Final answer:
Heterochromatin is a form of tightly packed DNA that is transcriptionally inactive and includes regions like centromeres and telomeres. Common examples include the nucleolus, the Barr body in female mammals, and the protective structures at chromosome ends.
Step-by-step explanation:
Heterochromatin is a tightly packed form of DNA, which is characterized by its dense appearance under a microscope and by being generally transcriptionally inactive. The nucleolus is a notable example of heterochromatin. During the interphase stage of the cell cycle, eukaryotic chromosomes consist of two types of chromatin: heterochromatin and euchromatin. Whereas euchromatin is less dense and contains genes that are often transcriptionally active, heterochromatin is tightly packaged and typically includes regions like the centromeres and telomeres where genes are not usually expressed.
Heterochromatin also plays a role in maintaining the stability of these chromosomal regions. Examples of heterochromatin include the inactivation of one of the X chromosomes in female mammals (known as the Barr body), centromeric heterochromatin, and telomeric heterochromatin that protect the ends of chromosomes. These structures are crucial for the proper segregation of chromosomes during cell division and the preservation of genomic integrity.