Chapter 23 Flashcards
Constitutive Genes
genes that are always on in every cell (important for regular cell functions)
Regulated Genes
get switched on and off based on what the cell needs
negative control
regulatory proteins stop transcription
positive control
regulatory proteins turn on transcription
operons
groups of genes with related functions driven by a single promoter; meaning one segment of DNA turns on the function in a cell
Inducible genes
genes are mostly off, and turned on when needed
which pathways are mostly inducible and which repressible
catabolic = inducible anabolic = repressible
repressible genes
genes mostly on and turned off when needs are met
5 levels of eukaryotic control
genomic, transcription, RNA processing and export, translation, and posttranslational
The trp operon
controls tryptophan synthesis;
repressed when trp levels are high
how does tryptophan function in trp operon
when there is enough trp, it binds to the repressor and activates it to repress the operon. when more trp is needed, it is removed and synthesis resumes
what type of control and what type of operon is trp operon
negative control and repressible operon
what is the lac operon
the 3 genes involved in lactose metabolism
Lac I
expressed constitutively and binds to operator to prevent transcription
what happens when lactose is present
lactose binds to repressor and removes it
what happens when glucose and lactose are present
glucose is preferred so it will be metabolized first but the presence of lactose releases the repressor so some lactose will be metabolized
what is cyclic AMP’s role
it reflects glucose levels so when it gets high cAMP activates the lac operon by binding to the cAMP regulatory protein
function of CRP (cAMP regulatory protein)
recruits polymerase to the promotor which increases the transcribing of the lac genes
high glucose means
low cAMP levels
what type of control is cAMP/CRP complex
positive because it activates transcription
what type of control is the lac operon
negative because the repressor blocks the polymerase
what kind of operon is the lac operon
inducible because it is only turned on when lactose is available
3 kinds of genomic control
gene amplification, gene deletion, chromatin structure (histones and inaccessible DNA)
gene deletion
loss of genetic info (like RBC)
gene amplification
copies of same gene
the histone code regulation
addition of methyl, acetyl, and phosphate groups to histones which alters chromatin structure (can activate or repress genes)
chromatin structure regulation that’s not histones
compact regions are inaccessible to transcription machinery (DNA methylation)
barr bodies
inactivated X chr
proximal control elements
binding sites for regulatory transcription factors that affect the initiation of transcription; located upstream but only 100-200 base pairs away from the promoter
distal control elements
function either upstream or downstream from promoter at highly variable distances; either silencers or enhancers
silencer
distal control element that inhibits transcription as a binding site for repressors
enhancers
distal control element that stimulates transcription as the binding site for activator proteins
transcription factor activation via phosphorylation
- CREB is bound to DNA near cAMP activated genes on the CRE
- cAMP facilitates phosphorylation of transcription factor CREB
- activated CREB recruits CBP which then remodels chromatin and recruits polymerase
what is CREB
a transcription factor
CBP
CREB-binding protein
CRE
cAMP response element
3 broad ways transcription is controlled
Proximal control elements, distal control elements, and mRNA is produced in different amounts in different cells
in what ways does RNA processing and transport control gene expression
alternative splicing, alternative poly-A sites, and export is dependent on several factors being present
what factors is RNA export dependent on
Proper processing (5’ cap and poly-A tail), or it may require a nuclear export signal
ways translation is controlled (6)
initiation factors, phosphorylation of eIFs, control elements, mRNA stability, RNA interference, miroRNAs
control elements
located in untranslated regions and regulate binding of transcription factors
initiation of transcription requires
several initiation factors (eIFs)
what does phosphorylation of eIFs result in
either activation or inactivation
where is iron response element found (IRE) that increases transcription
5’ UTR of ferritin in mRNA
how does IRE work without enough iron
IRE-binding protein binds to IRE in the absence of iron which represses translation
what happens with IRE when there is enough iron
iron binds to IRE-binding protein to activate it and starts translation
mRNA stability
if RNAs are around longer, they can make more proteins
factors affecting mRNA stability
length of poly-A tail, half life ranges from 30 min - 10 hrs, control elements in UTRs
mRNA stability example of IRE in 3’ UTR of transferriten receptor mRNA
iron binds to IRE-binding protein to inactivate it so the mRNA can be degraded; in the absence of iron, the IRE-binding protein binds to IRE which protects the mRNA from degradation
RNA interference (RNAi)
uses dsRNAs to target specific mRNAs for degradation or translation inhibition
RNAi pathway
- dsRNA is cleaved by Dicer into siRNAs
- siRNAs assembled into RISC and 1 strand is retained as the guide strand
- guide strand targets mRNA based on regions of homology
perfect homology
RISC degrades mRNA and targets new mRNA
imperfect homology
RISC remains bound and blocks ribosomes
microRNAs
they fold into hairpin structures and mimic dsRNA then hop into the RNAi pathway
3 kinds of posttranslational control
protein degradation, modification to protein structure and function, ubiquitin targets proteins for destruction
ubiquitin targets proteins for destruction
short chains of ubiquitin are added to lysine residues of proteins; recognition of proteasomes results in protein destruction
half-life of proteins
range from minutes to weeks
importance of protein half-life
proteins with shorter half-lives can respond to change more quickly
