

lac

operon has three genes that code for enzymes that break down lactose.

8.6 Gene Expression and Regulation

• The

lac

operon acts like a switch. – The

lac

operon is “off” when lactose is not present.

– The

lac

operon is “on” when lactose is present.

8.6 Gene Expression and Regulation Eukaryotes regulate gene expression at many points.

• Different sets of genes are expressed in different types of cells.

• Transcription is controlled by regulatory DNA sequences and protein transcription factors.

8.6 Gene Expression and Regulation

• Transcription is controlled by regulatory DNA sequences and protein transcription factors.

– Most eukaryotes have a TATA box promoter.

– Enhancers and silencers speed up or slow down the rate of transcription.

– Each gene has a unique combination of regulatory sequences.

8.6 Gene Expression and Regulation

• RNA processing is also an important part of gene regulation in eukaryotes.

• mRNA processing includes three major steps.

8.6 Gene Expression and Regulation

• mRNA processing includes three major steps.

– Introns are removed and exons are spliced together.

– A cap is added.

– A tail is added.