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Constitutive heterochromatin can affect the genes near itself (e.g. position-effect variegation). It is usually repetitive and forms structural functions such as centromeres or telomeres, in addition to acting as an attractor for other gene-expression or repression signals.
During mitosis it is believed that constitutive heterochromatin is necessary for proper segregation of sister chromatids and centromere function. [6] The repeat sequences found at the pericentromeres are not conserved throughout many species and depend more on epigenetic modifications for regulation, while telomeres show more conserved sequences.
The family of heterochromatin protein 1 (HP1) ("Chromobox Homolog", CBX) consists of highly conserved proteins, which have important functions in the cell nucleus.These functions include gene repression by heterochromatin formation, transcriptional activation, regulation of binding of cohesion complexes to centromeres, sequestration of genes to the nuclear periphery, transcriptional arrest ...
By contrast, CMT2 and CMT3 preferentially function in constitutive heterochromatin and depend strongly on DDM1 to maintain silencing over these regions. [ 131 ] [ 5 ] [ 3 ] Similarly, MET1, which maintains DNA methylation at CG sites after replication, requires DDM1 to access heterochromatin and maintain CG methylation in those regions. [ 132 ]
Heterochromatin is further separated into facultative and constitutive heterochromatin, the former of which is found in DNA sequences that code for developmental proteins. Non-coding repetitive DNA associated with specific chromosomal locations, such as centromeres and telomeres, is referred to as constitutive heterochromatin.
The level of nucleosomal packaging can have profound consequences on all DNA-mediated processes including gene regulation. Euchromatin (loose or open chromatin) structure is permissible for transcription whereas heterochromatin (tight or closed chromatin) is more compact and refractory to factors that need to gain access to the DNA template.
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A constitutive gene is a gene that is transcribed continually as opposed to a facultative gene, which is only transcribed when needed. A housekeeping gene is a gene that is required to maintain basic cellular function and so is typically expressed in all cell types of an organism.