Primary and secondary constrictions of a chromosome

Question

Define the primary and secondary constrictions of a chromosome? Write the other name provided to the secondary constriction?

Answer 1

Primary constriction is the narrower area of a condensed chromosome where the centromere, the structure which unites similar chromatids, is situated. Secondary constriction is an area identical to the primary constriction, narrower than the normal thickness of the chromosome as well, and in common it is associated to genes which coordinate the formation of the nucleolus and control the ribosomic RNA (or rRNA) synthesis. For this cause the secondary contrictions (which can be one or more in chromosome) is termed as nucleolus organizer region (or NOR).