Nucleolus is a dense, rounded, dark-staining, granular structure without a limiting membrane. It consists largely of RNAs and proteins. Nucleolus synthesises and stores RNA. It also receives ribosomal proteins from the cytoplasm for storage. It forms ribosomal subunits by wrapping the rRNAs with ribosomal proteins. The ribosomal subunits later leave the nucleus through the nuclear pores.