Heterochromatin Topics

• x chromosome inactivation
• chromosome centromere
• fission yeast schizosaccharomyces
• epigenetic inheritance
• female mammals
• human chromosomes
• dna ends
• y chromosome
• ribosomal rna
• double stranded dna
• daughter cells
• dna repair
• euchromatin
• pombe
• hml
• rdna
• rnai
• telomeres
• morphogenesis

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Heterochromatin, Heterochromatin

For example, naked double-stranded DNA ends would usually be interpreted by the cell as damaged DNA, triggering cell cycle arrest and DNA repair.Some regions of chromatin are very densely packed with fibres displaying a condition comparable to that of the chromosome at mitosis.Heterochromatin is generally clonally inherited; when a cell divides the two daughter cells will typically contain heterochromatin within the same regions of DNA, resulting in epigenetic inheritance. Variations cause heterochromatin to encroach on adjacent genes or recede from genes at the extremes of domains. Transcribable material may be repressed by being positioned (in cis ) at these boundary domains. This gives rise to different levels of expression from cell to cell, ).

For example, all human chromosomes 1, 9, 16, and the Y chromosome contain large regions of constitutive heterochromatin. In most organisms, constitutive heterochromatin occurs around the chromosome centromere and near telomeres.

However, the formation of facultative heterochromatin is regulated, and is often associated with morphogenesis or differentiation. An example of facultative heterochromatin is X-chromosome inactivation in female mammals: one X chromosome is packaged as facultative heterochromatin and silenced, while the other X chromosome is packaged as euchromatin and expressed.

Although most of its genome can be characterized as euchromatin, S. cerevisiae has regions of DNA that are transcribed very poorly. These loci are the so-called silent mating type loci (HML and HMR), the rDNA (encoding ribosomal RNA), and the sub-telomeric regions.Fission yeast ( Schizosaccharomyces pombe ) uses another mechanism for heterochromatin formation at its centromeres. Gene silencing at this location depends on components of the RNAi pathway. Double-stranded RNA is believed to result in silencing of the region through a series of steps.

Source: Wikipedia > Heterochromatin