Nucleolus

4. Nucleolus :

Within each nucleus, there is a darkly stained, granular, naked and large organelle without limiting membrane. It was discovered by Fontana in 1781.
The term nucleolus was coined by Bowman (1840).
The size of nucleolus is comparatively larger in those cells which have rapid rates of protein biosynthesis.

The position of the nucleolus is generally definite within the nucleus.
It is associated with the nucleolar organizer region (NOR) of the nuclear chromosome.
It is absent in muscle fibers , RBC, Yeast, sperm and prokaryotes.
In general, each nucleus has one or two nucleoli.
Its number depends on the number of chromosomes in the species.
For each haploid set of chromosomes in the nucleus, there is a single nucleolus.
However, a pair of nucleoli may be found in haploid nuclei.
In human beings, two pairs of nucleoli are found in each diploid nuclei.
In human beings, two pairs of nucleoli are found in each diploid nucleus Xenopus oocytes may contain upto 1000 nucleoli in the nucleoli in the nucleus.

Ultrastructure :

The ultrastructure of nucleolus was studied by Borysko and Bang in 1951 and again by Berhard in 1952.
On the basis of electron microscopic studies of the structure of nucleolus, de Robertis et al., (1971) described it to be made up of four parts:

This part has many granules each having the diameter of 150-200 Å.
These are derived from nucleolar fibers, chemically, these granules are also ribonucleoproteins.

It forms a covering or envelope around the nucleolus.
It may have ingrowths at certain places inside the nucleolus, which are called trabeculae.

Each nucleolus has a dense fibrillar region due to the presence of which it is associated with the nuclear organizer region of chromosomes.
These regions have been reported to contain many copies of DNA responsible for synthesis of ribosomal RNA.
These rRNA molecules are rapidly synthesized in this region.
The proteins of ribosomes are synthesized in the cytoplasm which is transported to the nucleus and finally to the nucleolus.
The rRNA and protein molecules combine to form complete ribosome molecules.

These newly synthesized ribosomes are associated with thin fibrils of RNA and look like beaded strings.
This structure is called nucleo-nema.
On the basis of the presence and structure of nucleo-nema, following three types of nucleoli may be recognized:

Nucleolus with nucleo-nema which is more common is all types of cells.
Nucleolus without nucleo-nema which is commonly found in salivary gland cells.
Ring shaped nucleolus containing ribonucleoprotein granules and RNA fibrils.
This is common in endothelial cells and muscle cells.

It controls all the cellular functions.
It controls the synthesis of all the structural and enzymatic proteins.
Synthesis of all the 3 types of RNA (mRNA, tRNA and rRNA) takes place in the nucleus.
It plays important role in cell division.
Cell growth is controlled by nucleus
Nucleus controls cellular differentiation by regulating differential gene expression

It induces genetic variation and thus helps in organic evolution.
Sexual reproduction happens due to fusion of two nuclei gametes of opposite sex.
Due to presence of all these organelles and other structures, a cell functions as self-regulatory systems and provides a definite set of characteristics to different organisms.

Nucleolus can be seen as a very conspicuous structure in the interphase nucleus. It disappears during mitosis and reappears at the next interphase.
The process by which the nucleolus is formed, is described as nucleologenesis.
During prometaphase to early telophase, when the nucleolus remains disappeared, a number of non-ribosomal nucleolar proteins as well as U3 s- RNA are found in
(i) the peripheral regions of chromosomes and in the
(ii) nucleolus derived foci (NDF) found as cytoplasmic particles 1-2 in diameter;