NUCLEOLUS AND RIBOSOMAL RNA GENES

NUCLEOLUS AND RIBOSOMAL RNA GENES

Nucleolus

Introduction

Nucleolus is the most prominent sub-structure with in the nucleus. It is the site of rRNA transcription and processing and of ribosome assembly. Nucleolus is a ribosome production factory, designed to fulfill the large scale production of rRNAs and assembly of the ribosomal subunits. It is a rounded colloidal body without  surrounding membrane.

History

Nucleolus was discovered by Fontana in 1781.It is a spheroidal body situated with in the nucleus, either in central or peripheral position. It is found in close association with the nucleolar organizer region(NOR) of two or more chromosome.It is also called as plasmasomes and is mainly consist of rRNA and protein. Actively synthesizing cells have large nucleoli. Nucleolus disappears during late prophase and reappears during telophase of mitosis and meiosis. Nucleolus stains with acidic dyes.

Evolution

Its development is relatively late in evolution as many lower species like yeasts and bacteria lack nucleoli. Nucleolus is also absent in some algae and undifferentiated embryonic cells. Also absent in certain mammalian cells like erythrocytes, reticulocytes and spermatozoa.

Number and Structure

One or many may be present in a nucleus.

Morphologically nucleolus consists of three regions:

a. The fibrillar centre:-

It is pale staining part of nucleolus. It is the innermost region and rRNA genes of NOR is located in the fibrillar centre. Transcription of these genes initiated in this region.

b. Dense fibrillar component:-

It surrounds the FC region and RNA synthesis progresses in this region.70S ribosomal protein also binds to transcript in this region.

Transcription occur primarily at the boundary of the fibrillar centre and DFC, called as transcription zone (txn)

c. Granular component:-

It is the outermost region. Here processing and maturation of pre-ribosomal particles occur. Processing of the pre rRNA is initiated in the DFC and continues in GC, where rRNA is assembled with ribosomal protein to form nearly completed preribosomal subunits, ready for export to the cytoplasm.

These different regions are thought to represent the sites of progressive stages of rRNA transcription, processing and ribosome assembly.

Chemical Composition of nucleolus:

Nucleolus is not bounded by any limiting member and Ca++ ions are supposed to maintain its intact organization. Chemically, nucleolus contains DNA of “NO” , 4 types of rRNAs, 70 s types of ribosomal proteins, RNA binding proteins (e.g., nucleolin) and RNA splicing nucleoproteins (U1, U2, …..U12) .It also contains phospholipids, orthophosphates and Ca++ ions. Also contain some enzymes such as acid phosphatase, nucleoside phosphorylase and NAD+ synthesizing enzymes for the synthesis of some coenzymes, nucleotides and ribosomal RNA.RNA methylase enzymes which transfers methyl groups to the nitrogenous bases, occurs in the nucleolus of some cells.

Function of Nucleolus:

It is site of biogenesis of ribosomal subunits (i.e. 40s and 60s).3 types of rRNAs, namely 18s, 5.8s and 28s are transcribed as parts of much longer precursor molecules (45s transcript) which undergoes processing (RNA splicing) by the help of 2 types of proteins such as nucleolin and U3 snRNP (small nuclear ribonucleoprotein).5S rRNA is transcribed on the chromosome existing outside the nucleolus and 70 types of ribosomal proteins are synthesized in the cytoplasm.

All these components migrate to the nucleolus, where they are assembled into 2 types of ribosomal subunits which are transported back to the cytoplasm. Smaller subunits (40S) are formed and transported much earlier than 60S.Such a time tag in migration of 40S and 60S prevents functional ribosome from gaining access to incompletely processed HnRNA inside the nucleolus.

  

Ribosomal RNA Genes

Nucleolus is organized around the chromosomal regions that contain the genes for the 5.8S, 18S and 28S rRNA which are therefore called as Nucleolar Organizing Regions (NOR).

All organisms have multiple rRNA genes. In the case of Xenopus, each nucleolar organizer contains 450 rRNA genes. These genes are tandemly repeated along the DNA molecule and separated from each other by stretches of spacer DNA which is not transcribed. These rRNA genes are being actively transcribed and the nascent RNA chains are spread perpendicularly to the DNA axis. Each gene is transcribed into a RNA molecule which will eventually be processed giving rise to 18S, 28S and 5.8S rRNA. As each gene has fixed initiation /promoter site and fixed termination site, the transcript adopts the characteristic”Christmas tree” configuration. Nucleolar rRNA genes are transcribed by RNA polymerase I.

Eukaryotic ribosomes contain 4 types of RNA i.e. 5S, 5.8S, 18S and 28S.Out of these 4, 5.8S, 18S and 28S transcribed as single unit within the nucleolus by RNA polymerase I as 45S ribosomal precursor RNA. The 45S rRNA processed to the 18S rRNA of 40S and 5.8S and 28S of 60S .5S transcribed out of nucleolus by RNA polymerase III and is part of 60S.

Ribosomal proteins are imported to the nucleolus from the cytoplasm and begin to assemble on pre-rRNA prior to its cleavage. As pre-rRNA is processed, additional ribosomal protein and 5S rRNA assemble to form preribosomal particles.

Final step of maturation follows the export of preribosomal particles to the cytoplasm yielding 40S and 60S ribosomal subunits.