Final answer:
Heterochromatin is the transcriptionally inactive and highly condensed form of chromatin, characterized by tightly packed DNA regions such as centromeres and telomeres.
Step-by-step explanation:
The highly condensed and transcriptionally inactive form of chromatin is called heterochromatin. This form of chromatin is associated with regions of DNA that are not being transcribed into RNA, meaning that the genes within these regions are not expressed. Unlike its less condensed counterpart, euchromatin, which contains actively transcribed genes, heterochromatin is characterized by being densely packed and is often found in regions of the genome like the centromeres and telomeres. The condensation of heterochromatin involves high levels of DNA methylation and deacetylation of histones, which leads to a tightly coiled structure of DNA around the histone proteins, forming nucleosomes. These nucleosomes then form higher-order structures through the additional binding of non-histone proteins, eventually giving rise to metaphase chromosomes during mitosis and meiosis.