BIOL 370 – Developmental Biology
Topic #2
Differential Gene Expression:
The Gene’s Role in Development
Lange
Cloning - the process of producing similar populations of
genetically identical individuals that can occur in nature
when organisms such as bacteria, insects or plants
reproduce asexually.
Cloning (using modern biotechnology) also refers to
scientific processes used to create copies of DNA
fragments (molecular cloning), cells (cell cloning), or
organisms (organismal cloning) in the lab or in a clinical
setting.
Cloned mammals have been created using nuclei from adult somatic cells
The basic process of
producing “Dolly”… the
first cloned mammal.
Murray Barr
Canadian physician and medical researcher who discovered the Barr Body
in 1948. He also played a key role in understanding
the genetics behind calicoism in cats.
Mary Lyon
In 1961 she proposed the concept of X-inactivation: one of the two X chromosomes
inside a female mammal shuts off under normal circumstances. Yet in some
circumstances different X chromosomes will shut off, resulting in the calico
appearance. She observed this in the coat color patterns in mice.
The kitten “CC”
In image (a) we see the kitten named “CC” who is a clone of the material
cat in image (b) named “Rainbow”. The kitten appears different and
behaves differently from the mother due to a non-randomized Xinactivation, but they are genetically identical.
What
Timing of X-inactivation:
In studies using mouse cells to study the process of X-inactivation:
•
it has been determined that an early, imprinted inactivation of the paternally-derived X
chromosome in two-cell or four-cell stages of development occurs.
•
However, in the early blastocyst, this initial, imprinted X-inactivation is reversed in cells of
the inner cell mass, and in these cells both X chromosomes become active again.
•
Each of these cells then independently and randomly inactivates one copy of the X
chromosome. This inactivation event is irreversible during the lifetime of the cell, so all the
descendants of a cell which inactivated a particular X chromosome will also inactivate that
same chromosome.
•
X-inactivation is an epigenetic change that results in a different phenotype and is not a
change at the genotypic level.
•
For an individual cell or lineage the inactivation is therefore skewed or 'non-random', and
this can give rise to mild symptoms in female 'carriers' of X-linked genetic disorders.
It is hypothesized there is an autosomally produced 'blocking factor' that binds to an X
chromosome after fertilization and prevents its inactivation. Potentially, the blocking factor may
affect the selection of which X chromosome at an early cell stage or perhaps at a later stage.
The later the stage of effect, the more likely the display of calicoism.
So, what is the way in which cells, which are identical
genetically, will differentially grow and develop into specific
cell types, tissues, organs, etc.?
• Differential gene expression – this can arise at a variety
of levels including: gene transcription, nRNA processing,
and mRNA translation.
• Histone protein differences - histones are highly alkaline
proteins found in eukaryotic cell nuclei that package and
order the DNA into structural units called nucleosomes.
• Methylated histones often inhibit gene expression
• Acetylated histones often activate gene expression
The genetic elements regulating tissue-specific transcription can be identified by fusing reporter
genes to suspected enhancer regions of the genes expressed in particular cell types
A reporter gene is a gene that
researchers attach to a
regulatory sequence of
another gene of interest.
Enhancer region modularity
Silencers. Analysis of β-galactosidase staining patterns in 11.5-day embryonic mice
Here we see the
NRSE (neural
restrictive silencer
element) sequence of
a gene for the
development of the
nervous system.
The lack of the NRSE
sequence in (b)
results in the mouse
displaying excessive
B-galactosidase (the
product produced by
the lacZ gene
sequence.
Methylation of globin genes in human embryonic blood cells
Methylation typically
results in inhibition of
a gene.
DNA methylation can block transcription by preventing transcription factors from binding to the
enhancer region
X chromosome inactivation in mammals
Prader-Willi Syndrome and Angleman’s Syndrome
Two genetic disorders that affect development and are caused by issues
associated with chromosome 15.
Prader-Willi Syndrome (PWS) - a rare, genetic disorder in which seven
genes on Chromosome 15 are deleted or unexpressed on the paternal
chromosome. It was first described in 1956 and characteristic of PWS
include:
•
•
•
•
•
low muscle tone,
short stature,
incomplete sexual development,
cognitive disabilities,
a chronic feeling of hunger that can lead to excessive eating and lifethreatening obesity.
Angelman Syndrome (AS) a neuro-genetic disorder characterized by:
•
•
•
•
•
severe intellectual and developmental disability,
sleep disturbance,
seizures,
jerky movements (especially hand-flapping),
frequent laughter or smiling, and usually a happy demeanor.
AS is a classic example of genomic imprinting in that it is caused by deletion or
inactivation of genes on the maternally inherited chromosome 15 while the
paternal copy, which may be of normal sequence, is methylated and therefore
silenced.
Inheritance patterns for Prader-Willi and Angelman syndromes (Part 1)
Note that
the region
can be
deleted or
methylated
into
inactivation.
Inheritance patterns for Prader-Willi and Angelman syndromes (Part 2)
The Dscam gene of Drosophila can produce 38,016 different types of proteins by alternative nRNA
splicing
This Dscam gene in the fruit fly is homologous to a DNA
sequence for nervous system development on human
chromosome 21.
In cases where this sequence is disrupted in humans
embryologically, the offspring will develop a form of
Downs Syndrome.
Dscam protein is specifically required to keep dendrites from the same neuron from adhering to
each other
Muscle hypertrophy through mispliced RNA
A television program on TLC focused on a child who has a believed to
be similar mutation causing muscle hypertrophy. The television show
was called “The World’s Strongest Toddler”
Degradation of casein mRNA in the presence and absence of prolactin
The lack of hormone resulted in rapid decay of mRNA, while the
presence of the hormone exerted the oposite effect. How might this
relate to developmental expression of milk in females?
The lymphoid precursor cell can generate B cells or T cells
B cells will form antibodies, whereas T cells primairly attack directly.
End.