Transcript Gene Control

Gene Control
Turning off and on
Gene Control
• Molecular
mechanisms that
govern when and how
fast genes will be
transcribed and
translated. Not all
genes are working all
the time in all cells
Controls are triggered by…
• Programmed
schedules of
development (think of
a fetus developing in
the womb)
• In response to
chemical conditions
• Receiving a signal,
such as a hormone
Examples of controls
• Programmed
schedule: the
changes that take
place in puberty
• Chemical conditions:
Methylation of DNA
often inactivates a
gene; demethylation
might turn it on later
• Acetylation; acetyl
group attaches to a
histone, loosens the
wrap and makes it
easier to transcribe
• Receiving a signal: a
hormone triggers a
response from a cell
What does the controlling?
• Mostly regulatory
proteins
• Negative control:
slows or stops gene
activity (methylation)
• Positive control: turns
on or enhances gene
activity (acetylation)
Gene Control
• Is different in
prokaryotes versus
eukaryotes
• Less complicated in
prokaryotes
• Often controlled by
operons: one
promoter and a set of
2 operators. Operator
is a binding site for an
oppressor.
Prokaryote Example
• Lac operon: controls the
production of enzymes
that digest lactose. Only
turned on when lactose
and present and glucose
is absent.
• Glucose is the preferred
sugar, only make
provisions for lactose if
it’s the only energy
source available
Eukaryote examples
• All cells contain all genes.
Many are basic cell
function genes that
operate regularly.
• Cells also differentiate by
activating only certain
genes
• For example, a neuron
does not do the same
thing as a cell in the
pancreas
When do the controls happen?
• Page 242-243
• Can occur during
transcription, in
mRNA processing, in
translation, and
following translation
Types of
Controls in
Eukaryotes
• Homeotic genes:
interact with others to
control development
• X chromosome
inactivation makes
sure only one X is
working in both sexes
• This is dosage
compensation
(females are mosaics)
More Examples
• Signaling:
hormones in
animals
• Protein hormones
attach to a receptor
outside the cell
membrane and
stimulate a chain of
events known as
the 2nd messenger
system
More Examples
• Steroid hormones
pass through the cell
membrane to find its
receptor inside
• An activator protein
winds up next to the
promoter for the
targeted gene.
• A hormone always
triggers a response
Loss of control
• Can mean cancer
• Mutations in the
checkpoint genes of
the cell cycle and/or
repressor genes or
enhancer genes
lead to too much
mitosis