Q. What are the secondary and the primary constrictions of a chromosome? And what is the other name given to the secondary constriction?

Primary constriction is the narrower region of a condensed chromosome where the centromere, the structure that are unites identical chromatids, is located. Secondary constriction is a region alike to the primary constriction, narrower than the normal thickness of the chromosome too, and in general it is related to genes that coordinate the formation of the nucleolus and control the ribosomic the RNA (rRNA) synthesis. For this reason the secondary contrictions that can be one or more in chromosome is called nucleolus organizer region (NOR).