Structure of Nucleolus

Structure of Nucleolus

Structure of Nucleolus

In higher organisms, every cell nucleus has a spherical, colloidal body called nucleolus, which is associated with a particular nucleolar organizing chromosome. A special region in this chromosome is known as the nucleolar organizing region (NOR) to which ally the nucleolus is associated. Quite often, more than one nucleoli in the same nucleus may also be observed. These several nucleoli may subsequently coalesce and give rise to a larger single nucleolus. While chromatin mainly consists of DNA, nucleolus mainly consists of RNA, acidic dyes and basic dyes; phospholipids and alkaline phosphatase are also found. Nucleolar DNA had also been reported in a number of cases. This DNA is believed to represent the nucleolar organizer.

The nucleolus was first described in 1781 by Fontana. Ever since then, vast literature has been published on the nucleolus which has been adequately covered in a monograph entitled. “The nucleolus and ribosome biogenesis” published in 1985. This monograph was written by the distinguished French molecular biologist, Asen Hadjiolov, who suddenly passed away in April 1996. Since the publication of the above monograph, considerable work has been done, which has been covered recently in a special issue of the journal chromosoma brought out in the memory of Dr. Hadjiolov. The present status of the biology of nucleolus is briefly summarized here.

(I) 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 (e.g. B23, fibrillarin, nucleolin and p52) as well as U3 snoRNA are found in (i) the peripheral regions of chromosomes and in the (ii) nucleolus derived foci (NDF) found as cytoplasmic particles 1-2μm in diameter, the number of these NDF’s can reach as many as 100 per cell at mid-to late-anaphase, but later their number declines to few or none at telophase. The decline in the number of NDF approximately coincide with appearance of prenucleolar bodies (PNBs) and reforming of the nucleoli. These observations suggest that the NDF are the precursors of prenucleolar bodies, which later form nucleoli, but the forces that bind them together to form nucleoli and still unknown.

(II) Nucleolus and the ribosome biogenesis

The nucleolus is the site of ribosome biosynthesis, where the synthesis of ribosomal RNA (rRNA) and the assembly of ribosome takes place. It has been shown that the genes for ribosomal. RNA are clustered at the nucleolar organizer region in the form of tandemly repeated ribosomal DNA (rDNA) units. The initiation, production and maturation of ribosomes in the nucleolus seem to proceed from centre to the periphery in the followin three distinct regions:

  • fibrillar centre (FC), where rRNA genes of NOI (nucleolar organizing region) are located; the transcription of rRNA gene (RNA synthesis on DNA template) also takes place in this region.
  • Dens fibrillar component (DFC), which surrounds the fibrillar centre and wher RNA synthesis progresses; the 80S ribosomal proteins (rps) also bind to the transcripts in this region.
  • Cortical granular component (GC), which is the outermost region and where processing and maturation of pre-ribosoma particles occurs. These three regions of the nucleolus and their roles in ribosome formation are shown in Figure. The transcription for the synthesis of pre-rRNA actually occurs in FC and particularly at its edge in a region described a transcription zone (txn). In this region are observed the structures describes as “Christmas trees” by O. Miller. The ribosomal precursors (pre-rRNAs) move from one compartment to the next for different stages of processing and are subsequently transported to the cytoplasm to make mature ribosomes.

(III) Small nucleolar RNAs (snoRNAs) and rRNA processing

During the last more than a decade, the list of snoRNAs has grown to over 50, of which seven snoRNAs (U3, U7, U14, U22, SnR10, SnR30,RNA component of RNAase MRP) are known to be required in the processing of pre-rRNA. Of these seven U3 snoRNA associates with the nascent pre-rRNA and accompanies it, when it passes through the different domains of the nucleolus during its processing. This U3 snoRNA recycles from the granular component (GC) to the dense fibrillar component (DFC) for association with another nascent pre-rRNA.

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