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Gene splicing
 “Gene splicing is the removal of introns from the
primary trascript of a discontinuous gene during
the process of Transcription.”
 Gene
splicing
is
a
post-transcriptional
modification in which a single gene can code for
multiple proteins.
 Gene splicing is an important source of protein
diversity.
 One gene can lead to different mature mRNA
molecules that generate multiple functional
proteins.
 Thus, gene splicing enables a single gene to
increase its coding capacity.
 Introns:
The areas of the gene that are spliced out are
representing noncoding regions that are intervening
sequences also known as introns.
 Exons:
The DNA that remains in the processed RNA is
referred to as the coding regions and each coding
regions of the gene are known as exons.
 There are two phenomenons by which gene
splicing occur one is natural i.e.
1. Post transcriptional modification.
2. Artificially or by chemical agent used generally
in recombinant technology.
History of gene splicing:
Berg was the first scientist
to splice together segments
of DNA from different
organisms and his work was
published in a landmark
paper in the Proceedings of
the National Academy of
Science in 1972.
Gene Splicing Mechanism
 There are several types of common gene splicing
events.
1. Exon Skipping:
 In which exons are included or excluded from the
final gene transcript leading to extended or
shortened mRNA variants.

The exons are responsible for producing proteins
that are utilized in various cell types for a number
of functions.
2.Intron Retention:
 In which an intron is retained in the final
transcript.
 In humans 2-5 % of the genes have been reported
to retain introns.
 It leads to a demornity in the protein structure and
functionality.
3.Alternative 3' splice site and 5' splice site:
 Alternative gene splicing includes joining of different
5' and 3' splice site.
 Two or more alternative 5' splice site compete for
joining to two or more alternate 3' splice site.
 “Introns are first cut at their 5' end and then at their 3'
end.
Spliceosomes
 “The molecules or molecular complexes that
actually splice RNA in the cellular nucleus are
called spliceosomes”.
 The spliceosome is an enzymatic complex.
Structure of spliceosomes
 Spliceosomes are made of small sequences of
RNAs bound by additional small proteins.
 The small RNAs which make up the spliceosome
are small nuclear RNAs (snRNA's).
 The snRNAs combine with proteins to comprise,
small nuclear ribonuclearprotein particles.
 The absence of individual snRNP components can
inhibit splicing.
Alternative splicing
 “A single gene can be processed to create numerous
gene products, or proteins and this process is referred
to as alternative splicing”.
 In eukaryotes information can be stored much more
economically.
 Several proteins can be encoded by a single gene, thus
allowing a more varied proteome from a genome of
limited size.
 Alternative splicing was first observed in the late
1970s.
 In humans, over 80 % of genes are alternatively
spliced.
 Alternate splicing is used to create the five
antibody-types from the same gene.
 Alternate splicing controls sex determination
in Drosophila melanogaster flies.
 The gene Tra encodes a protein that is expressed
only in females.
Alternative splicing and disease
 over
60%
of
human
disease
causing mutations affect splicing rather than
directly affecting coding sequences.
 Cancerous cells show higher levels of intron
retention than normal cells, but lower levels of
exon skipping.
Splicing out introns
 Most introns begin with the nucleotide sequence G-T
and end with the sequence A-G.
 Are described as conforming to the GT-AG rule.
 Within the intron is another highly conserved
sequence this region (called the branch site) is the area
that connects to the 5' end of the intron as it is cut and
then curls around to form a lariat shape.
 which is removed from the maturing RNA.
mRNA splicing mechanism
 Splicing of mRNA is performed by spliceosome,






containing snRNPs designated
U1
U2
U4
U5
U6
(U3 is not involved in mRNA splicing).
Self-splicing
 “Self-splicing occurs for rare introns that form




a ribozyme, performing the functions
spliceosome by RNA alone”.
There are three kinds of self-splicing introns.
Group I
Group II
Group III
of
the
Other splicing events
tRNA splicing
 tRNA (also tRNA-like) splicing is another rare
form of splicing that usually occurs in tRNA.
 The
splicing reaction involves a different
biochemistry
 Ribonucleases cleave the RNA and ligases join the
exons together.
Protein splicing
 Proteins can also undergo splicing.
 The biomolecular mechanisms is different, but
principle is the same.
 Inteins instead of introns, are removed.
 The remaining parts, called exteins instead of
exons.
 Protein splicing has been observed in all sorts of
organisms, including bacteria, archaea, plants,
yeast and human.
Common errors
 Mutation of a splice site resulting in loss of
function of that site.
 Mutation of a splice site reducing specificity.
 Displacement of a splice site, leading to inclusion
or exclusion of more RNA than expected, resulting
in longer or shorter exons.
Applications of gene splicing
 Using gene-splicing technology, vaccines have
been produced.
 Another application of gene spicing technology is
related to the gene involved in Vitamin B
production.
 Human insulin-producing genes have been spliced
into plasmids.
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