Thursday, December 10, 2015
Wednesday, December 9, 2015
18.1 Bacteria often respond to environmental change by regulating transcription Notes
18.1 Bacteria
often respond to environmental change by regulating transcription
o
Bacterial cells that can conserve resources and
energy have a selective advantage over cells that are unable to do so.
o
Natural selection has favored bacteria that
express only the genes whose products are needed by the cell.
o
Metabolic control occurs on 2 levels.
§
1st cells can adjust the activity of
enzymes already present. This is a fairly fast response, which relies on the
sensitivity of many enzymes to chemical cues that increase or decrease their
catalytic activity. The activity of the first enzyme in the tryptophan
synthesis pathway is inhibited by the pathway’s end product.
§
2nd cells can adjust the production
level of certain enzymes; that is, they can regulate the expression of the
genes encoding the enzymes. Many genes of the bacterial genome are switched on
or off by changes in the metabolic status of the cell.
Operons: The Basic Concept
o
E. coli synthesizes the amino acid tryptophan
from a precursor molecule
o
Transcription gives rise to one long mRNA
molecule that codes for the five polypeptides making up the enzymes in the
tryptophan pathway
o
When an E. coli cell must make tryptophan for
itself because the nutrition medium lacks his amino acid, all the enzymes for
the metabolic pathway are synthesized at one time.
o
The switch is a segment of DNA called an
operator.
o
Altogether, the operator and the promoter, and
the genes they control constitute and operon
o
The operon can be switched off by a protein call
the trp repressor
o
The trp repressor is the protein product of a
regulatory gene called trpR, which is located some distance from the trp operon
and has its own promoter
o
Tryptophan functions in this system as
corepressor, a small molecule that cooperates with a repressor protein to
switch an operon off.
Repressible and Inducible operons: Two Types
of Negative Gene Regulation
o
The trp operon is said to be a repressible
operon because its transcription is usually on but can be inhibited when a
specific small molecule binds allosterically to a regulatory protein.
o
An inducible operon is usually off but can be
stimulated when a specific small molecule interacts with a regulatory protein
o
Inducer inactivates the repressor
o
FIGURE 18.4
o
For the lac operon, the inducer is allolactose,
an isomer of lactose formed in small amounts from lactose that enters the cell.
o
In gene regulation, the enzymes of the lactose
pathway are referred to as inducible enzymes because their synthesis is induced
by a chemical signal
Positive Gene Regulations
o
When glucose and lactose are both present in its
environment, E coli preferentially uses glucose.
o
Mechanism depends on the interaction of an
allosteric regulatory protein with a small organic molecule, in this case
cyclic AMP
o
The regulatory protein, called catabolite
activator protein (CAP), is an activator, a protein that binds to DNA and
stimulates transcription of a gene
Virus Structure- 19.1
Virus Structure Notes- 19.1
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Wednesday, December 2, 2015
17.2 Transcription Notes
17.2 Transcription
- RNA polymerase: put together RNA nucleotides to DNA strand
- In bacteria:
- Promoter- where RNA starts
- Terminator- ends
- Transcription unit- piece that is transcribed
- 3 stages of Transcription
- Initiation
- Elongation
- Termination
- Promoter called a TATA box is crucial in forming the initiation complex in eukaryotes
ELONGATION OF THE RNA STRAND
- As RNA Polymerase moves along the DNA, it untwists the double helix, 10-20 bases at a time
- Transcription progresses at a rate of 40 nucleotides per second in eukaryotes
- Gene can be transcribed simultaneously by several RNA polymerases
- Nucleotides are added to the 3' end of the frowing RNA molecule
TERMINATION OF TRANSCRIPTION
- In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequences
- The RNA transcript is poreleased 10-35 nucleotides past this polyadenylation sequence
Tuesday, December 1, 2015
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