Download 8.6 Gene Expression and Regulation

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
•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
•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
•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
•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- intervening sequences of bases within genes that are not part of the
code for a protein
Exons- 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.