mechanisms that control when and how fast specific genes will be transcribed and translated, and whether gene products will be switched on or silenced
Usually exhibit transcriptional level control by organizing related groups of genes into groups that can be rapidly turned on and off (operons)
Jacob and Monod-
discovered mutant E. coli strains in which a single genetic defect wiped out the activity of three enzymes that helped to digest lactose. Therefore- DNA coding sequences for all three enzymes must be linked together as a unit and controlled by a common mechanism
an arrangement in which a promoter and a set of operators control access to more than one prokaryotic gene. no operons in eukaryotes.
Regulatory (or repressor) gene
produces a repressor protein- substance that can prevent gene expression by blocking the action of RNA polymerase. operon.
is a sequence of DNA that RNA polymerase can attach to in order to begin transcription. operon.
can block the action of the RNA polymerase if the region is occupied by a repressor gene. operon.
Structural genes contain DNA sequences that code for several related enzymes that direct the production of an endproduct. operon.
diagram inducible operon
meaning that a repressor is bound to the operator region, preventing the transcription of genes that code for enzymes that break down the substance the operon targets.
if substance the operon targets is required to induce, or turn on the gene, this operon is said to be an inducible operon. substance is usually repressed, but when the substance is present, it acts as an allosteric regulator that combines with an allosteric site on the repressor, making the repressor inactive. When the repressor is inactive, RNA polymerase is able to transcribe the genes that produce the enzymes that break down substance.
usually in induced state. these genes only stop producing enzymes in the presence of an active repressor
diagram repressible operon
meaning that the regulatory gene produces an inactive repressor that does not bind to the operator
detail repressible operon
RNA polymerase transcribes the genes necessary to produce enzymes that synthesize substance the operon targets. However, if the bacteria is in regions of abundance of substance, it no longer needs to synthesize its own. Therefore, some substance reacts with the inactive repressor and makes it active. substance acts as a corepressor. This active repressor can now bind to the operator regions, preventing the transcription of the genes.
regulatory proteins slow down gene activity
regulatory proteins enhance gene activities. Regulated by activator proteins that bind to DNA andstimulate more efficient transcription of the gene
Positive control–E. coli
E. coli cells will not typically break down lactose that much unless there is no glucose. If no glucose- an activator called CAP will bind to cAMP, a second messenger, which allows RNA polymerase to bind more efficiently.
Positive control diagram
Do not normally form operons- instead, each gene its own has specific regulatory sequences
type of transcriptional control. methylation- methyl groups (-CH 3 ) can be “painted” on parts of DNA to block access to the genes. Acetylation- acetyl groups (-CH 3 CO–) are attached to DNA to make the genes more accessible
upstream promoter element
Promoter (TATA box)-
RNA polymerase binds here to begin transcription–always TATA sequence. RNA polymerase can only initiate transcription after various regulatory proteins called transcription factors have assembled on the chromosome. part of regulatory molecules and control sites
Transcription initiation site
where transcription begins. part of regulatory molecules and control sites
regulatory molecules and control sites
type of transcriptional control.
tata box diagram
DNA sequences that increase the rate of RNA synthesis. enhancer type of UPE . part of regulatory molecules and control sites
(types of regulator proteins) can bind here and help activate transcription part of regulatory molecules and control sites
DNA sequences that turn off transcription by binding to proteins called repressors
at terminator sequence, transcription is completed and mRNA is released
Post translational control
Allosteric enzymes regulate rate of metabolic pathways through feedback inhibition
Post translational control. proteins are synthesized in inactive form but become active by removal of a portion of the polypeptide chain
Post translational control–chemical modification
adding or removing functional groups to alter the activity of an enzyme
mRNA can be destroyed by ribonuclease enzymes in order to control the number of protein molecules translated from it
part of transcriptional control. DNA wound tightly around histones prevent polymerases from accessing the genes; Inactive genes lie in heterochromatin; Active genes–euchromatin.
Inactive genes lie in compacted chromatin called heterochromatin. Heterochromatin is not transcribed- ex- Barr bodies.
active genes lie in loosely packed chromatin called euchromatin.
What makes histones lose their grip? (Chromosome Organization)
Acetylation can make histones lose their grip
What blocks gene’s influence on trait? (Chromosome Organization)
Methylation can block a gene’s influence on a trait
Post- transcriptional control-
Introns are spliced out by snRNPs, Differential mRNA processing, Ribozymes, Nuclear envelope controls when mRNA reaches a ribosome
Differential mRNA processing-
the same pre-mRNA can be spliced in different ways in different cells- one sequence can be an intron in one type of cell but an exon in another, producing different versions of mRNA
Ribozyme- some intron RNA’s can self-splice, acting as an enzyme to catalyze the splicing process
Nuclear envelope controls when mRNA reaches a ribosome
mRNA can only pass through the nuclear pores if there are specific proteins attached to the mRNA, Proteins help move the mRNA to its specific location to be translated or stored, Each mRNA has its own proteins attached, coded for by the untranslated end of mRNA