Chromatin is essentially composed of DNA and basic proteins called histones. It also contains RNA and some non-histone proteins. The histone proteins are the packaging proteins that are associated with the packaging of DNA into compact structures called chromosomes. During certain stages of cell division, the cells show structured chromosomes in place of the nucleus.
It is observed that when chromosomes are stained with dyes like acetocarmine or Feulgen stain, two types of regions can be observed: (a) Heterochromatin region: This region gets a dark stain during interphase. It is a genetically inactive and highly condensed region with tightly packed DNA. (b) Euchromatin region: This region gets a light stain during interphase. This portion is genetically active
and is rich in loosely packed DNA. Transcription occurs in this region.
In higher organisms, the well-organised nucleus contains a definite number of chromosomes of definite size and shape. For example, a single human cell has an approximately two-metre-long thread of DNA distributed among its 46 chromosomes (23 pairs of chromosomes).