Anti-Human Nucleolus

In cell biology, the nucleolus (plural nucleoli) is a sub-organelle of the cell nucleus, which itself is an organelle. A main function of the nucleolus is the production and assembly of ribosome components. The nucleolus is roughly spherical, and is surrounded by a layer of condensed chromatin. No membrane separates the nucleolus from the nucleoplasm. Nucleoli are made of protein and ribosomal DNA (rDNA) sequences of chromosomes. The rDNA is a fundamental component since it serves as the template for transcription of the ribosomal RNA (rRNA) for inclusions in new ribosomes. Most plant and animal cells have one or more nucleoli, but some cell types do not have any. Since nucleoli carry out the production and maturation of ribosomes, large numbers of ribosomes are found inside them. In addition to ribosome biogenesis, nucleoli are believed to have other roles in cellular activity. Nucleoli fragment during cell reproduction (they can no longer be seen in metaphase of mitosis, when the chromosome copies separate). After the daughter cells complete their separation, the fragments of nucleoli fuse together around the nucleolus organizer regions (NORs) of the chromosomes.