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.
- 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:
I. Fibrillar regions:
- This part is made up of ribonucleoprotein fibers.
- It is also called nucleolemma.
- Each fiber has a length of around 50-80 Å.
Ii. Granular regions :
- 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.
Iii. Protein region:
- This proteinous part is also called pars amorpha.
- This is the fluid part of the nucleolus in which other parts are found.
- It is made up of chromatin fibers containing DNA.
- These DNA molecules function as templates for RNA synthesis.
- The chromatin part may be differentiated into two parts
a) Perinucleolar Chromatin:
- It forms a covering or envelope around the nucleolus.
- It may have ingrowths at certain places inside the nucleolus, which are called trabeculae.
b) Intra-nucleolar chromatin:
- These chromatin fibers are found in the internal protein region.
- These form many septa-like structures.
- 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.
Functions of Nucleolus
- Nucleoli are the site of rRNA biosynthesis.
- It stores rRNA.
- It helps in the biogenesis of ribosomes.
- It helps in the formation of spindle fibers.
- It plays an important role in mitosis.
Functions of Nucleus
- 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 cycle & NUCLEOLOGENESIS
- 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;