Transcript GENE EXPRESSION - Doctor Jade Main

REGULATION of GENE
EXPRESSION
GENE EXPRESSION
• all cells in one
organism contain
same DNA
• every cell has same
genotype
• phenotypes differ
• skin cells have
different structure &
function from muscle
cells
GENE EXPRESSION
• differences -due to
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differences in gene
expression
some genes are turned
on
others are turned off in
different cells
functionally eliminates
particular cell from doing
certain functions
cell cannot make proteins
needed to do certain
functions
GENE EXPRESSION
• expression of most genes is
controlled at transcription
• some genes are actively
transcribed
• others remain quiescent
• some function at all times
• 30,000 are expressed in
nearly all cell types
• housekeeping genes
– carry out basic metabolic
processes
• called constitutive
• other genes are regulated
– turned on or off as
needed
Transcription Factors
• proteins which bind to
promoter & enhancer regions
of DNA to turn on (or off)
genes
• ability to be turned on is
inducible
• ability to be turned off is
repressible
• genes are most often
regulated as a group
• located next to one another
on a chromosome
• these genes along with their
regulatory sequences of DNA
are called an operon
The Lac Operon
• E. coli cells
– use different sugars for
energy
– glucose & lactose
– ability to use lactose
requires special
enzymes
– transacetylase
– lactose permease
– beta-galactosidase
• genes for these enzymes
are found on a single
unit-operon
The Lac Operon
• tells cell machinery to
make or not to make
enzymes
• Consists of genes that
make enzymes , promoter
& operator-control
sequences
– promoter region
• transcription
enzyme-RNA
polymerase attaches
• begins transcription
– operator
• functions as switch
• determines if RNA
polymerase can
attach to promoter
region
Lac Operon
• transcription of 3 enzymes is
repressed-turned off by
repressor protein
– binds to operator
– blocks attachment of RNA
polymerase
– regulatory gene located
outside operon codes for
repressor
• regulatory gene is expressed all
the time
• if regulatory gene is always
being transcribed
• there is always repressor
protein to stop transcription of
enzymes needed to use lactose
• How is lac operon turned on?
• lactose in environment
Lac Operon
• lactose binds to repressor
protein changes its shape.
• new shape means it cannot bind
to active site of operatorsite
is turned on
• RNA polymerase attaches
• transcription of enzymes
needed to metabolize lactose
begins
• genes that code for enzymes
that lets cell use lactose are
made only when lactose is
present
• induction
– presence of a small molecule
causes enzymes to be made
trp operon
• bacteria
• repressor-inactive alone
• to be active combines with specific
small molecule
• that small molecule is amino acidtryptophan
• E. coli can make tryptophan using
enzymes in trp operon but if
tryptophan do not make their own
• tryptophan binds to repressor
• activates repressor
• turns off operon
• when tryptophan is not present
repressor is not active operon is
turned ontryptophan is made
Repressor Operon
• arginine is an essential
amino acid
• when plentifule. coli
cells use it
• arginine not presente.
coli must make it
• requires enzymes
• mechanism allows e. coli
cells to save cellular
resources by shutting
genes off for particular
substance when
substance is available
Gene Regulation in Eukaryotes
• cells differ in appearance &
function
• inherit same, complete set
of genetic information
• differences in appearance &
function is not due to
different genes
• differences due to genes
being turned on or off
• cells performing particular
functions are termed
specialized
• during development cells
differentiate & stay
differentiated
• terminally differentiated
Gene Expression-Eukaryotes
• begins at
chromosome level
• DNA in one
chromosome is
about 4 cm long
• entire amount can
fit into nucleus
because of way it is
packaged
DNA PACKAGING
• DNA helix is wound
around small proteinshistones
• DNA-histone complex
looks like beads on a
string
• each bead-nucleosome
• segment of DNA wound
around 8 histones
• short DNA segmentslinkers make up string
part between
nucleosomes
DNA PACKAGING
• beaded strings are
wrapped into tight
helical fibers
• which in turn are
coiled into
supercoils
• looping & folding
further compacts
DNA
DNA PACKAGING
• extreme packaging is important
in gene regulation
• prevents gene expression by
preventing transcription proteins
from contacting DNA
• some regions-heterochromatin
• so condensed-never transcribed
– 10% of genome
• remainder of complexeuchromatin
• less condensed
• can be transcribed
• 10% is active at any given time
Fine Control of Transcription in
Eukaryotic Cells
• fine tuning is done with control of
RNA synthesis-transcription
• most important way of regulating
gene expression
Control of Transcription in Eukaryotic
Cells
• regulatory proteins bind to
DNA to turn transcription of
genes on & off
• each eukaryotic gene has its
own promoter & other control
sequences
• Activator proteins are more
important in eukaryotic cells
than in prokaryotic cells
• in most eukaryotic organisms
genes are turned off
• small percentage of genes
must be turned on for any one
particular cell to make
proteins required to carry out
its particular job
Control of Transcription in Eukaryotic
Cells
• regulatory
proteins in
eukaryotic cells
are transcription
factors
• required for RNA
polymerase to
transcribe DNA
Control of Transcription in
Eukaryotic Cells
• first step in gene transcription is
binding of transcription factors to
DNA sequences-enhancers
– usually far away from genes
they regulate
• binding of activators to enhancers
causes DNA to change shape
• it bends
• with bending bound activators can
interact with transcription factor
proteins which act as a complex at
promoter area of gene
• this complex promotes attachment
of RNA polymerase to promoter
transcription begins
• there are also repressor proteinssilencers
• inhibit transcription
Splicing & Regulation
• transcription of
DNA  mRNA
• used to make a
specific protein by
translation
• mRNA can be
regulated by
splicing
Splicing & Regulation
• during splicing certain
segments of RNA are
eliminated
• the way a piece of
mRNA is spliced giving
rise to different types
of mRNA
• gives rise to different
proteins
Regulation of Translation
• after mRNA has been fully processed
and is in the cytoplasm other
regulatory processes may occur
• mRNA breakdown
• initiation of translation
• protein activation
• protein breakdown
mRNA Breakdown
• mRNA molecules do
not stay intact forever
• broken down by
enzymes
• time of breakdown is
important
• regulates amount of
protein that is made
• longer living mRNAs
can make more
protein
Initiation of Translation
• many proteins control
initiation of translation
of RNA
• in red blood cells,
translation does not
occur unless heme is
present
Protein Activation
• after translation is
complete proteins
often need altering
to become
functional
• many made as
proenzyme
• Inactive
• cleaving part of
protein makes it
functional
Protein Breakdown
• proteins can be
broken down after
a short or after a
long time
• broken down after
short timehave
limited time to
carry out functions
• may be important in
short term regulatory
activity in cells