Chromosome Structure: Euchromatin and Heterochromatin

Abstract

Publisher Summary This chapter discusses the terms “euchromatin” and “heterochromatin” that have long been familiar for expressing primarily the state of chromatin condensation during the cell cycle. The term “heterochromatin,” in particular, has been used to describe such chromatin or chromosome segments that did not go through the uncoiling process toward the end of mitosis and so remained condensed throughout interphase. Usually this condensed state of chromatin results in a heteropyknotic property. On the other hand, the chromatin that uncoils in the subsequent interphase is considered euchromatin. The earlier studies on different chromatintypes were based on the staining variation in interphase nuclei. Similar studies using metaphase chromosomes were limited to those species where the heterochromatic segments were recognizable from the staining differences by conventional methods. However, recent advances in banding technology and in sequencing DNA replication using isotype-labeled nucleotides and BrdU have rendered the metaphase chromosomes amenable for such investigations. In metaphase chromosomes, the regions stained by the C-banding method and the latereplicating X chromosome (in females) are regarded as heterochromatic. The remaining regions are referred to as “euchromatic.”