Unit 3 - Gene Expression Flashcards
Gene regulation
involves turning genes on or off depending on their products r required
Regulators (2) types
r the proteins that switch genes on or of:
1) activators (gene on)
2) repressors (gene off)
Housekeeping genes
ALWAYS NEEDED
- constantly being transcribed/ translated (ssbp)
- most genes r not HK genes
Transcription
in the nucleus = product mRNA
Translation
in the cytoplasm = product protein
4 levels of control in gene expression
1) transcriptional
(mRNA being synthesized)
2) posttranscripional
(mRNA being made)
3) translational
( protein being synthesized)
4) posttranslational
(after protein has been synthesized
Transcriptional regulation
- regulation of which genes r transcribed
- can involve cortrol of the rate t occurs
a) controlling chromatin structure
b) enhancer regions on DNA
Chromatin Accessinilty
the structure of chromatin (DNA and its organizing proteins) can be regulated. More open or relaxed chromatin makes a gene more available for transcription (+ of acetyl group to histones loosens their association w DNA and promoter bc accessible)
Methylation transcriptional regulation
a methyl group is + to the cytosine bases in the promoter of a gene inhibiting transcription This silences the genes for a period of time
bone marrow
produce red blood cells, use specific enzyme to remove methyl group and allow transcription
Alternative splicing
perhaps 75% of human gens r alternatively splices at the pre-mRNA level
binding masking proteins to mRNA
when the mRNA is associated w a masking protein it does not undergo protein synthesis
regulation changes the rate of degradation of mRNAs
-regulatory mols such as a hormone, will directly or indirectly affect the rate of mRNA breakdown
ie. mammary gland of a rate it takes 5h for half of the mRNA for milk protein to break down and in the presence of the hormone prolactin time increases to 92h
mRNA transcription r translated into proteins
ie. length of Poly A tail increases or decreases how long translation take place but scientists r not really sure how this works
Posttranslational Regulation
controls when proteins bc fully functional, how long they r functional and there degradation (modifications made to the polypeptide chain)
ie. proteins go from inactive to active from processing mechansims (special chemical group can be added or removed from protein )
Prokarytotic; gene regulation
gene expression in P is regulated in response to the concentration of two mols:
- lactose
- tryptophan
Lac operon vs Trp operon
both - feedback
lac = uses a single mol (lactose ) that induces the expression of operon genes
trp = a single mol of (tryptophan) that represses the expression of the operon genes
Operon in prokaryotes
a cluster of genses under the contril of one set of regulatroy sequence
regulatory sequence (2)
1) promoter = the site where DNA transcription begins
2) operator = the sequence of bases that control transcription
Strucural genes
coding regions which code for the specific proteins
Operator sequence
regulator protein binds
- depending on the regulator, binding to the operator causes either activation transciption or repression transcription
effector of operator (3)
1) acts on the regulator protein
2) can be an inducer (stimulates)
3) can be a corepressor (inhibits)
Lac Operon (-) regulation
the sugar lactose, a potential source of energy from prokaryotes must be acquired directly from the environment. required for lactose metabolism, prokaryotes use lac operon
Lac Operon (3)
1) lactose = glucose + galactose
2) dissacharide in milk
3) e.coli: b-galactoside catalyses the cleavage of the bond in lactose
structure of lac operon
cluster of genes: lacZ, lacy, lacA
- each codes a different part of the enzyme
- all r under the control of one promoter
lacoste absent
negative control mechanism prevents production of B-galactosidase
Lacl Protein
repressor protein; blocks transcription of the lac (physical blockage; covers part of lac promoter)
mechanism of regulation (1) lac
when lactose is not present in the cell environment, the lacl protein binds to the lac operator coverint part of the promoter blocking transcription
mechanism of regulation (2) lac
lactose binds to the Lacl protein, changing its shape, it can no longer bing to the lac operator and transcription proceeds
Lactose absent
= no transcription
Lactose is present
- lactose binds to Lacl protein
- Lacl changes conformation = unable to bind to lac operator
- RNA polymerase can access lac promoter
- transcription occurs
what is the effector mol
inducer
Inducer
a single mol that triggers the expression of an operon’s genes by deactivating the repressor protien
(-) control
transcription is off, due to binding of a repressor protein
- presence of an effector (inducer) removes the repress form operator region
transcription can now occur
Trp Operon (+) regulation
tryptophan is an important aa that is used to build proteins. Most prokaryotic cells are able to synthesize tryptophan independently, but they can also take it up directly if it is available in the environment. The operon that regulates the production of tryptophan in a cell is called trp operon
operon in trp + transcription
cluster of 5 genes encoding enzymes required for trp synthesis and transcription of operon is repressed when concentrations of trp in the cell r high
why is trp +
positive control mechanism prevents production of tryptophan, if tryptophan levels r high
Mechanism of regulation (1) trp
lack of tryptophan inactivates the repressor and transcription proceeds
Mechanism of regulation (2) trp
tryptophan acts as a corepressor and binds to the tryptophan repressor. the complex can now bind to the trp operator and transcription is blocked.
trp absent
trp operon is transcribed (default)
trp present
- trp itself will bind to the repressor protein
- conformational shape occurs
- allows repressor to bind to the trp operator = shuts off transcription
effector mol of trp
corepressor
corepressor
it binds to a repressor to activate it in order to repress transcription
positive control
transcription is on so presence of an effector (co-repressor) cause binding of the repressor to the operator so transcription does not occur
Positive vs Negative control
+ = transcription on allows the repressor to bind
- = transcription is off so removes repressor
Gene Regulation
describes any process that alters the rate of gene expression
operon
a cluster of closely related genes. they are all controlled by one set of regulatory sequences
promoter
DNA sequence to which RNA polymerase binds to being transcription
operator regulatory DNA sequence
DNA sequence to which a transcription factor binds in order to alter transcription = close to the promoter
