Changes in Purkinje cell firing and gene expression precede ...
Ch. 18 Regulation of Gene Expression · expression of the genes on the chromosomes is different for...
Transcript of Ch. 18 Regulation of Gene Expression · expression of the genes on the chromosomes is different for...
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Human genome has around 23,688 genes (Scientific American 2/2006)
Essential Questions:
How is transcription regulated?
How are genes expressed?
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Bacteria regulate transcription based on their environment
1. can adjust activity of enzymes already present
Ex. enz 1 inhibited by final product
2. Adjust level of certain enz. regulate the genes that
code for the enzyme
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Operon model the tryptophan example
The five genes that code for the subunits of the enzymes are clustered together.
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Grouping genes that are related is advantageous only need one "switch" to turn them on or off
"Switch" = the operator (segment of DNA)located within the promotercontrols RNA polymerase's access to the genes
operon = the operator, the promoter, and the genes they control
trp operon is an example in E.coli
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operon can be switched off by a repressor protein
binds to operator and blocks attachment of RNA polymerase to the promoter
trp repressor is made from a regulatory gene calledtrpR
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trpR has its own promoter
regulatory genes are expressed continuously:binding of repressors to operators is reversiblethe trp repressor is an allosteric protein has
active and inactive shapestrp repressor is synthesized in inactive form
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if tryptophan binds to trp repressor at an allosteric site, then becomes active and can attach to operator
in this case tryptophan is a corepressor a small molecule that cooperates with a repressor protein to switch operon off.
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Two types of negative gene regulation:
Repressible operons transcription is usually on, but is inhibited by the corepressor
Ex. Trp operon
Inducible operon transcription is usually off, but can be stimulated by when a corepressor interacts with a regulatory protein
Ex. lac operon
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Example of an inducible operon: lac operonif lactose absent regulatory gene lacI, located outside operon codes for allosteric repressor protein that switches off lac operon
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lac repressor is active by itself, binds to operator and switches lac operon off
to turn lac operon on, need an inducer (allolactose) the inactivates the repressor
the enzymes of the lac operon are inducible enzymes synthesis is induced by a chemical signal
usually function in catabolic pathways break down nutrient
repressible enzymes (ex. trp operon) usually function in anabolic pathways make products
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Positive gene regulationE. coli prefers glucose over lactose if both present
when glucose is scarce cyclic AMP (cAMP)a small organic molcule accumulates
CAP (catabolite activator protein) is a regulatory protein an activator binds to DNA and stimulates transcription
when cAMP binds to regulatory protein CAP becomes active shape and attaches upstream of
lac promoterhelps RNA polymerase bind
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How is eukayotic gene expression regulated?
All cells have the same genome (exception immune cells)
only 20% of genes are expressed at any given time
but get differentiated during developmentexpression of the genes on the chromosomes
is different for each differentiated cell = differential gene expressionEx. in a muscle cell a certain gene may be
turned on in a skin cell, same gene may be turned
off
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Only 1.5% of DNA is coding DNA for proteinsa small amount is used to make RNArest is "noncoding"
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control of gene expression in eukaryotic cells
each stage represents a place where regulation can happen
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Factors that affect transcription regulation:
Chromatin structure:1. if in heterochromatin genes are not
expressed
2. where promoter is in relation to nucleosomes and DNA
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3. chemical modification to histoneshistone acetylation acetyl group attached to pos charged Lysine in histone tailsif histone tails are acetylated, become neutral no binding with other nucleosomesgives chromatin a looser structuretranscription proteins have access to genesmay be involved in transcription factors
attaching to promoter site
methylation to histone tails can lead to condensation of chromatin
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4. DNA methylation
to cytosine after DNA synthesis
heavily methylated = genes not expressedEx. inactivated X chromosome in mammals
important for embryonic development to form specialized tissues
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5. epigenetic inheritance inheritance traits transmitted by mechanisms not involved in nucleotide sequence
Ex. chromatin modifications that affect gene expression in future generations of cells
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Regulation of transcription initiation:6. chromatin modifying enzymes make DNA more or less able to bind to transcription complex
7. interactions between enhancers (control elements far upstream from promoter) and activators (protein that binds to an enhancer and stimulates transcription)
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1. Activator proteins bind to distal control elements (enhancer)
2. DNA bending protein brings activators closer to promoter
3. activators bind mediator proteins and transcription factors to form initation complex
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8. Some transcription factors function as repressors inhibit expression of a genea. can block activatorsb. can bind to enhancer elements to turn off transcription even if activators are present
