The nucleolus is an approximately spherical region within the nucleus of eukaryotic cells. It consists of nucleolar organizers? and ribosomal subunits.
Nucleolar organizers are special regions on some chromosomes that contain multiple copies of the genes encoding for rRNA (ribosomal RNA), which is a functional part of the ribosomes and which are involved in protein biosynthesis.
     Following synthesis the rRNA is attached to the
ribosomal subunits in the nucleus before transport through the nuclear pores into the cytosol.

A. ell's cytoplasm
B. nucleus of cell
C. nucleolus labelled with radioactive RNA precursors.