Transcript Determination powerpoint

If induction tells cells
what to do
How do they remember
these instructions
for a lifetime??
Cloning:
The ultimate
Transplant
experiment
Dolly
Cloning:
The ultimate
Transplant
experiment
Oops-Here’s the real Dolly
(guess which tissue
she was cloned from!)
Cloning:
The ultimate
Transplant
experiment
So HOW
does
cloning work?
Blastula stage cells
Can be easily re-programmed
But cloning success drops dramatically as
the age of the nuclear donor increases
Gilbert Fig. 4.6
But cloning success drops dramatically as
the age of the nuclear donor increases
Why??
Gilbert Fig. 4.6
All cells have the same DNA
and thus the same library of genes
yet different mRNAs
are made by different cells
Methylation is HERITABLE at the cell level
and thus could be used by cells
to remember decisions they have made
ECB 3e 8-22
DNA imprinting: an example
of how DNA methylation
influences gene expression
during development
Two maternal genomes:
“normal” embryo,
Poor development of
Extraembryonic tissue
Two paternal genomes:
Very small embryo,
“normal” development of
Extraembryonic tissue
Imprinting results
From pre-programmed
Differences in
DNA methylation
Of selected genes
in the sperm and egg
The DNA within cells is NOT naked!
Instead it is wrapped around
histone proteins as chromatin
Inactive genes are thought to be wrapped up
in an inactive "chromatin conformation"
Bone specific
genes are
wrapped up and
inaccessible
Skin cells
Skin specific
genes are not
wrapped up and
are thus
accessible
Muscle specific
genes are
wrapped up and
inaccessible
mRNA
Gene
expression
Post-translational
modification of
histones plays
a key role
in regulating
gene expression
Post-translational
modification of
histones plays
a key role
in regulating
gene expression
To understand a third mechanism of
large scale gene regulation, let’s return to this experiment
If one transferred chromatin
from a de-methylated fat cell
that had become a myoblast
it could tranform untreated fat cells.
However frequency suggests
Only one or a few of the genes have this ability
They ultimately identified a single genemyoD
- whose expression could convert
,any other cell types into muscle cells
MyoD is induced
by treatments
that induce
mesoderm
MyoD is expressed at the
right place and time
to be a master regulator of
muscle development
But is this true???
Is myoD necessary for muscle formation?
Mario Capecchi
Oliver Smithies
UNC
Gene knockout
Nobel prize in
physiology and
medicine 2007
Martin Evans
MyoD works with related transcription factors
to regulate the muscle cell fate
MyoD works with related transcription factors
to regulate the muscle cell fate
I can tell you and even better story
How to Remember a Developmental Decision
Remember that
Wingless is
expressed by
specific cells
within each segment
Engrailed is
expressed by cells
Anterior
posterior to
those that express
Wingless
Posterior
Wg En
Here’s the real thing
Engrailed
expression
initially
requires
Wingless
signaling
by the
neighboring
cells
Later, however, Engrailed becomes autoregulatory
i.e., it turns on its own synthesis
In cells where engrailed is turned off, a set of special chromatin
proteins bind to it and wrap it up in a permanently inactive state.
We also regulate stem cells
And are mutated in cancer
We also regulate stem cells
And are mutated in cancer