Transcript Control Mechanisms - Earl Haig Secondary School

Control Mechanisms
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(Laugh)
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Review…
What do genes ultimately encode?
 What is the intermediate between gene
and polypeptide chain (protein)?
 What are the various functions of
proteins?
 How many genes encode proteins?
 Do you think all proteins are needed to be
transcribed/translated at all times of the
day? Why or why not?

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So, cells need a way to turn genes
on and off!
Transcription factors turn genes ‘on’
when required – what do we mean by
‘on?’  genes are expressed (transcribed
and translated to protein)
 This method of controlling the transcription
and translation of genes is called gene
regulation
 Some proteins are always needed – called
housekeeping genes

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Levels of Gene Regulation

On your own, think of a way (identify a
step or mechanism) where transcription or
translation can be controlled (Hint: think of
molecules that are involved in the
production of proteins from genes)
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As information flows from DNA to proteins…
DNA  mRNA  protein activated protein
transcription
translation
Post-translational modifications
Transcriptional Control
DNA  mRNA  protein  activated protein
Translational Control
DNA  mRNA  protein  activated protein
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Four levels of Gene Regulation
Transcriptional – regulates which genes or
the rate at which they are transcribed
 Posttranscriptional – after the mRNA is
transcribed, the modifications it undergoes
can be regulated
 Translational – regulates how often and
how quickly mRNA is translated, which
affects the time to activate/destroy mRNA
 Posttranslational – regulate modifications
that are made to newly formed proteins

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Levels of Gene Regulation
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Got Milk?
• It was previously thought
that the E. coli in our
digestive system was
responsible for our ability to
digest lactose
• However, it is now known
that the enzyme lactase
digests lactose
• Nonetheless, the lac
operon serves as an
excellent model for
understanding the concept
of gene regulation
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superlaugh.com/dan/lactose.htm
A Prime Example – lac Operon
Primary energy source in E. coli: Glucose
However, E. coli may use other carbon sources (i.e.: lactose) if
glucose is absent.
lactose = Glucose + Galactose
Therefore, E. coli can indirectly obtain glucose via cleavage of
lactose by the enzyme β-galactosidase
Note:
β-galactosidase is only produced in the presence of lactose
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 WHY?
A Prime Example – lac Operon
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Only in prokaryotes
The lac gene codes for a protein called βgalactosidase
To conserve energy, this enzyme is only made
when lactose is present in the cell
When lactose is unavailable, the cell must block
the production of β-galactosidase
What’s an operon? A region of bacterial DNA
that codes for a series of functionally related
genes that are transcribed into one RNA
transcript
Let’s look at the anatomy of the lac gene to
understand how this happens
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The lac Operon
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The gene that codes for β-galactosidase is part of
an operon
An operon consists of structural genes that code for
enzymes, promotor (P), and an operator (O)
P
O
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The lac Operon

Followed by the operon are a cluster of
genes that code for proteins involved in
the breakdown of lactose: lacZ, lacY, lacA
P
O
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The lac Operon
lac Z
-Z codes for the β-galactosidase enzyme that
breaks down lactose
lac Y
-Y codes for a protein called galactoside
permease which transports lactose into the cell
lac A
-A codes for transacetylase (function unknown
)
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There’s more to this operon…
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lacI is a gene that is part of the operon but codes for
a repressor protein; it has its own promoter
This repressor blocks the transcription of βgalactosidase by binding to the lactose operator
(this prevents RNA polymerase from binding to the
gene)  will any genes be transcribed?
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P
O
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An Analogy…
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Here's an analogy. A promoter is like a
door knob, in that the promoters of many
operons are similar. An operator is like the
key hole in a door knob, in that each door
is locked by only a specific key, which in
this analogy is a specific regulatory protein
(i.e. the repressor made by lacI).
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So what does this all mean?
If lactose is not present, the repressor
protein will be made to prevent the
production of β-galactosidase and its
associated proteins
 What if lactose is present? β-galactosidase
will be required!
 What happens?
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In the presence of lactose…
• Lactose interacts with the repressor and causes it to
change shape and release from its binding site
(allosteric enzyme – what is this again?)
• Thus, lactose itself is called the signal molecule or
inducer
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Allosteric Regulation
http://library.thinkquest.org/28751/media/review/figure/allosteric.jpg
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Think for a minute…

If scientists grew a culture of E.coli
bacteria who are known to have a
mutation in their lacI gene (so that the
gene is defective), what would they
observe in terms of lactose metabolism?
 Loss
of regulation, which causes constitutive
production of β-galactosidase at all times
despite the absence of lactose
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Finally! It’s time to test your
knowledge…
Will you ace the test?
Which one will it be?
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Let’s see what you understood!
1. What are the stages of translation?
– initiation, elongation and termination
2. What is it called when DNA is made into mRNA?
mRNA to protein?
– transcription and translation
3. Where does RNA Polymerase bind?
– the promoter
4. What is an operon?
– a region of bacterial DNA that codes for a series of
functionally related genes that are transcribed into one
RNA transcript
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1. If a certain gene contains 2 exons (each with 5
nucleotides) and 1 intron (containing 2 nucleotides),
how many nucleotides will be in the mRNA transcript
after splicing?
a. 4
b. 8
c. 10
d. 12
2. From the following three nucleotides, what would be the
mRNA codon and tRNA anticodon, respectively?
5’…….GCT…….3’
a. AGC, UCG
b. UCG, AGC
c. CGA, UCG
d. TCG, TCG
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3. The following sequence depicts:
DNA --X---> mRNA ------> protein ------> activated protein
(NOTE: X = block in the pathway)
a. Transcriptional control
b. Post-transcriptional control
c. Translational control
d. Post-translational control
4. The following sequence depicts:
DNA -- ---> mRNA --- X ---> protein ------> activated protein
(NOTE: X = block in the pathway)
a. Transcriptional control
b. Post-transcriptional control
c. Translational control
d. Post-translational control
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5. The processes of transcription and translation occur
in which parts of the eukaryotic cell, respectively?
a. nucleus and cell membrane
b. cytoplasm and extracellular space
c. cytoplasm and nucleus
d. nucleus and cytoplasm
6. Lactose cannot accumulate in an E. coli cell. What
type of mutation has occurred?
a. lacZ
b. lacI
c. lacY
d. lacA
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7. Why is the lac operon called an operon?
a. The genes are under the control of the same promoter.
b. The genes are transcribed into the same molecule of mRNA.
c. The genes are commonly regulated.
d. All of the above.
8. The regulatory protein of the lac operon is:
a. lacI
b. lacZ
c. lacY
d. lacA
9. Lactose induces the transcription of:
a. lacZ, lacY
b. lacZ, lacI
c. lacI, lacY
d. lacX, lacA
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How is the trp operon different?
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Read p256-257 and take notes on the trp
operon (also take notes of the similarities
and differences between the lac and trp
operons, Table 2 on p258)
Homework:
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Do questions #1-6 on p258
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