This presentation is about Split genes and mRNA splicing.

2. Split Genes
mRNA Splicing
Mubaika seher
BS Zoology 5th (Morning)
BSF1800548
Group No. 08

3. Contents
• Split gene
• Exons
• Interons
• Discovery of split genes
• mRNA Splicing
• Chemistry of mRNA splicing
• Spliceosome Machinery
• Significance of mRNA splicing

4. Split Genes
• Definition
“The structural genes are composed of coding or essential
regions and non-coding or non-essential regions that are alternatively
separated”.
 Split genes most commonly found in eukaroytes
 Also found in some bacteriophage genes and in some genes in archea

5. Split Genes
Exons
 Exons are the coding
sequences that appear on split
genes and primary transcripts,
and will be expressed to
matured mRNA.
 Exons are normal sequences
 They code for a particular type
of protein.
Introns
 Introns are the non-coding
sequences that are transcripted
into primary mRNAs, and will be
cleaved out in the later splicing
process.
 Interrupted or intervening
sequences
 They do not code for a protein.
• Donor site(5’ end of the interon )
• Branch site(near the 3’ end of
interon)
• Accepter site (3’ end of the
interon)

6. Cont.

7. Discovery
 Three groups
1. Philip A.Sharp and Richard J.Roberts working on adenovirus
In 1993 Nobel Prize in Physiology and medicine was awarded
to Richard Roberts and Phillip Sharp for their work on split
genes and RNA splicing.
2. Research group of D.S.Hogness ,I.B.David and N.Davidson
studied genes of 28s RNA in drosophila
3. Research group of P.Chambon ,P.Leader and R.A. Flavell
studied B-globin genes,ovalbumins genes and t-RNA genes

8. mRNA Splicing
• Definition:
 mRNA splicing is a form of mRNA
processing in which a newly made
precursor messenger RNA (mRNA) is
transformed into a mature mRNA by
removing the non-coding sequences
termed introns.
 RNA splicing allows the cell to
remove the interon sequences and
join the exons to make coding
nucleotide sequences.
 This spliceosomal action create
mature mRNA from interon loss that
continue on to translation.

9. Spliceosome
• Splicseosome
 A spliceosome is a large and
complex molecule formed of RNAs
and proteins that regulate the
process of RNA splicing.
 The spliceosome is composed of five
small nuclear RNAs (snRNA) and
about 80 protein molecules.
 The combination of RNAs with these
proteins results in the formation of
an RNA-protein complex termed as
small nuclear ribonucleoproteins
(snRNPs).
 All spliceosomes are involved in both
the removal of introns and the
ligation of remaining exons

10. Roles of snRNP
i. They recognize the 5' splice site and the
branch site
ii. They bring those sites together as
required.
iii. They catalyze (or help to catalyze) the
RNA cleavage and joining reactions.
iv. Specialized RNA molecules recognize the
nucleotide sequences that specify where
splicing is to occur and also participate in
the chemistry of splicing.

11. Mechinery of
spliceosome
Step:1 Step:2
Step:3

12. Spliceosomal
machinery
Step:4
Step:5

13. Chemistry of mRNA splicing
Transesterification
 The bonding of the guanine
and adenine bases takes place
via a chemical reaction known
as transesterification ,in which
a (OH) hydroxyl group on a
carbon atom of the adenine
attacks the bond of the
guanine nucleotide at the
splice site

14. mRNA Splicing Process/Mechanism
• The process of RNA splicing begins with the binding of the
ribonucleoproteins or spliceosomes to the introns present on the
splice site.
• The binding of the spliceosome results in a biochemical process
called transesterification between RNA nucleotides.
• During this reaction, the 3’OH group of a specific nucleotide on the
intron, which is defined during spliceosome assembly, causes a
nucleophilic attack on the first nucleotide of the intron at the 5’
splice site.
• This causes the folding of the 5’ and 3’ ends, resulting in a loop.
Meanwhile, the adjacent exons are also brought together.
• Finally, the looped intron is detached from the sequence by the
spliceosomes.

15. Cont.
• Now, a second transesterification reaction occurs during the
ligation of adjacent exon segments.
• In this case, the 3’OH group of the released 5’ exon then
performs an electrophilic attack on the first nucleotide
present just behind the last nucleotide of the intron at the 3’
splice site.
• This causes the binding of the two exon segments along with
the removal of the intron segment.
• Besides the spliceosomes, another group of protein/ enzymes
termed ‘ribozymes’ are also involved in the control and
regulation of the splicing process

16. mRNA editing
mRNA splicing
• Significance: gene sequences, after post-transcriptional
modification, can be multiple purpose differentiation.
• Splicing makes genes more "modular," allowing new
combinations of exons to be created during evolution.
• RNA splicing also helps in the regulation of gene and protein
content in the cell.
• Pre-mRNA splicing is a fundamental process in cellular
metabolism that plays an essential role in generating protein
diversity.

17. References
• Hsu, S. N., & Hertel, K. J. (2009). Spliceosomes walk the line: splicing
errors and their impact on cellular function. RNA biology, 6(5), 526–
530. https://doi.org/10.4161/rna.6.5.9860
•
• https://www.khanacademy.org/science/biology/gene-expression-
central-dogma/transcription-of-dna-into-rna/a/eukaryotic-pre-
mrna-processing
• https://www.slideshare.net/msaltyy/rna-splicing-65637051
• (IWASA, July,2013)