MCB 2050 Molecular Biology of the Cell -
W21 complete midterm exam notes (get well
prepared) University of Guelph
Gene Regulation
Gene Expression, RNA polymerases and Promoters
Control of Gene expression in Bacteria
- Prokaryotic gene expression is regulated by simple mechanisms that control transcription by
RNA polymerase
- A group of proteins called σ factors recognize promoter elements (-10, --35) and load RNA
polymerase at the start site
- Transcription activators and repressors regulate the loading and release of RNA polymerase
o Most bacteria and single-celled organisms regulate gene expression to adjust cell
enzymatic machinery and structural components to change in nutritional and physical
environment
o Bacteria cells normally synthesize only proteins that are required for survival under
current conditions
The lac operon (structural gene) is a classic example of a regulated prokaryotic promoter
- activated by an activator protein (CAP, catabolite activator protein), that binds next to a
promoter and recruits RNA polymerase
- the lac operon is repressed by a repressor protein (lac repressor)
RNA polymerase
- Binds to -10 position
- Needs a σ factor to initiate transcription
- σ factor associates with -30 protein
- -10 and -30 elements act as basal
elements found in all promoters
Bacterial gene expression is regulated by simple
transcriptional activators and repressors that
bind promoters of the gene
- CAP activator: binds to CAP site
- Lac repressor: binds to operator
, lOMoARcPSD|6353920
Lac repressor (Lac I) binds to DNA downstream of
transcription initiation site and prevents
transcription by RNA polymerase
- Presence of lactose drives the binding of
Lac repressor to its binding site
CAP (catabolite activator protein)
- Activated by cAMP (glucose -)
- Binds to DNA upstream of initiation site
and stimulate recruitment of RNA
polymerase
, lOMoARcPSD|6353920
In absence of glucose (↑cAMP, CAP binds the
promoter) and in presence of lactose (LacI
doesn’t bind to promoter)
- The lac operon is expressed
, lOMoARcPSD|6353920
Eukaryotic gene Control
- The primary purpose of gene control in multicellular organisms is the execution of precise
developmental programs
o So that the proper genes are expresses in the proper cells at the proper times during
embryologic development and cellular differentiation
o In eukaryotes we have three RNA polymerases
o In eukaryotes, general transcription factors are involved in elaborate process of
recognition of promoter element and assembly of pre-initiation complex then load RNA
polymerase
- In eukaryotes, transcription takes place on DNA that is wrapped in chromatin
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