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Biogenesis of Ribosomes

	Biogenesis of Ribosomes - In eukaryotes the site of synthesis of most of the ribosomal RNA (rRNA) is the nucleolus. The nucleolar organiser contains many copies of ribosomal DNA (repetitive DNA). Several distinct types of rRNA have been isolated from cells. Of these only four classes, namely 28S 5.8S and 5S have been found in the ribosomes. The other types are intermediate stages in the formation of the RNA of the ribosomes. The scheme for the biosynthesis of ribosomes is as follows: (1) The RNA cistron of nucleolar DNA transcribes 45S precursor RNA in the presence of the enzyme RNA polymerase.
Observations on Xenopus laevis oocytes indicate that each rDNA cistron is simultaneously acted upon by about 100 RNA polymerases, each of which transcribes one 4SS RNA (2) The ribose sugar of certain regions of 45S RNA undergoes methylation (addition of methyl groups). It is the methylated regions which gives rise to 28S and 18S RNA of the ribosomes. The non methylated regions have a higher content of guanine and cytosine than the methylated regions.
(3) Cleavage at site removes the transcribed spacer sequence from the 5'P end of 45S RNA. Cleavage at site 2 separates 18S rRNA. Cleavage at site 3 results in a large segment containing 28S RNA and 5.8S RNA along with spacer segments. Cleavage 4 results in the final trimming of this segment. (4)5S RNA is synthesized outside the nucleolus. Treatment of cultured cells with actinomycin D inhibits production of 28S and 18S rRNA but not 5S RNA. Also, in Xenopus laevis 5S rRNA is present in mutants without nucleoli, but not 28S and 18S RNA These observations indicate that the genes for 5S rRNA are not located in the nucleolar organiser.
The genes coding for 5S rRNA have a large number of copies in X. laevis (27,000) and in HeLa cells (10,000), much larger than 28S and 18S genes. In Drosophila, however, the number of 5S genes and 18S and 28S genes is about the same. (5) Ribosomal proteins are synthesized in the cytoplasm and translocated to the nucleus were they become associated with RNA. Structural core proteins first associate with 45S RNA to form 80S ribonucleoprotein particles. Other proteins are probably bound later. The series of degradations by which 45S RNA forms 18S, 5.8S and 28S RNA occur within the ribonucleoprotein particles.
	Ultimately the 40S and 60S subunits of the ribosome are formed, containing 18S and 28S+5.8S RNA, respectively. 5S RNA, which is synthesized outside the nucleolus, also becomes associated with the large 60S subunit. The two types of subunits pass through pores in the nuclear membrane to the cytoplasm. (6) The 40S subunit binds to mRNA in the cytoplasm to form a 40S-mRNA complex. The 60S subunit now becomes associated with the 40S-mRNA complex to form the 80S ribosome with bound mRNA. (7) In prokaryote cells the subunit RNA precursors are trimmed to form 16 S and 23S RNA. These become associated with protein to form the 30S and 50S subunits, respectively, of the 70S ribosome.

The RNA genes are present in only a few copies closely grouped together, i.e. the ribosomal genes occur in a single operon.

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Ribosomes of Chloroplasts Sedimentation Coefficients of Chloroplast and Cytoplasmic ribosomes and their Subunits in Plants RNA Content of Chloroplast Chloroplast and Cytoplasmic Ribosomes Homology Between Chloroplast and Bacterial Ribosomes Ribosomal RNA Interactions Polyribosome Biogenesis of Ribosomes Functions of Ribosomes

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