mRNA This splicing

This presentation is about Split genes and mRNA splicing.

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

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

3. 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

4. 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 )

5. Cont.

6. 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

7. 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.

8. 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

9. Roles of snRNP They recognize the 5' splice site and the branch site T hey bring those sites together as required. T hey catalyze (or help to catalyze) the RNA cleavage and joining reactions. Specialized RNA molecules recognize the nucleotide sequences that specify where splicing is to occur and also participate in the chemistry of splicing.

10. Mechinery of spliceosome

11. Spliceosomal machinery

12. 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

13. 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 .

14. 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

15. 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.

16. 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 )