1. M. Bilal
2019-ag-5279
UAF Subcampus Depalpur
Topic:.
RNA PROCESSING
Introduction:.
RNA processing is the term collectively used to describe the sequence of events through
which the primary transcript from a gene acquires its mature form. Very soon after synthesis by
RNA polymerase II begins, transcripts from nuclear protein-coding genes acquire a 5′ cap
structure.
What are the processing steps for messenger RNAs?
In eukaryotic cells, pre-mRNAs undergo three main processing steps:
Capping at the 5' end
Addition of a poly(A) tail at the 3' end
Splicing to remove introns
Summary of steps in processing eukaroytic messenger RNAs
2. Capping
In the capping step of mRNA processing, a methylated-guanosine (7-methyl-G) is linked
to the phosphates at the 5' end of the mRNA. The cap protects the 5' end of the mRNA
from degradation by nucleases and also helps to position the mRNA correctly on the
ribosomes during protein synthesis.
The mRNA capping structure is a methylated G nucleotide that is linked with triphosphates to
the 5 ' nucleotide instead of via a phosphodiester bond.
Poly(A) tail addition
The 3' end of a eukaryotic mRNA is first trimmed, then an enzyme called Poly(A) Polymerase
adds a "tail" of about 200 ‘A’ nucleotides to the 3' end. There is evidence that the poly(A) tail
plays a role in efficient translation of the mRNA, as well as in the stability of the mRNA. The
cap and the poly(A) tail on an mRNA indicate that the mRNA is complete (i.e., not defective).
Splicing
In many cases, splicing now appears to occur as the transcript is being synthesized. Introns are
removed from the pre-mRNA by the activity of a complex called the spliceosome. The
spliceosome is made up of proteins and small nuclear RNAs (snRNAs) that associate to form
protein-RNA enzymes called small nuclear ribonucleoproteins or snRNPs (pronounced
3. SNURPS). The splicing machinery must be able to recognize sequences that are specific to
splice junctions (i.e., the end of each exon and the start of the next) in order to correctly cut out
the introns and join the exons together to make the mature, spliced mRNA.
