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Silence of the Genes
Genetics
• The study of inheritance
Gregor Mendel
(1823-1884)
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Mendelian Ratios
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Mutation
Phenotype
ATGCGATCGAGT
Genotype
ATGCGAGCGAGT
Epigenetics
• Heritable changes in gene expression that
do not involve changes in DNA sequence.
Epi-mutation
Phenotype
ATGCGATCGAGT
Genotype
ATGCGAACGAGT
DNA Modifications
Histone Modifications
Proteins
Nucleosome
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DNA
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histones
DNA
Methylation
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CH3
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histones
Histone
Methylation
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CH3
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histones
Examples of Epigenetics
X-Inactivation
male
XY
female
unequal expression
XX
inactivation
XY
equal expression
X
X
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Barr Body
Barr Body
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Calico Cat
black
allele
o
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X
o
XO
black sector
X X
O orange
allele
oX
X
O
orange sector
Other Examples of Epigenetics
Imprinting
• Imprinted genes are expressed differently
depending on whether they are inherited
through the maternal or paternal parent.
Horses and Donkeys
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male
female
male
female
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Gynogenotes
• Embryos containing two female genomes
– do not develop normally
– fail due to underdeveloped extraembryonic
placental tissue
Androgenotes
• Embryos containing two paternal
genomes
– result in abnormal (often overgrown) embryo
– display overdeveloped extraembryonic
placental tissue
Maternal vs. Paternal Imprinting
• Male genome wants to promote growth.
• Female genome wants to inhibit growth.
Resetting Methylation Patterns
Primordial Germ Cells
methylation
male
female
developmental time
Battle for Maintaining DNA
Methylation
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Battle to Maintain Methylation
methylation
Fertilization
developmental time
Female Strategy
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Genes that promote growth
Fertilization
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Male Strategy
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Genes that inhibit growth
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Antisense
Fertilization
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Resetting Imprints is Important
for Proper Development
Dolly
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• First mammal to be
cloned from an adult
cell.
• Developed illness
common in older
sheep (arthritis)
• Probably due to
abnormal imprinting
Gene Silencing
Heterochromatin
Densely staining condensed chromosomal regions;
believed to be transcriptionally inert.
Euchromatin
A chromosomal region that stains normally;
thought to contain the normally functioning genes.
Heterochromatin
B. McClintock
Paul Fransz
IV
Heterochromatin
• Centromeric regions
• Telomeric regions
Position Effect Variegation
W+
heterochromatin
W+
spreading
W+
suppressed
W+
enhanced
Post-Transcriptional Gene
Silencing
Anti-Sense
Sense Also Works?
Guo and Kemphues, (1995)
What is Causing Silencing?
?
dsRNA
Post Transcriptional
?
Fire et al. (1998)
RNAi
?
Fire et al. (1998)
RNAi
dsRNA
Dicer
RISC
AA
A AA
A
RNAi
dsRNA
Dicer
RISC
AA
A AA
A
RNA Silencing
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cosuppression
RNA Silencing
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quelling
RNA Silencing
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Post Transcriptional
Gene Silencing
(PTGS)
RNA Silencing
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RNA Interference
(RNAi)
RNA Interference
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DNA Methylation
Mette et al. (2000)
RNAi and Transcriptional
Silencing
Transcriptional Silencing
Mette et al. (2000)
Micro RNAs
dicer
Grishok et al. (2001)
RNAi Genes in S. pombe?
RNAi Genes in pombe?
• Yes!!
• Mutation of these genes results in loss of
centromeric silencing.
• Loss of RNAi reveals centromeric
transcripts.
• dsRNA from centromere targets
transcriptional silencing.
Cen
Cen
RNAi
Small RNAs
Cen
RNAi
Silencing
Machinery
Small RNAs
Silencing
Machinery
Cen
RNAi
Small RNAs
Cen
RNAi
• Is important for initiation and maintenance
of heterochromatin at the centromere.
• Could be involved in other silencing
phenomena
Where else may RNAi be
functioning to silence genes?
X Chromosome Inactivation
(RNAi?)
Xist
Tsix
Heard et al (2001)
X Chromosome Inactivation
(RNAi?)
Silencing
Machinery
RNAi
Xist
Tsix
Heard et al (2001)
X Chromosome Inactivation
(RNAi?)
Silencing
Machinery
RNAi
Xist
Tsix
Heard et al (2001)
Imprinting
(RNAi?)
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Silencing
Machinery
RNAi
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Antisense
Small RNAs
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Imprinting
(RNAi?)
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Antisense
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Imprinting
(RNAi?)
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Silencing
Machinery
RNAi
Heterochromatin
Densely staining condensed chromosomal regions;
are not necessarily transcriptionally inert. RNAi
is important for its initiation and maintenance.
Euchromatin
Chromosomal regions that do not densely stain;
thought to contain functioning genes that may
be transcriptionally active.