The document summarizes RNA synthesis and splicing. It discusses that in eukaryotes, RNA is processed after synthesis from DNA in the nucleus. There are three main types of RNA splicing - self-splicing via ribozymes, spliceosomal splicing involving protein complexes, and polyadenylation of mRNA. Introns are non-coding regions that are spliced out, while exons are coding regions that are joined to form mature mRNA. Small nuclear RNAs and proteins form spliceosomes that catalyze splicing of spliceosomal introns in eukaryotic pre-mRNA.

1. SARAH ASHFAQ
BSADP-BT51F21
BS 6th Sem. ADP Replica
Presented To : Prof. Dr. Zahoor Ahmad Sajid
Topic: RNA Synthesis and Splicing
Institute of Botany , University of the Punjab 1
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2. ‫يم‬ ِ‫ح‬َّ‫الر‬ ِ‫ن‬ ٰ‫حم‬َّ‫الر‬ ِ‫ہللا‬ ِ‫سم‬ِ‫ب‬
Contents
1. Introduction 11. Spliceosomal introns and spliceosome
2. RNA synthesis 12. Small nuclear RNAs
3. Ribozymes 13. Splicing mechanism in mRNA transcipt
4. Primary transcript 14. Polyadenylate polymerase
5. RNA splicing 15. Splicing Diversity
6. Eukaryotic mRNA 16. Cellular RNA
7. Role of Intron and Exons contents flow chart
8. Self-splicing 17. Conclusion
9. Group 1 Introns and reactions 19. References
10. Group 2 Introns and reactions
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3. 1.Introduction
• Many of the RNA molecules in bacteria and virtually all RNA molecules in
eukaryotes are processed to some degree after synthesis. Some of the most
interesting molecular events in RNA metabolism occur during the post
synthetic processing. Synthesis takes place within the nucleus of eukaryotic
cells or in the cytoplasm of prokaryotes and converts the genetic code from
a gene in deoxyribonucleic acid (DNA ) to a strand of RNA that then directs
protein synthesis.
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2.

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7. 3.Ribozymes
• Several of the enzymes that catalyze the
chemical reactions consists of RNA
rather than proteins.
• The discovery of these catalytic RNAs or
ribozymes has brought a revolution in
thinking about RNA function and about
origin of life.
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Source:
https://en.wikipedia.org/wiki/Ribozyme

8. 4.Primary transcript
• A newly synthesized RNA molecule is called as primary transcript.
• The primary transcript for eukaryotic mRNA typically contains sequences
encompassing one genes, although the sequence encoding the polypeptide
may be contiguous.
• Special function RNAs are also processed.
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9. 5.RNA splicing
• “RNA splicing is the process by which the newly synthesized pre-mRNA,
also known as hnRNA, (heterogeneous nuclear RNA) is processed and
forms the mature mRNA.”
• Exons are the coding segments.
• Noncoding tracts that breakup the coding region of the transcript are called
as introns.
• During RNA splicing the primary transcript and the exons are joined to form
a continuous sequence that specifies a functional polypeptide.
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10. 6.Eukaryotic mRNA
• Eukaryotic mRNAs are also modified at each end.
• A modified residue called a 5' cap is added in the 5' end.
• The 3' end is cleaved and 80 to 250 residues are added to create poly (A)
tail.
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Source:https://www.researchgate.net/publication/48376766_Lehninge
r_Principles_of_Biochemistry

12. 7.Role of Introns and Exons
• The introns in the primary RNA transcript are spliced and the exons are
joined to form mature functional RNA.
• In eukaryotic mRNAs most exons are less than 1000 nucleotides long wit
many to the 100 to 200 nucleotide size range encoding 50 to 20000
nucleotides.
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13. 8. Self-splicing
• In self splicing no protein enzymes are involved.
• Group 1 and Group 2 introns classes are self-splicing.
• Neither class requires a high co-factor ATP for splicing.
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Group 1 introns In Nuclear, Mitochondrial and chloroplast genes that code for rRNAs,
mRNAs and tRNAs.
Group 2 introns In primary transcript of mitochondrial or chloroplast mRNAs in fungi,
algae and plants.

14. 9.Group 1 introns and reactions
• It requires a guanine nucleoside or nucleotide co factor but cofactor is not used as a
source of energy.
• The 3' – hydroxyl group of guanosine is used as a nucleophile in the first step of
splicing pathway.
• The 3',5'-phosphodiester bond with the 5' end of the intron.
• The 3' hydroxyl of the exon that is displaced in the step than act as nucleophile in
the similar reaction at the 3' end of intron.
• Result: Excision of intron and ligation of exons.
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15. 10. Group 2 introns and reactions
• In Group 2 introns the reaction pattern is similar except for the nucleophile
in the first step.
• In this case 2'-hydroxyl group of an a residue within the intron.
• A branched lariat structure is formed as intermediate.
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Source:https://j.p.gogarten.uconn.edu/mcb3421_2
013/class19_2013.htm

17. 11. Spliceosomal introns and
spliceosome
• The third and largest class of
introns includes those found in
nuclear mRNA primary
transcripts . These are called as
spliceosomal introns.
• Their removal occurs within and
is catalyzed by a large proteins
complex called as Spliceosome.
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18. 12.Small nuclear RNAs
• The spliceosome is made up of specialized RNA-protein complexes,
snRNAs.
• Each snRNP contains one of a class of eukaryotic RNAs 100 to 200
nucleotides long known as small nuclear RNAs.
• Five snRNAs (U1,U2,U3,U4,U5 and U6) involved in splicing reactions found
in eukaryotic nuclei.
• RNAs and proteins are highly conserved.
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19. 13.Splicing mechanism in mRNA primary
transcripts
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20. 14.Polyadenylate polymerase
• Polyadenylate polymerase (PAP) is one of the enzymes involved in the
formation of the polyadenylate tail of the 3' end of mRNA.
• Poly (A) tail formation is a significant component of 3' processing, a link in
the chain of events, including transcription, splicing, and
cleavage/polyadenylation of pre-mRNA.
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Source:https://www.researchgate.net/publication/48376766_Lehnin
ger_Principles_of_Biochemistry

21. 15.Splicing diversity
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PROKARYOTES EUKARYOTES
SPLICEOSOMAL + _
SELF-SPLICING + +
tRNA + +

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CELLULAR RNA CONTENT FLOW CHART
TOTAL RNA
CODING RNA
4% OF TOTAL
NON-CODING RNA
96% OF TOTAL
hnRNA
mRNA
PRE-rRNA PRE-tRNA snRNA SnoRNA scRNA
mRNA tRNA
Source: Sarahashfaq51

23. References
• Lehninger Principles of Biochemistry Eighth Edition.
• Berget, S. M., et al. Spliced segments at the 5' terminus of adenovirus 2 late mRNA. Proceedings of
the National Academy of Sciences 74, 3171–3175 (1977)
• Chow, L. T., et al. An amazing sequence arrangement at the 5′ ends of adenovirus 2 messenger
RNA. Cell 12, 1–8 (1977)
• Darnell, J. E., Jr. Implications of RNA–RNA splicing in evolution of eukaryotic cells. Science 202,
1257–1260 (1978) doi:10.1126/science.364651
• Early, P., et al. Two mRNAs can be produced from a single immunoglobulin chain by alternative RNA
processing pathways. Cell 20, 313–319 (1980)
• Knapp, G., et al. Transcription and processing of intervening sequences in yeast tRNA
genes. Cell 14, 221–236 (1978)
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AHUMDULILLAH