Download Lecture#5 - Introduction to gene regulation and operons in

6/28/17
BIOLOGY 207 - Dr.Locke
Lecture#5 - Introduction to gene regulation and operons in
Prokaryotes.
Required readings and problems:
Reading: Open Genetics, Chapter 12
Problems: Chapter 12
Optional
Griffiths (2008) 9th Ed. Readings: - Ch. 10 pp 351-363
Problems: 9th Ed. Ch. 10 1-5, 7, 9-12 (for lect#6, too)
Campbell (2008) 8th Ed. Readings: Concept 18.1
Concepts:
How are prokaryotic genes expressed?
1. The basics of protein coding genes
2. Transcriptional regulation involves the binding of proteins (regulatory proteins) to the
DNA.
3. In prokaryotes, operons are groups or clusters of genes that are transcribed as a
polycistronic mRNA.
4. The lac operon is a classic prokaryote gene (operon) and was first described using
genetics.
5. Positive regulation of the lac operon
Biol207 Dr. Locke section
Lecture#5
Fall'11
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Central Dogma - Genetic Information and Genes
Information flows:
DNA -------> RNA ------> Protein
Gene expression involves:
1) Transcription - Information is transferred from the DNA sequence into an RNA
sequence - messenger RNA.
2) Translation - Information is transferred from the mRNA to protein sequence.
3) Proteins function to carry out the final actions of expression. Enzyme activity ->
structure/function of cell or organism etc.
Phenotype
What is a gene? Definition=
Biol207 Dr. Locke section
Lecture#5
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Basic Types of genes:
1- Transcribed – polypeptide coding - mRNA
-> translated into polypeptides
e.g. - typical protein coding genes
2- Transcribed – Structural - transcribed into RNA used directly
e.g. - rRNA, tRNA, snRNA (others, too).
3- Non-transcribed - Regulatory elements
- serve to bind proteins
- involved in DNA replication, transcription, and chromosome transmission
e.g. - origin of replication, centromere, telomere
Gene Regulation is frequently at the transcription step:
First step -> fewest resources wasted.
Status:
on - RNA polymerase transcribes the DNA sequence and transfers the information to
mRNA -> protein
off - RNA polymerase doesn't transcribe the DNA sequence.
Biol207 Dr. Locke section
Lecture#5
Fall'11
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Gene Regulation involves the activity of proteins
Transcription depends upon:
1) RNA polymerase and
2) other proteins that either assist (promote/enhance) or
prevent (repress/silence) transcription.
3) These proteins bind to the DNA (directly)
or to proteins that bind to DNA (indirectly).
Basic Structure of a protein coding gene:
Biol207 Dr. Locke section
Lecture#5
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Prokaryote Gene Regulation - the lac operon
Enzymes in the same metabolic pathway are often encoded by genes that are
clustered and transcribed as a single unit
Called - an operon.
Transcription of mRNA- polycistronic
- single mRNA that encodes multiple polypeptides
- AUG start-codon to UAG stop-codon (start to stop etc.)
- polycistronic mRNA; cistron = gene
- provides co-ordinate control of multiple gene products (polypeptides) that are needed
at the same time in the same process
Only some prokaryote genes are operons – not all.
Diagram of lac operon
Biol207 Dr. Locke section
Lecture#5
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Positive regulation at the lac operon
- promoter sequences – CAP and polymerase
- polymerase transcribes operon
- CAP protein
– active complex requires cAMP
- binds to DNA and to polymerase to increase level of transcription
Catabolite Repression – Positive control
Glucose levels in the cell affect transcription of the lac operon
LOW glucose levels: Adenylate cylcase (on) ATP ------->
HIGH glucose levels: Adenylate cylcase (off) ATP ----|
Biol207 Dr. Locke section
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cAMP
cAMP
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Genetics of the lac operon
Mutations
Location of mutation:
Result:
Not in gene
Positive regulation - Structural
Positive regulation - Regulatory
Negative regulation - Regulatory
Biol207 Dr. Locke section
Lecture#5
Fall'11
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Negative Regulation
First understanding of gene regulation comes from the work of Jacob and Monod in
the 1950's and ‘60's -> Nobel prize in 1965.
Inducers - specific substrates that induced the appearance of specific enzymes (new
synthesis of the enzymes).
beta-galactosidase could be induced with several types of beta-galactosides – IPTG
– Fig The enzyme is not normally present in absence of an inducer.
Biol207 Dr. Locke section
Lecture#5
Fall'11
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Constitutive mutants of the I locus
J & M found a mutant that produced beta-galactosidase at
full level all the time (even in the absence of inducer).
It is called a Constitutive mutant
- always "on" - expressed
- unregulated by inducers.
Genetically mapped close to, but distinct from, the ZYA genes
These mutants defined the I locus.
I
I
+
cells are inducible to full lac ZYA levels only in presence of inducer
-
mutants are not inducible but instead, always express full levels of ZYA in either
presence or absence of inducer
See Fig
Biol207 Dr. Locke section
Lecture#5
Fall'11
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Lac operon: Fig
The I gene product is a
repressor protein which
acts by binding to the
operator DNA sequence
thereby preventing the
RNA polymerase (which
binds to the
promoter-P sequence)
from transcribing a
polycistronic messager
RNA of the cistrons Z, Y,
A.
Example of negative regulation
- repressor protein acts to prevent transcription
- transcription would take place in the absence of the repressor.
Biol207 Dr. Locke section
Lecture#5
Fall'11
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