Assignment No. 2

Fall 2018

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Assignment
Questions

Attempt all of the
following questions.  (Total marks 10)

Question no. 1

What is RNA splicing? Describe the major events that occur
in splicing pathway.
 ANS:RNA SPLICING
Eukaryotic
pre-mRNAs are transcribed from intron containing genes,the sequences encoded
by the intronic DNA must be removed from the primary transcript prior to the
RNAs becoming biologically active,the process of intron removal is called RNA
splicing.
MAJOR EVENTS
IN SPLICING PATHWAY
1.In a splicing
pathway a number of steps may differ slightly in their order.But the
pathwayreveals the series of events under taken by the spliceosome to drive
the splicing reaction in the cell
2.Initially the 5
splice site is recognized by U1 Sn RNP
3.U2 AF is made up
of two sub units the larger of which (65) binds to the Py tract and the
smaller (35) binds to the 3′ splice site.
4.The former
subunit interacts with BBP (SF1) and helps that protein bind to the branch
site. This arrangement of proteins and RNA is called the early ( E) complex.
5.U2 SnRNP then
binds to the branch site aided by U22AF and displacing BBP  (SF1). This arrangement is called A
complex.
6.Th base pairing
between U2 SnRNA and the branch site is such that the branch site A residue
is extruded from the resulting stretch of double helical RNA as a single
nucleotide bulge.
7.This A residue is
thus unpaired and available to react with 5 splice site.
8.The next step is
rearrangement of the A complex to bring together all three splice sites.
9.This is achieved
as fallows: the U4 and U6 SnRNPs along with the U5SnRNP join the complex.
10.Together these
three SnRNPs are called tri SnRNP particle.
11.Within this particle
the U4 and U6 SnRNPs are held together by complementary base pairing between
their RNA components and the U5 SnRNP is more loosely associated through
protein-protien interaction.
12.With the entry
of tri-SnRNP, the A complex is converted into the B complex.
13.In the next step
U1 leaves the complex and U6 replaces it at 5′ splice site.
14.This requires
that the base paring between the U1 SnRNA and pre mRNA be broken allowing the
U6 RNA to annual with the same region.
15.Those steps
complete the assembly pathway.
16.The next
rearrangement triggers catalysis and aoccurs as fallows.
17.U4 is released
from the complex allowing U6 to interact with U2 (through the RNA : RNA base
pairing)
18.this arrangement
is called C complex,produces the active site.

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