Regulation of Gene Expression (4) Flashcards
genes are regulated …
… by proteins that bind to regulatory sequences that are upstreams or downstreams of the transcribed sequence
what is the name of the units that prokaryotic genes are organized in?
operons
what are prokaryotic genes regulated by?
metabolites binding to proteins that recognize and bind to operators
where does regulation of gene expression in eukaryotes occur?
various levels like transcriptional, post-transcriptional, translational, and post-translational
what does the bacterial genome include?
chromosomal DNA and episomes (extrachromosomal DNA in plastids)
transduction
transfer of a part of a chromosome from one bacterium to another through a phage
- can be general or specific
transfection
experimental method to introduce dna fragments into bacterial using phage as vectors
transformation
introduction of dna from an external medium into the bacterial
gene expression
-synthesis of RNA from a DNA template
- includes translation of mRNA into specific proteins resulting in specific genotypes
- constitutive or regulated
constitutive genes
expressed without much regulation
- expressed continuously
Transposable elements (Transposons)
- mobile genetic elements/ jumping genes
- move from one chromosome to another part of a different chromosome
- important in new combinations of genes, mutations, and microevolution of bacteria
How is bacterial dna introduced from one bacterium to another?
conjugation, transduction, and transformation
conjugation
sexual mating of bacterial through f-pili and the genes present in the F-factor are transferred from an F+bacteria to an F-bacteria
what does an operon constitute?
- the coding sequences of the genes in that unit
- a promoter
- operator
what does the promoter in an operon do?
determine tha accuracy of transcription
what does the operator in an operon do?
determine the amount of transcription
- serves as the on/off switch to regulate transcription in response to an enviromental stimuli
polycistronic
many transcripts are made from one operon simultaneously as a single mRNA in a single transcription
operons can be …
induced (positive regulation) or repressed (negative regulation)
Inducible operon
- mostly turned off
- turned on only when necessary
- present to control catabolic pathways used to break down compounds
inductible operon example
lac operon
- induced when lactose is present with glucose or alone
- expression of genes for lactose-utilizing enzymes is activated by removing a repressor that is bound to the operator of the lac operon
- the lactose binds to the repressor molecule, and the operon is derepressed to start transcription
-If there is no glucose and only lactose is present, it results in an increase of cyclic AMP (cAMP) levels in the cells. The cAMP binds to a catabolite activator protein (CAP) and that binds to the promoter to further enhance the activity of the lac operon much more than the basal level at derepression alone
repressible operon
- used for anabolic pathways to synthesise compounds
- mostly turned on
repressible operon example
trp-operon to synthesize tryptophan
- the enzymes in the Trp biosynthetic pathway are expressed only when Trp is absent
- when trp is absent a repressor protein is inactive and it does not bind to the operator to repress transcription
- when sufficient levels of Trp are made, it binds to the repressor molecule and makes it an active repressor that now binds to the operator and stops transcription
- feedback inhibition at gene level
sigma factors
- can control gene regulation
- can bind to the regulatory elements of bacteria and activate transcription
- several types of sigma factors and other DNA-binding proteins that regulate prokaryotic gene expression
- factors can transmit environmental signals to the genes to express the needed proteins immediately in response to the stimulus
how is chromatin made?
the double helix of dna wraps around histone proteins to form nucleosomes, which condenses further to make chromatin
Repetitive sequences
found near the telomere
- useful in dna replication because they fold onto themselves to provide a -OH group for dna synthesis
gene families
- result of gene replication during crossing over
- genes essential for survival are highly duplicated
coordinated gene expression
expression of multigene family members is regulated by common cis-acting elements recognized by crommon trans-acting factors specific for that gene family
how are eukaryotic genes regulated?
by a set of regulatory DNA sequences/ cis-acting elements (recognized by trans-acting factors)
in prokaryotes there are no introns to be removed (T/F)
T
Where can gene expression be regulated in eukaryotes?
transcriptional level, translational level, posttranslational level
the rate of transcription is regulated at the ____ level
gene with diff mechanisms for long and short term controls
Long term controls (2)
- compaction
- methylation
Compation
- chromosomes condensed during cell division and later uncoil to chromatin
- but some chromatin remain condensed and not accesible by RNA polymerase
Methylation
- to control transcription on selected chromosomes
- by DNA methylase
- adds methyl group to the C and A bases, preventing regions form being transcribed
- helps DNA polymerase distinguish the old strand from the new
short-term control
- some regions of chromosomes are actively transcribed
- some factors directly bind to RNA polymerase or other DNA binding proteins and can affect transcription through protein-protein interaction
regulatory elements
- promoter (upstream of the start site)
- enhancers
- repressors
transcription factors
- have unique secondary structures (motifs)
- helix-turn (or loop) - helix proteins
- zinc finger proteins
- leucine zippers
All these DNA binding proteins act as _____, which interact with each other along with these _____ (unique secondary/tertiary strictures) and with DNA to regulate _____.
All these DNA binding proteins act as dimers, which interact with each other along with these motifs (unique secondary/tertiary strictures) and with DNA to regulate transcription.
Some _____ can bind to these transcription factors in the ____ and then migrate to the nucleus, bind with specific ____-acting elements, and activate the ____ of certain genes.
Some hormones can bind to these transcription factors in the cytoplasm and then migrate to the nucleus, bind with specific cis-acting elements, and activate the transcription of certain genes.
Post-transcriptional regulation
- limited regulation
- alternate splicing
- mRNA stability
alternate splicing
- during mRNA processing (where introns and exons …)
- some exons are joined differently to yield different proteins
mRNA stability
- some mRNA are long-lived, and some short-lived
- mRNA stability is altered by RNA binding proteins that can degrade the mRNA
Translational regulation
- in eukaryotes in the cytoplasm
- involves initiation factors and elongation factors
- blocks translation of processed mRNA until need arises
Examples of translational regulation
- mRNA storage
- hormonal regulation
- effect of cofactor on translation
Post-translational modification
- last stage of controlling gene expression at protein level
- zymogen activation
- selective targeting
- chemical modification
- glycosylation
Totipotency
single-cell having the potential to become a whole new organism
- zygotes
