Download Worksheet/Learning Aid Key

Bio 210A
Organization of Bacterial Genomes and Control of Gene expression
(Pre-lecture/lecture Worksheet)
Use the following animations, the lecture and its PowerPoint, and your book to understand the
process and answer the question below organization of bacterial genomes and mechanism of
prokaryotic gene regulation
The animations:”The Tryptophan Operon” and “Combination of Switches- The lac Operon” are
accessible through
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter18/animations.html#
FILL IN THE BLANKS
1. Bacterial genes coding for individual proteins needed for the same cellular process are organized as
operons that share the same non-coding regulatory DNA sequence known as the promoter region.
2. Each of the “operon” genes is known as a structural
gene.
3. Binding of the RNA polymerase to the promoter region followed by active transcription leads to the
formation of a polycistronic mRNA, which represents an RNA molecule with coding sequences for
protein synthesis of separate proteins.
4. Separate ribosomes are assembled on the polycistronic
mRNA and initiate translation
followed by termination of individual polypeptides or proteins
.
5. Bacterial gene expression is controlled by modulation of specific allosteric
interactions.
6. Different operons are regulated differently. Some code for proteins that are normally needed by the
cell and they are usually actively transcribed. However, transcription can be inactivated by
preventing RNA polymerase from transcribing the mRNA. Such operons and their genes are
repressible (repressible/inducible)
7. Other operons code for proteins that are normally not produced by the cell except when they are
needed. Such operons and their genes are inducible (repressible/inducible)
Regulatory non-coding regions of bacterial genes
8. A non-coding DNA sequence known as the operator separates the promoter regions from the
transcription initiation site of the “operon”. This region acts as the binding site of a protein known as
the repressor protein. The gene encoding for the “repressor” protein is separate from the operon it
controls.
9. Binding of the repressor protein to the operator DNA region prevents RNA polymerase from
initiating transcription. Therefore, repressor proteins actively prevent transcription. The repressor is
an allosteric protein whose shape can be modulated by the non- protein metabolites known as
either the co-repressor or the inducers.
10. A second non-coding sequence located up-stream of the promoter regions is the binding site of the
activator protein. The gene encoding for the “activator” protein is separate from the operon it
controls.
11. Binding of the activator protein to the Activator binding DNA region is necessary for RNA
polymerase to bind to the promoter . Therefore, activator proteins allow active transcription to
proceed.
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P.T.O.
Learning Aid
Control of Bacterial Gene Expression
1.
Use information from your textbook and/or from “The Tryptophan Repressor” animation from
the website http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter18/animations.html# to demonstrate your
understanding of regulation of the trp operon by filling-in the following table:
Tryptophan
level
Repressor
State of
operator
Transcription
Present
CoState of
repressor
repressor
(tryptophan)
Absent
Inactive
Low
Unbound
ON
High
Present
Present
Bound
OFF
Active
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2.
Use information from your textbook and/or from the “Combination of Switches - the Lac
Operon” animation from the following website
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter18/animations.html# to
demonstrate your understanding of regulation of the lac operon by filling-in the following table:
Lactose &
glucose
levels
Inducer
(allolactose)
(present or
absent)
State of
repressor
(active or
inactive)*
State of
Transcription
operator
(active or
(Is it
inactive)
(occupied
by a bound
repressor?)
No,
present
glucose is
high
absent
active
Repressor
bound
No,
present
glucose is
high
present
inactive
Repressor
unbound
Yes,
present
glucose is
low
absent
active
Yes,
present
glucose is
low
present
CAP
Protein
(present
or
absent)
CAP
protein
bound to
CAPbinding
site (Yes
or No)
present
Glucose High present
Lac
Repressor
(present
or absent)
Lactose Low
Glucose High
Inactive,
glucose
high &
lactose low
Lactose High
Inactive,
glucose is
high
Lactose Low
Glucose Low present
Repressor
Inactive,
lactose is low
bound
Lactose High
Glucose Low present
inactive
Repressor
unbound
active, low
glucose and
high lactose
(*) An active repressor protein has the conformation that allows it to bind to the operator.
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