9. activators and repressors can influence chromatin structuresome activators get proteins that acetylate histones
near promoters to promote transcriptionsome repressors get proteins to deacylate histones
and reduce transcription = silencing
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Eukaryotic organisms usually don't have operons like bacteria, however some genes are coexpressed
found in clusters that have their own promoter and individually transcribed
some are found on different chromosomesexpression depends on a combination of elements that recognize control elements
and bind to them, so all genes are transcribed at the same time
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can be initiated by chemicals such as steroids (binds to an intracellular receptor protein) which then serves as a transcription activator
nonsteroid signal molecules that don't enter cell but bind to surface, use signal transduction pathways
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Mechanisms of post transcriptional regulationgene expression can be regulated at any post transcriptional step
1. RNA processing Alternative RNA splicing can produce different mRNA
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2. mRNA degradation prokaryotic mRNA degrades by enzymes in a few minutes
eukaryotic mRNA survives, hours, days or weeks
gets degraded by shortening polyA tail and removal of 5' cap
when cap removed nuclease enzymes can attack
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3. Initiation of Translation can regulate genes
during initiation stage
regulatory proteins bind to specific sequences within the untranslated region of the 5' end (5' UTR)
prevent ribosome attachment
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4. Protein processing and degradation to control gene expression
chemical modifications that make them functional
need to be transported to particular places to function
protein degradation ubiquitin attaches to protein, proteasomes recognize the ubiquitin and degrade them
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Noncoding RNAs can control gene expression
ex. small RNAs
occurs at two points:1. mRNA translation
2. Chromatin configuration
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MicroRNAs (miRNAs)= single stranded RNAformed from longer RNA precursors that fold back on themselves, forming short doublestranded hairpin structures held with hydrogen bonds
dicer = enzyme that cuts RNA hairpin outone strand is degraded, other (miRNA)forms a complex with proteinscan bind to mRNA with complementary
sequencecauses degration of mRNA or blocks
translation
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Remodeling Chromatin structure
siRNAs = small interfering RNAs used in heterochromatin condensing
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Regulation of gene expression during cell Differentiation
determination = the events that lead to the observable differentiation of a cell
once initiated embryonic cell is "committed to its fate"
happens in "tissue specifc proteins" found only in a cell type and give the cell is structure and function
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pattern formation spatial organization of tissues and organs
in animals begins in early embryo
due to positional information provided by cytoplasmic determinants and inductive
signals
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Cancer
Types of genes associated with cancer1. tumor viruses transform cells through integration of viral nucleic acid into host cell
Ex. Epstein Barr virus that causes mono has been linked to Burkett's lymphoma
Papilloma virus linked to cervical cancer
HTLV1 (retrovirus) causes adult leukemia
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2. Oncogenescancer causing genesnormal cellular genes = protooncogenes (code for proteins that stimulate cell growth and
division)can become oncogenes via:
a. movement of DNA within genomeb. amplification of a protooncogene
(more copies of a gene than normal)c. point mutations
1. within control element2. within the gene
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3. Tumorsuppressor genes code for proteins that normally inhibit cell division
any mutation that decreases the activity of these genes can cause cancer
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other functions of proteins made from these genes:1. repair damaged DNA
2. control adhesion to cells to each other or to extracellular matrix anchorage to cells is important
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3. regulate cellsignaling pathways that inhibit cell cycle
a. ras gene = G protein that relays a signal from growth factor on membrane to protein kinases
at end of pathway stimulates cell cyclewill not operate unless correct amt. of
growth factormutation leads to hyperactive ras gene = increased cell divisionmutated in 30% of human cancers
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b. p53 gene codes for tumorsuppressor protein that synthesize growth inhibiting proteins
in about 50% of human cancers
1. activates p which halts cell cycle
2. turns on genes involved in DNA repair
3.if cell is too damaged, signals "cell suicide" genes via apoptosis
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So how does cancer develop?
more than 1 somatic cell mutation is neededlonger we live the more chance for mutationshave done studies on colorectal cancer gradual developmentmost cancers must have at least:
one active oncogene mutation or loss of tumorsuppressor genes (need to block both alleles
recessive)gene for telomerase is activated lengthens the DNA
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Can cancer be inherited?
if an oncogene is inherited or mutants allele of a tumorsuppressor gene15% of colorectal cancers inherited mutations510% of breast cancers mutations in BRCA1 or BRCA2 gene
found in 1/2 of inherited cancerif inherits 1 mutant BRCA1 allele = 60% chance of getting breast cancer by age of 50
normal = 2% chancemutant alleles are recessivewild type protects against breast cancer