Silence of the Genes

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Silence of the Genes
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Genetics

• The study of inheritance

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Gregor Mendel(1823-1884)
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Mendelian Ratios
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Mutation
Phenotype
Genotype
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Epigenetics

• Heritable changes in gene expression that do not
  involve changes in DNA sequence.

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Epi-mutation
Phenotype
Genotype
DNA Modifications
Histone Modifications
Proteins
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Nucleosome
DNA
histones
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DNAMethylation
histones
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HistoneMethylation
histones
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Examples of Epigenetics
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X-Inactivation
unequal expression
equal expression
Barr Body
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Barr Body
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Calico Cat
orange allele
black allele
X
X
o
O
X
X
o
O
O
o
X
X
orange sector
black sector
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Other Examples of Epigenetics
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Imprinting

• Imprinted genes are expressed differently
  depending on whether they are inherited through
  the maternal or paternal parent.

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Horses and Donkeys
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

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Androgenotes

• Embryos containing two paternal genomes
• result in abnormal (often overgrown) embryo
• display overdeveloped extraembryonic placental
  tissue

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Maternal vs. Paternal Imprinting

• Male genome wants to promote growth.
• Female genome wants to inhibit growth.

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Resetting Methylation Patterns
Primordial Germ Cells
male
methylation
female
developmental time
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Battle for Maintaining DNA Methylation
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Battle to Maintain Methylation
Fertilization
methylation
developmental time
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Female Strategy
Genes that promote growth
Fertilization
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Male Strategy
Genes that inhibit growth
Fertilization
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Resetting Imprints is Important for Proper
Development
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Dolly

• First mammal to be cloned from an adult cell.
• Developed illness common in older sheep
  (arthritis)
• Probably due to abnormal imprinting

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Gene Silencing
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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.
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Heterochromatin
B. McClintock
Paul Fransz
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Heterochromatin

• Centromeric regions
• Telomeric regions

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Position Effect Variegation
W
heterochromatin
W
spreading
W
suppressed
W
enhanced
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Post-Transcriptional Gene Silencing
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Anti-Sense
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Sense Also Works?
Guo and Kemphues, (1995)
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What is Causing Silencing?
?
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dsRNAPost Transcriptional
?
Fire et al. (1998)
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RNAi
?
Fire et al. (1998)
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RNAi
dsRNA
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RNAi
dsRNA
RISC
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RNA Silencing
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RNA Silencing
quelling
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RNA Silencing
Post Transcriptional Gene Silencing (PTGS)
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RNA Silencing
RNA Interference (RNAi)
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RNA Interference
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DNA Methylation
Mette et al. (2000)
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RNAi and Transcriptional Silencing
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Transcriptional Silencing
Mette et al. (2000)
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Micro RNAs
dicer
Grishok et al. (2001)
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RNAi Genes in S. pombe?
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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.

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Cen
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Cen
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Cen
RNAi
Silencing Machinery
Small RNAs
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Silencing Machinery
Cen
RNAi
Small RNAs
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Cen
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RNAi

• Is important for initiation and maintenance of
  heterochromatin at the centromere.
• Could be involved in other silencing phenomena

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Where else may RNAi be functioning to silence
genes?
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X Chromosome Inactivation(RNAi?)
Xist
Tsix
Heard et al (2001)
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X Chromosome Inactivation(RNAi?)
Silencing Machinery
RNAi
Heard et al (2001)
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X Chromosome Inactivation(RNAi?)
Silencing Machinery
RNAi
Xist
Tsix
Heard et al (2001)
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Imprinting(RNAi?)
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Imprinting(RNAi?)
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Imprinting(RNAi?)
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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.