Epigenetic mechanisms of gene regulation Chromatin structure Chromatin structure Slows transcription Slows transcription Hypercondensation stops transcription Hypercondensation stops transcription Position effect variegation Position effect variegation X inactivation X inactivation Inactivation of one X chromosome in female mammals Inactivation of one X chromosome in female mammals Genomic imprinting Genomic imprinting Silences transcription selectively if inherited from one parent Silences transcription selectively if inherited from one parent

Normal chromatin structure slows transcription Fig

Remodeling of chromatin mediates the activation of transcription Fig

Studies using DNase identify decompacted regions Fig a

Extreme condensation silences expression Heterochromatin Heterochromatin Darkly stained region of chromosome Darkly stained region of chromosome Highly compacted even during interphase Highly compacted even during interphase Usually found in regions near centromere Usually found in regions near centromere Constitutive heterochromatin remains condensed most of time in all cells (e.g., Y chromosomes in flies and humans) Constitutive heterochromatin remains condensed most of time in all cells (e.g., Y chromosomes in flies and humans) Euchromatin Euchromatin Lightly stained regions of chromosomes Lightly stained regions of chromosomes Contains most genes Contains most genes

Heterochromatin versus euchromatin Heterochromatin is darkly stained Heterochromatin is darkly stained Euchromatin is lightly stained Euchromatin is lightly stained C-banding techniques stains constitutive heterochromatin near centromere C-banding techniques stains constitutive heterochromatin near centromere Fig

Hypercondensation over chromatin domains causes transcriptional silencing Fig

Position effect variegation in Drosophila: moving a gene near heterochromatin prevents it expression Facultative heterochromatin Facultative heterochromatin Moving a gene near heterochromatin silences its activity in some cells and not others Moving a gene near heterochromatin silences its activity in some cells and not others Fig a

A model for position-effect variegation A model for position-effect variegation Heterochromatin can spread different distances in different cells Heterochromatin can spread different distances in different cells Position effect variegation in Drosophila: moving a gene near heterochromatin prevents it expression Fig b

In mammals hypercondensation is often associated with methylation It is possible to determine the methylation state of DNA using restriction enzymes that recognize the same sequence, but are differentially sensitive to methylation 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

X inactivation Inactivation of one X chromosome to control for dosage compensation in female mammals Inactivation of one X chromosome to control for dosage compensation in female mammals One X chromosome appears in interphase cells as a darkly stained heterochromatin mass-Barr body One X chromosome appears in interphase cells as a darkly stained heterochromatin mass-Barr body

Experiments with transmission of Ig f 2 deletion showed mice inheriting deletion from male were small. Mice inheriting deletion from female were normal. Experiments with transmission of Ig f 2 deletion showed mice inheriting deletion from male were small. Mice inheriting deletion from female were normal. Figure a

Figure b

H19 promoter is methylated during spermatogenesis and thus the H19 promoter is not available to the enhancer and is not expressed H19 promoter is methylated during spermatogenesis and thus the H19 promoter is not available to the enhancer and is not expressed Figure d

Methylation can be maintained across generations by methylases that recognize methyl groups on one strand and respond by methylating the opposite strand Methylation can be maintained across generations by methylases that recognize methyl groups on one strand and respond by methylating the opposite strand Fig c

Figure e