The nucleolus is organized from the "nucleolar organizing regions" on different chromosomes. A number of chromosomes get together and transcribe ribosomal RNA at this site. The nuclear organizing ( NO ) regions are seen as circular areas ( pale ) surrounded by a rim of electron dense filaments. These filaments collectively are called the pars fibrosa ( PF ). This is formed from newly transcribed ribosomal RNA. This diagram shows the parts of the nucleolus.

After the ribosomal RNA is transcribed, it is linked to proteins and one can see accumulation of ribonucleoprotein particles in the pars granulosa ( PG ). These particles form the two types of ribosomal subunits ( large and small ) which are then transported out of the Nuclear Pores separately. The pores do not accommodate the assembled ribosomes, therefore, they cannot reenter. This means that translation of RNA and synthesis of proteins must occur outside the nucleolus. For more information follow the link to ribosomes

After the ribosomal subunits are in the cytoplasm, they are connected and then, they may attach to endoplasmic reticulum . They provide an attachment and "code reading" site for the messenger RNA. If they attach to rough endoplasmic reticulum, they form a pore that moves the newly synthesized proteins into the cisterna of the rough endoplasmic reticulum.