The document summarizes the process of post-transcriptional modification of proteins in eukaryotes. It describes how the primary transcript (hnRNA) undergoes three major processes - capping, polyadenylation, and splicing. Capping adds a 7-methyl guanosine cap to the 5' end. Polyadenylation adds around 200-210 adenine nucleotides to the 3' end. Splicing removes introns and joins exons to produce a mature mRNA by recognizing GU and AG sequences at intron-exon boundaries.

1. Sijo.A
B.sc. Botany & Biotechnology
Mar ivanios College
Tvm,Kerala
Post Transcriptional Modification of
Proteins

2.  The RNA produced during transcription are called
primary transcript.
 hnRNA( heterogenous nuclear RNA ) is the
primary transcript produced by RNA polymerase II
in eukaryotes.
 It undergoes chemical modification inside the
nucleus and becomes a mature functional mRNA.
This is called mRNA processing or post
transcriptional modification.
 Mature RNA then leaves the nucleus.
 The processing of mRNA involves three major
events, namely
 1)Capping
 2)Tailing or polyadenylation
 3)Splicing

3. 1)Capping
 Capping is the formation of a “protective cap” at
the 5’end of pre-mRNA by the addition of 7-
methyl guanosine with 5’ 5’ triphosphate
linkage.
 It stabilizes the structure of mRNA.
 2)Tailing or Polyadenylation
 To the 3’end of pre-mRNA about 200-210 adenine
nucleotides are added with the help of enzyme
adenylic acid transferase. This is known as
poly(A)tail.
 It protects the mRNA from rapid enzymatic
degradation.
 It allows the passage of mRNA through the
nuclear pore to the cytoplasm.

4. Splicing
 The primary transcript contain two types of
nucleotide sequences.
 The expressing sequence are called exons and
intervening sequence are called introns.
 The exons and introns are alternately arranged.
 The exons of mRNA possess genetic code and
are responsible for protein synthesis.
 So the introns are not required in genetic
translation.
 The introns are removed from the primary RNA
transcript by a process called RNA splicing.

5.  The exon intron junction have a GU sequence at
5’boundary and AG sequence at its 3’end. That
will define the splice junction.
 The SNRNP recognises the splice junction and
ensures splicing accuracy. For example
 i) U1SnRNP recognises 5’ splice junction
 ii)U5SnRNP recognises 3’ splice junction
 The association of snRNP with hnRNA at the
exon-intron junction is called spliceosome.
 The RNA actually catalyses the splicing reaction
and it is called ribozyme.