Provide a comprehensive definition of heterochromatin, and list as many examples as you can.

Heterochromatin is a tightly packed form of DNA, which is transcriptionally inactive due to its condensed structure. This condensed state prevents the access of the transcription machinery to the DNA, thus inhibiting gene expression. Heterochromatin can be classified into two types: constitutive heterochromatin and facultative heterochromatin.

Constitutive heterochromatin is a stable, permanently condensed form of DNA that is found mainly at the centromeres and telomeres of chromosomes. It is formed mainly of repetitive sequences and does not contain many protein-coding genes. Its primary function is to maintain chromosome stability and integrity during cell division.

Facultative heterochromatin is a more dynamic form of heterochromatin, which can switch between a condensed and a more relaxed, transcriptionally competent state. Its formation is often associated with the regulation of gene expression during development and in response to environmental stimuli.

1. In eukaryotes, heterochromatin is found at the centromeres and telomeres of chromosomes, ensuring proper chromosome segregation during cell division and protecting the ends of chromosomes from degradation. 2. In mammals, the inactive X chromosome in female cells undergoes X chromosome inactivation, which involves the formation of facultative heterochromatin to silence most genes on one of the X chromosomes. 3. In Drosophila, heterochromatin is responsible for the phenomenon known as position-effect variegation (PEV). When a gene is placed near heterochromatin due to a chromosomal rearrangement, it may be stochastically silenced due to the spreading of heterochromatin. 4. In plants, heterochromatin is often found in condensed regions called chromocenters and is composed of repetitive DNA sequences. These chromocenters have been shown to play a role in the regulation of gene activity, especially during developmental transitions. 5. In yeast, silent mating type loci HML and HMR are examples of heterochromatin, where specific genes are silenced to maintain a specific mating type. The silent information regulator (SIR) protein complex is responsible for the formation of heterochromatin at these loci.