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Epigenetic mechanisms of gene regulation. Chromatin structure Slows transcription Hypercondensation stops transcription Position effect variegation X inactivation Inactivation of one X chromosome in female mammals Genomic imprinting
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Epigenetic mechanisms of gene regulation • Chromatin structure • Slows transcription • Hypercondensation stops transcription • Position effect variegation • X inactivation • Inactivation of one X chromosome in female mammals • Genomic imprinting • Silences transcription selectively if inherited from one parent
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 • Heterochromatin • 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 • Lightly stained regions of chromosomes • Contains most genes
Heterochromatin versus euchromatin • 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 • Facultative heterochromatin • Moving a gene near heterochromatin silences its activity in some cells and not others Fig. 12.14 a
Position effect variegation in Drosophila: moving a gene near heterochromatin prevents it expression • A model for position-effect variegation • Heterochromatin can spread different distances in different cells Fig. 12.14 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 Fig. 17.14
X inactivation • 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
Experiments with transmission of Ig f2 deletion showed mice inheriting deletion from male were small. Mice inheriting deletion from female were normal. Figure 17.15 a
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
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