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:

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.

iv.Chromatin part:  

  • 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 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;
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