Transcript Epigenetic mechanisms

Epigenetic mechanisms of gene regulation
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Chromatin structure
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X inactivation
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Slows transcription
Hypercondensation stops transcription
Position effect variegation
Inactivation of one X chromosome in female mammals
Genomic imprinting
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Silences transcription selectively if inherited from one
parent
Normal chromatin structure slows
transcription
Fig. 17.13
Remodeling of chromatin mediates
the activation of transcription
Fig. 17.13
Studies using DNase identify
decompacted regions
Fig. 12.12 a
Extreme condensation silences
expression
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Heterochromatin
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Darkly stained region of chromosome
Highly compacted even during interphase
Usually found in regions near centromere
Constitutive heterochromatin remains condensed most
of time in all cells (e.g., Y chromosomes in flies and
humans)
Euchromatin
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Lightly stained regions of chromosomes
Contains most genes
Heterochromatin versus euchromatin
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Heterochromatin
is darkly stained
Euchromatin is
lightly stained
C-banding
techniques stains
constitutive
heterochromatin
near centromere
Fig. 12.13
Hypercondensation over chromatin domains
causes transcriptional silencing
Fig. 17.14
Position effect variegation in Drosophila: moving a
gene near heterochromatin prevents it expression
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Facultative
heterochromatin
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Fig. 12.14 a
Moving a gene
near
heterochromatin
silences its
activity in some
cells and not
others
Position effect variegation in Drosophila: moving a
gene near heterochromatin prevents it expression
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A model for
position-effect
variegation
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Heterochromatin
can spread
different distances
in different cells
Fig. 12.14 b
In mammals hypercondensation is
often associated with methylation
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It is possible to determine the methylation state of
DNA using restriction enzymes that recognize the
same sequence, but are differentially sensitive to
methylation
Fig. 17.14
X inactivation

Inactivation of one X chromosome to
control for dosage compensation in female
mammals
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One X chromosome appears in interphase cells
as a darkly stained heterochromatin mass-Barr
body
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Experiments with transmission of Ig f 2
deletion showed mice inheriting deletion from
male were small. Mice inheriting deletion from
female were normal.
Figure 17.15 a
Figure 17.15 b
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H19 promoter is methylated during
spermatogenesis and thus the H19 promoter
is not available to the enhancer and is not
expressed
Figure 17.15 d
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Methylation can be maintained across generations
by methylases that recognize methyl groups on one
strand and respond by methylating the opposite
strand
Fig. 17.15 c
Figure 17.15 e