7.4 Flashcards
(17 cards)
Controlling Gene expression
- Not all proteins are required by cells all the time
Housekeeping genes: genes that are always required
Ex. Genes for metabolism, growth, replication
Prokaryotic Gene Control Mechanisms
- Regulated by concentration of Trp, and Lactose
- Both are negative feedback Mechanisms
The Lac Operon
- Lactose is an energy source for prkaryotes
- Regulates expression of gene that metabolizes lactose
- Lac Operan consists of 3 genes
What does the Lac Operon Consists of
Promoter: Where transcription begins
Operator: sequence of bases that controls transcription
Coding Regions: Enzymes that metabolizes Lactose
Lac Reproessor:
-Upstream from operon
-Always transcribed
-takes cues from the env. ([lactose]) and regulates the production of lactose-metabolizing proteins
How the protein behaves
depends on [lactose] in the cell
In the Absence of Lactose
- Lac repressor is active, and binds to operator
- Inhibits RNA polymerase from binding to promoter region
- Inhibits the production of enzymes that metabolizes lactose
In the Presence of Lactose
- Lactose (inducer) binds to lac repressor, making it inactive
- Lac repressor cant bidn to operator
- RNA polymerase can bind to the promoter region
- transcription of the lac genes begins
The trp Operon
- Tyrptophan is a.a that builds protons
-* Prokaryotes can make tryptophan
directly or take it up in their env.
- Tyrptophan synthesizing enzymes
- The lac repressor protein is
inactivated by lactose - trp repressor protein is activated in the presence of tryptophan
In the Absence of Tyrpt
- Repressor protein is in an inactive state and does not bind to the operator
- RNA polymerase can bind to the promoter
- Transcription to synthesis tyrpt begins
In the Presence of Tyrpt
- Tryptophan acts as a corepressor b/c it serves to repress expression of tyrpt
- Cell conserves energy by using the available tryptophan
and stopping the transcription of the genes that code for
the enzymes used in making tryptophan - Reprossor protien is active and provents RNA Poly to make enzymes for synt. trypt
Eukaryotic Gene Control
- Does not use operon systems
- Transcriptional (as mRNA is being made)
- Post-Transcriptional (as mRNA is being processed)
- Translational (as the protein is being made)
- Post-Translational (after the protein has been made)
Transcriptional Regulation
- Most common type
- DNA is wrapped around histone (making the promoter unavailable)
- Promoter must be exposed
- Activator molecules bind near the gene and signal proteins to loosen DNA structure
- Histones loosens, promoter exposed, allows transcription to begin
- General transcription factors attach to the TATA box, enabling RNA polymerase to bind.
- Regulation of transcription: Activators can increase transcription rates, while repressors can slow them down.
Methylation
- Methyl group is added to Cytosine bases in the
promoter of a gene - Inhibits trancription
- silencing , turning on or off a gene
Gene Methylation in Agouti Mice
- In normal, healthy mice, the agouti genes are kept “off”
(silenced) - In yellow/obese mice, the same genes are NOT methylated
- Thus genes are turned “on”
- These mice have a higher risk of cancer and diabetes
Post-Transcriptional Regulation
- Alternative splicing (75% of human genes)
- Masking proteins can bind to mRNA, blocking translation (e.g., common form
of control in animal eggs) - Regulatory molecules like hormones can influence the rate at which mRNA
persists in the cytosol
Translational Regulation
Length of the poly(A) tail can be modified by enzymes
- Increase/decrease the time required to translate the mRNA into a protein
Post-Translational Regulation
- Processing
* Initially, proteins built are inactive and must be activated by various processing
mechanisms (e.g., proinsulin is the inactive precursor to insulin) - Chemical Modification
* Chemical groups are added/removed from the protein affecting its function
* Can put the protein “on hold” until its needed - Degradation
* Proteins can last from minutes to a lifetime
* Rate of degradation is under control
* Short-lived proteins are tagged with a small protein called ubiquitin
* Adding/removing tags can either shorten or extend the functional life of a protein
Cancer
- Lack regulator mechanisms
- In cancer cells, genes are mutated to become
oncogenes - Oncogenes cause the constant and undifferentiated cell
division that creates tumours
