Within the nucleus are several organelles with specialized functions. Of these organelles, the nucleolus is one of the most prominent. 

A nucleolus is the site of ribosomal RNA, or rRNA, transcription and processing and ribosome assembly. Hence it is also referred to as a ‘ribosome producing factory.’

A membraneless organelle, the nucleolus is an aggregate of rRNA genes and the various proteins and RNAs that are required for rRNA transcription and ribosome assembly. These include small nucleolar ribonucleoproteins, or snoRNPs, rRNA-processing enzymes, assembly factors, and partially assembled ribosomes. 

In eukaryotes, three rRNA genes, 18S, 5.8S, and 28S, are encoded by a single transcription unit. This transcription unit is tandemly repeated in the form of arrays in one to several chromosomes. 

The chromosomal regions which contain these rRNA gene clusters are known as ‘nucleolar organizer regions’, or N-O-Rs, and these are the regions around which the organization of nucleolus takes place. 

As a cell transits between the two major cell cycle stages, interphase and mitosis or M-phase, its requirement of protein synthesis changes drastically. 

It is high during interphase, becomes low during most of M phase, and becomes high again as the cell re-enters interphase.  

The size of the nucleolus, which reflects the number of ribosomes that a cell is producing, varies greatly during these cell cycle phases. During interphase, the nucleolus exists as a single large entity. 

At this stage, the chromosomes exist in a decondensed state and the NOR regions contribute DNA in extended, open loops inside the nucleolus. 

As the cell enters M phase, the chromosomes begin to condense and the nucleolus fragments into multiple smaller nucleoli. 

As mitosis progresses further, the pattern continues. The nucleoli gradually decrease in size and finally disappear. At the end of cell division, during telophase, the chromosomes start to decondense and tiny nucleoli begin to emerge. 

As M phase progresses further, the nucleoli progressively merge, through a process called nucleolar fusion. They first coalesce into larger nucleoli, and then into a single, large nucleolus as the cell enters interphase again.