rRNA Synthesis


Functional rRNA molecules are produced by processing larger primary transcript. In mammals, the primary transcript is a 45s RNA molecules, which has one copy of each 28s, 18s & 5.8s RNA’s.
Before, processing the primary transcript undergo certain modification.
1. Undergoes methylation of ribose residues (~110-CH3 groups per RNA molecule)
2. Binds to proteins to form a ribonucleo protein complex.

Processing:- 1) 1
st of all 5’ leader sequence is cleaved.
2) A cleaved now occurs in the region between 18s & 5.9s sequences. The 3’ end of 18s & 5’ end of 5.8s
molecules are most lightly sequences generated by trimming.
3) A cleavage now sepates the 5.8s RNA from 28s. The 5.8s RNA base pairs with a segment of the 28s

Fig: Processing of 45s primary transcript of rRNA to release the 28s, 18s, & 5.8s rRNA molecule (in HeLA cell)

Processing of mRNA:

It involves 3 steps. 1) Caping 2) Polyadenylation 3) Splicing
The primary transcript of a structural gene is called pre-mRNA. The pre mRNA are collectively known as
H-RNA( Heterogenous nuclear RNA) because of their large variation in sizes.

Caping: The 1st step of mRNA processing is caping. Methylated guanine cap is added to 5’ end of pre mRNA. 5’ end of pre mRNA contain 3 phosphate group along with nitrogenous base. Triphosphatage enzyme cleaved one phosphate group from the 5’ end. Then one enzyme known as guanalying transferage. Put one guanine residue with the phosphate group of 5’ end of pre Mrna. In some cases methylated guanine is added. This caping guided mRNA to entered cytosol from nucleus.

2) Polyadenylation: In this step the pre mRNA undergoes cleavage after binding of CPSP & CSTP factor.
This transcribe mRNA with this factor added with poly A tail with the help of the enzyme poly Adenine polymerase. After this one protein(PAB) poly Adenine binding protein added to it to chech wheather enough
poly Adenine added or not.

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