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Gene Regulation
Why do genes need to be “regulated”
or controlled?
-Cells don’t always need all of their genetic
information to be active all of the time.
For example:
1. In prokaryotes genes are regulated so the
cells don’t waste energy making proteins that
aren’t required all of the time
2. In multicellular (eukaryotic) organisms each
type of cell carries out specific activities
which require only a distinct set of proteins,
not all of the proteins in the entire genome.
• These cells all contain the same DNA, the
difference is caused by regulating gene
expression so that only the required genetic
information gets expressed in a cell at any
given time. (cell differentiation)
• Expressing a gene involves 3 main steps
– Transcription of DNA to RNA
– Translation of RNA to a polypeptide chain
– Activation of a protein
• So, the expression of a gene can be
regulated at several different times using
several different mechanisms
• Types of Control Mechanisms
– Regulatory proteins
• Negative control – slow down or stop gene activity
• Positive control – promote or enhance gene activity
– Non-coding DNA sequences
• Promoters – signal the start of a gene
• Enhancers – binding sites for some activator proteins
– Chemical modifications
• Methylation – adding CH3 to nucleotide bases causes
them to shut down or inactivate a gene
• Demethylation – removing CH3 can cause a gene to
activate
• Acetylation – attaching acetyl group (CH3CO) to
histones (proteins that DNA wraps around) make it
loosen its grip on its associated DNA so it can be
transcribed
• These control mechanisms respond to
various signals
• Some signals originate inside the cell
(intracellular) while others are from the cell
environment (extracellular)
– Examples – hormones and the absence or
presence of a certain chemical
• In prokaryotes:
– Some genes are constantly transcribed because
their proteins are always needed (constitutive
genes)
– Other genes are only transcribed when their
proteins are required
– Example: Bacteria growing in the colon of an
adult cow do not have lactose (milk sugar)
available as an energy source, but if they were in
the colon of a calf they would. So, should all of
those bacteria take the time and energy to
produce the enzymes (proteins) that digest
lactose all of the time. No, only when the
environment requires it.
– Most prokaryotic controls are transcriptional
controls
• In bacteria, functionally related genes are
regulated together in gene “complexes”
• This gene “complex” is called an operon
• An operon is several genes involved in the same
process and they function as a single unit
• Operons were discovered by French
microbiologists Francis Jacob & Jacques Monod
• Operons are composed of several parts
– Regulator gene – codes for repressor protein
– Promoters – sequence of DNA where RNA polymerase
binds to start transcription
– Operators – site where repressor proteins may bind
(“on/off” switch)
– Structural genes (several) – code for the proteins
• When a repressor protein binds to the operator,
transcription of the structural genes is inhibited.
The Lac Operon
• An inducible operon or an example of a
positive control mechanism
• The operon will not be transcribed unless an
inducer inactivates its repressor
• Its repressor is normally active, therefore the
operon is usually “turned off”
• It consists of 3 structural genes that code for
3 enzymes that breakdown lactose (lac z, lac
y, & lac a) - see book
• If lactose is absent:
– The lac-operon is “off” and the genes are not transcribed,
this is its usual state
– The repressor protein is bound to the operator(s) and
blocks the RNA polymerase
– it is referred to as inducible because it can be turned on
• If lactose is present:
– The operon is turned “on” by the removal of the repressor
– An inducer (allolactose) binds to the repressor and
inactivates it so that it no longer blocks the RNA
polymerase
– The operon can then be transcribed and the enzymes that
break down lactose can be produced
Show CD-ROM animation!!
The trp Operon
• A repressible operon or an example of a
negative control mechanism is usually on
but can be turned off.
• The operon will be transcribed unless a
corepressor activates its repressor
• Its repressor is normally inactive, therefore
the operon is usually “turned on”
• It consists of 5 structural genes that code
for enzymes the cell needs to synthesize
the amino acid tryptophan
• When trytophan levels are low, the repressor
protein is inactive and not bound to the
operon
– The RNA polymerase can bind and
transcribe the operon
– This is the “normal” state
• When tryptophan levels are high, tryptophan
(itself) binds to the repressor to activate it
– Tryptophan acts as a co-repressor
– The repressor is then able to bind to the
operon, blocking RNA polymerase and
stopping transcription
Eukaryotic Controls in General
• Have 4 levels of control
– Transcriptional – if a gene is not
transcribed, there is no gene product
– Post-transcriptional – processing of
RNA and speed it leaves nucleus
– Translational – life expectancy of mRNA
– Post-translational – activating or
modifying the polypeptide chain to make
an active protein or enzyme
Specific Eukaryotic Controls
• Transcriptional Controls
– Organization of chromatin
• Euchromatin –relaxed form, genetically active
• Heterochromatin – denser form, genetically inactive
– Ex. Mammalian females have 2- X chromosomes,
one of the X chromosomes is inactive in the cells (we
call these Barr bodies), it is inactivated by chance
– Telomeres – ends of chromosomes
• Non-coding portions
– Introns
– Tandom repeats
– Regulatory proteins
• Transcription factors and enhancers help RNA
polymerase bind to the promoter so transcription can
occur
Post-transcriptional Control
• Begins once there is an mRNA transcript
• Processing before mRNA leaves nucleus
– Differential excision of introns and
splicing of exons
– Affects the final protein product
• Also, the speed at which mRNA leaves the
nucleus affects how much product can be
made
Translational Control
• Begins when the mRNA reaches the
cytoplasm
• “Masking” of mRNA – just because mRNA is
in the cytoplasm doesn’t mean it gets
translated
• Life Expectancy of mRNA – the longer the
mRNA is active and in the cytoplasm, the
product can be made
• Influence of Hormones – hormones can
stabilize RNA and extend their life expectancy
(ex. Prolactin and casein)
Post-translational Control
• Begins after polypeptide is made
• Activation of protein product – some proteins
are not active after synthesis, they need to
be modified (have stuff added or removed)
• Degradation of a protein – many proteins are
short-lived in cells and are degraded so they
are no longer active