Download GENE REGULATION

Gene Regulation
Objectives: Understand how both
prokaryotes and eukaryotes control
the expression of genes
GENE REGULATION
Virtually every cell in your body
contains a complete set of genes
But they are not all turned on in
every tissue
Each cell in your body expresses
only a small subset of genes at any
time
During development different cells
express different sets of genes in a
precisely regulated fashion
Differentiation involves cell
specialization, in both structure and
function
Differentiation is controlled by turning
specific sets of genes on or off, NOT
by differences in DNA between cells
Copyright © 2009 Pearson Education, Inc.
GENE REGULATION
Gene regulation occurs at the level
of transcription or production of
mRNA
Occurs VERY differently in
prokaryotes and in eukaryotes
A given cell transcribes only a
specific set of genes and not
others
CENTRAL DOGMA
Genetic information always goes
from DNA to RNA to protein
Gene regulation has been well
studied in E. coli, especially
metabolic genes
When a bacterial cell’s food
changes it will alter the
manufacturing of the enzymes
necessary to metabolize that food
Prokaryotic Gene Regulation
An operon is a group of genes
under coordinated control in bacteria
–
Promoter sequence where RNA
polymerase binds
–
Operator sequence is where a
repressor can bind and block RNA
polymerase action
–
The actual gene/genes that make
an enzyme
Copyright © 2009 Pearson Education, Inc.
Prokaryotic Gene Regulation
Types of operon control
–
Inducible operon – usually OFF
– Takes advantage of an unusual
situation
– Active repressor binds to the
operator
– Inducer binds to and inactivates
the repressor
Copyright © 2009 Pearson Education, Inc.
OPERON
Regulatory
gene
Promoter Operator
Lactose-utilization genes
DNA
mRNA
Protein
Active
repressor
Operon turned off (lactose absent)
RNA polymerase
cannot attach to
promoter
DNA
mRNA
RNA polymerase
bound to promoter
Protein
Lactose
Inactive
repressor
Operon turned on (lactose inactivates repressor)
Enzymes for lactose utilization
Prokaryotic Gene Regulation
Types of operon control
–
Repressible operon – usually ON (trp
operon)
– Usually produces a necessary
product not encountered in
environment
– Repressor is initially inactive
– Co-repressor (tryptophan) binds to
the repressor and makes it active
–
For many operons, activators enhance
RNA polymerase binding to the promoter
Copyright © 2009 Pearson Education, Inc.
Left Page: Operon vehicles
Operons tend to either block RNA
Polymerase from binding to the DNA,
or help it.
Your job is to design a vehicle (and
name it) based off of an operon. You
must include the name, what the
vehicle looks like (4 colors), which
operon it is based on, what abilities
the vehicle has (based off of the
operon), and how you would convince
someone to buy it.
Eukaryotic Gene Regulation
In eukaryotic organisms like
ourselves there are several
methods of regulating protein
production
DNA/Gene packaging
Transciption controls (promoters,
activators, enhancers)
Alternative RNA splicing
DNA/Gene Packaging
Eukaryotic chromosomes undergo
multiple levels of folding and coiling,
called DNA packing
–
DNA is wrapped around proteins
– “Beads on a string” appearance
–
Supercoil is a coiling of the tight helical
fiber
DNA packing can prevent
transcription
Transcription Controls
Similar to prokaryotic gene regulation
Genes are controlled by regulatory
elements in the promoter region that act
like on/off or dimmer switches
Regulatory proteins that bind to control
sequences
– Transcription factors promote RNA
polymerase binding to the promoter
– Activator proteins bind to DNA
enhancers and interact with other
transcription factors
Enhancers
Promoter
Gene
DNA
Activator
proteins
Transcription
factors
Other
proteins
RNA polymerase
Bending
of DNA
Transcription
Alternative RNA Splicing
Eukaryotic DNA differs from
prokaryotic DNA in that the coding
sequences along the gene are
interspersed with noncoding
sequences
The coding sequences are called
EXONS
The non coding sequences are called
INTRONS (they INterfere with gene)
Alternative RNA Splicing
After the initial transcript is produced
the introns are spliced out to form the
completed message ready for
translation
Introns can be very large and
numerous, so some genes are much
bigger than the final processed mRNA
Alternative RNA Splicing
Exons can be spliced together in
different ways
This allows a variety of different
polypeptides to be assembled from
the same gene
Alternate splicing is common in
insects and vertebrates, where 2 or 3
different proteins are produced from
one gene – esp. for immune system
products
Exons
1
DNA
RNA
transcript
1
1
2 3
5
4
3
2
RNA splicing
mRNA
4
3
2
5
or
5
1
2 4
5
Differentiated Cells
Most differentiated cells retain a full
set of genes, even though only a
subset may be expressed
–
Plant cloning - A root cell can divide
to form an adult plant
–
Animal limb regeneration (i.e.
starfish)
–
De-differentiation followed by redifferentiation into specialized
cells
Copyright © 2009 Pearson Education, Inc.
Root of
carrot plant
Single
cell
Root cells cultured
in nutrient medium
Cell division
in culture
Plantlet
Adult plant
Left Page Assignment:
Answer the two questions below:
If all of the DNA in all of an individual’s
cells are the same, how is it possible to
have different cell types?
How is it possible that the cells lining
your stomach and the cells of your retina
(eyeball) have identical DNA in their
nuclei?
Pre-AP Bio
I will buy the class that
achieves the highest class
average on the test on Friday
donuts. So study hard!