Question

Describe formation of mature rRNA and explain the processing of tRNA

Answer 1

Ans-

rRNA :

Preribosomal RNA (pre-rRNA) is involved to form mature ribosomal RNA by forming a new bond between the exons.Pre-rRNA has three main sizes; 37S (yeast), the 40S (Xenopus) and 45S (mammals). To form mature rRNA, 18S, 5.8S, and 28S, pre-rRNA 40S (Xenopus) and 45S (mammals) involved to a series of cleavages for removing external and internal spacers (ETS/ITS) that is taking place in one of two pathways.
Pathway 1 cleavage at site 3 to separate the 5.8S and 28S rRNA coding regions in 32S pre-RNA from the 18S rRNA coding region in 20S pre-rRNA.
Pathway 2 starts by initial cleavage at sites A0, 1, and 2 before cleaving at site 3. U3 snoRNA is required for rRNA processing where U3 involved to base-pairing between the 3’ hinge region of U3 and complementary sequences of 5’-ETS.
After transcription, nucleolin immediately binds to the pre-rRNA to facilitate the base-pairing between the U3 snoRNA hinges and the ETS. The area where 5’-ETS is cross-linked to U3 is called site A and its cleavage dependent on U3, U14, E1 and E3 snoRNAs.
After the cleavage of A site, the 3’-ETS is cleaved at site T1 by U8 snoRNA. U3 snoRNA, U14 snoRNA snR30 and snR10 (yeast) and U22 snoRNA ( Xenopus) is required to cleavage at sites A0, 1, and 2 that result in a mature 18S rRNA.
For site A0 cleavage, it requires Box A of U3 snoRNA and it is unknown for site A cleavage. The site 2 cleavage requires the 3’-end of BoxA’ and U3 snoRNA.As the site 2 is cleaved, 18S rRNA released from the pre-rRNA.
For 18S rRNA, 5.8S and 28S rRNA formation, U3 snoRNA and U8 snoRNA are required respectively. Cleavage at site 3 forms 32S pre-rRNA, and at site 4 in ITS2, produces a precursor of 5.8S RNA. It is concluded that site 3 act as a link between 18S and 28S rRNA processing in higher organisms

tRNA processing:
In all organisms, tRNAs are transcribed from pre-tRNA. In bacteria, multiple tRNAs are transcribed as a single RNA where the first step is to digest the RNA and releasing individual pre-tRNAs. Each pre-tRNA is transcribed as a separate transcript in archaea and eukaryotes
The tRNA processing takes place in 5 steps that include:
1. Cleave off the 5? end of the pre-tRNA which is called the 5? leader sequence
2. Then the 3? end of the pre-tRNA is cleaved off.
3. Except the all eukaryotes, some bacterial and archaeal pre-tRNAs added a CCA sequence to the 3? end of the pre-tRNA after the original 3? end is removed. The tRNA’s amino acid is added to the CCA at the 3? end of the mature tRNA
4. Nucleotides in the pre-tRNA are modified by altering their nitrogenous bases in which the most common modifications include the conversion of adenine (A) to pseudouridine (?), the conversion of adenine to inosine (I), and the conversion of uridine to dihydrouridine (D).
5. The introns are spliced out from the eukaryotic and archaeal pre-tRNAs. In bacteria, introns are rarer but spliced out when occurring occasionally.
After processing, the mature pre-tRNA is ready to attach its cognate amino acid which is specified by its anticodon for a tRNA. After attaching this amino acid, it becomes the tRNA. In eukaryotes, the mature tRNA is produced in the nucleus and moves to the cytoplasm for charging.

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