Ribosomes. Coloured transmission electron micrograph (TEM) of ribosomes (blue) passing through pores in a cell's nuclear membrane (red). The nucleus is at top left and the cytoplasm is at bottom right. Ribosomes consist of two subunits composed of protein and ribosomal ribonucleic acid (rRNA). They are made in the nucleolus (not seen) and pass out of the nucleus to the cytoplasm where they translate mRNA (messenger RNA) into proteins.