Biogenesis of Ribosome

1. Biogenesis
Of
Ribosomes
By Miss Iram

2. Ribosomes
 Ribosomes are most important cell organelles composed of RNA and protein that
converts genetic code into chains of amino acids.”

3. Biogenesis of Ribosomes
 The ribosome is a complex molecular machine composed of numerous distinct
proteins and nucleic acids and is responsible for protein synthesis in every
living cell.
 Ribosome biogenesis is one of the most multifaceted and energy- demanding
processes in biology, involving a large number of assembly and maturation
factors, the functions of which are orchestrated by multiple cellular inputs,
including mitogenic signals and nutrient availability.
 The complete molecular organization of the ribosomes with heir three kinds
of rRNA molecules and their 50 or more specific proteins, implies that more
than 50 genes are involved in their for: mation. The biogenesis of bacterial
70S ribosomes differs markedly with that of eukaryotic 80S ribosomes.

4. Biogenesis of 70S Ribosome
 Smith have suggested that in bacteria the RNA genes coding for the 5S. 23S and 16S
ribosomal RNA are tightly Clustered in a region of the chromosomes and are present in
only few sequence .
 Thus, in bacteria a single gene transcript containing the sequences of 16S, 235 and 5S
rRNAS, is synthesized by a rRNA operon and this larger molecule is thought to undergo
both tailoring and chemical modifications before each RNA molecule assumes its
mature form.
 During tailoring of larger rRNA molecule, 16S rRNA sequence is first of all cleared off
and is separated from 23S and 5S sequences.
 The fragment containing 16S information is still larger than the mature 16S RNA by at
least 100 bases and is not methylated both the methylation and the tailoring of this
molecule takes place after it has associated with a number of proteins to form the
percursor ribosomal subunits.
 The 5S rRNA is found not to undergo the processes of tailoring and methylation,
before it becomes mature .
 The whole process of biogensis of 70S ribosome takes place In cytoplam.

5. 2. Biogenesis of 80S Ribosomes
 In eukaryotes, the biogenesis of ribosomes is much more complex and involves
a long-lasting process in which several regions of cell are involved.
 The 5-8S, 18S and 28S rRNAs are transcribed as a much larger molecule in the
nucleolar organizer (NO) which containing many copies of 5-8 S, 18S and 28S
rRNA genes or ribosomal DNA.
 The DNA coding for the 5S rRNA is also highly repetitive, but the molecule is
synthesized out side the nucleolus.
 It is in the nucleolus that newly synthesized rRNA accumulates and becomes
associated, presumably by a self- assembly process, with 50 or more
ribosomal proteins that have been synthesiz ed in the cytoplasm by usual
mechanism of protein synthesis and then migrate to the cytoplasm of cell, in
the form of ribosomal subunits.

6. The biogenesis of nucleolar organisation
 In eukaryotes 5.8S, 18S and 28S rRNA gene sequences are clustered in a
chromosomal region called the nucleolar organizer (NO).
 By definition a NO lies in heart of a nucleolus. Direct evidence that the
nucleolus is respo for the synthesis of rNA was obtained in 1964, when it
was discovered that an anucleolate mutant of amphibian Xenopus laeva
was incapable of RNA synthesis .
 The molecular structure of nucleolar organisation contain
 A Space DNA and satellite ribosomal DNA
 Amplification of the rDNA

7. B. Biogenesis of 5S RNA
 In contrast to prokaryotes, in eukar yotes the 5S rRNA is not related to the
nucleolar organizer. In X. laevis, by use of the DNA-RNA hybridization
technique.
 it was found that there are much more 5S genes (i.e., about 27,000) than 18S
and 28S rRNA genes and that they are not spatially related.
 Because of this fact, the presence of 5S rRNA was detected in the
anucleolate mutant of X. laevis. HeLa cells are found to contain about 10,000
copies of 5S rRNA genes. In Drosophila the number of 5S rRNA genes is about
the same as that of 18S and 28S genes .

8. C. Processing of the ribosomal RNA in
nucleolus.
 In eukaryotes original gene transcripts carrying 5.8S, 18S and 28S rRNA
sequences undergo both tailoring and chemical modifications before they
assume their mature forms. It is still not clear that how 58S CRNA is formed
from 45S rRNA molecule. However, processing of 28S rRNA, and 18S rRNA
includes following series of events
 (1) The original gene transcipt for 5-8S, 18S and 28S rRNA is a single large RNA
molecule with a sedimentation coefficient of 38 to 45S, depending on the
species.. It is transcribed from the rDNA of. nucleolar organizer in about 22
minutes (in Hela cells) and is present in the fibrillar region of the nucleolus. It
is assumed that 20 RNA polymerases are transcribing simultaneously on the
DNA cistron.
 (ii) During the transcription process certain regions of the 45S rRNA become
methylated in the ribose moiety producing 2'-O- methyl ribose. The regions of
the 45S RNA that are methylated correspond to the future 28S and 18S
ribosomal rRNsAs

9.  (iii) The 45S rRNA has a life time of about 15 minutes and is followed by
cleavage and degradation into smaller components. Consequently, 45S rRNA
molecule is cleaved into a 41S and 20S rRNA by the help of an enzyme called
endonuclease.
 (iv) The 20S rRNA is degraded in its non methylated region by an exonuclease
enzyme and thus, producing the 18S rRNA, which is rapidly released to the
cytoplasm
 (v) The 41S rRNA undergoes a series of degradations by exonuclease enzymes
which attack the non-methylated regions; byarises produced 36S and 32S
intermediates are produced.
 Vi) The 32S rRNA molecule remain in the granular part of nucleus and than is
degraded to 28 S rRNA.

11. Ribosome function
 The important ribosome function includes:
 It assembles amino acid to form proteins that are essential to carry out cellular functions.
 The DNA produces mRNA by the process of DNA transcription.
 The mRNA is synthesized in the nucleus and transported to the cytoplasm for the process of
protein synthesis.
 The ribosomal subunits in the cytoplasm are bound around mRNA polymers. The tRNA then
synthesizes proteins.
 The proteins synthesized in the cytoplasm are utilized in the cytoplasm itself, the proteins
synthesized by bound ribosomes are transported outside the cell.