Rna processing

Rna processing

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  RNAPROCESSING Mrs. Praveen Garg VITSCollege, Satna
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  INTRODUCTION • The majordifference in RNA processing between prokaryotes and eukaryotes, is in the processing of messenger RNAs. • In eukaryotic cells, RNA processing occurs in the nucleus, is separated from the protein synthesis process, which occur in the cytoplasm. • Eukaryotic genes have introns (noncoding regions) that interrupt the gene’s coding sequence. It must be removed before the mRNA is sent out of the nucleus. • The process of removing the introns and rejoining the coding sequence of the mRNA, is called splicing. • Once the mRNA has been capped, spliced and had a polyA tail added, it is sent from the nucleus into the cytoplasm for translation. • The initial product of transcription of a protein coding gene is called the pre-mRNA (or primary transcript). After it has been processed and is ready to be exported from the nucleus, it is called the mature mRNA or processed mRNA.
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  Eukaryotic mRNAs havethree main parts: – 5’ untranslated region (5’ UTR): varies in length. – The coding sequence: specifies the amino acid sequence of the protein that will be produced during translation. It varies in length according to the size of the protein that it encodes. – 3’ untranslated region (3’ UTR): also varies in length and contains information influencing the stability of the mRNA. AUG UAA
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  Steps of RNAprocessing
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   In prokaryotes,no RNA processing is necessary: • The nascent RNA is usually the mRNA.  In eukaryotes, the nascent RNA is called primary transcript-RNA • needs to be processed and transported to the cytoplasm for translation to occur.  In eukaryotic cells, pre-mRNAs undergo three main processing steps: • Capping (addition of methyl group at the 5' end) • Polyadenylation (addition of a poly-A tail at the 3' end) • Splicing (remove introns between exons)
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  CAPPING • In thecapping step of mRNA processing, a 7-methyl guanosine is added at the 5' end of the mRNA. • When the RNA chain is about 30 nucleotides long, the 5’ ends are modified.  Happens before transcription is finished.  Methyl transferase enzyme add methyl group in the 5’ ends. • The cap protects the 5' end of the mRNA from degradation by nucleases. • It also helps to position the mRNA correctly on the ribosomes during protein synthesis.
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  POLYADENYLATION • In theprocess of polyadenylation, the 3' end of a eukaryotic mRNA is firstly cleavage from the AAUAAA conserved esquence. • Then an enzyme called Poly-A Polymerase adds a "tail" of about 200 ‘A’ nucleotides to the 3' end. • The poly-A tail plays a role in efficient translation of the mRNA or transportation in to cytoplasm. • It also enhance the stability of the mRNA. • The cap and the poly-A tail on an mRNA are indications that the mRNA is complete,
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  SPLICING • Splicing isa process in which introns are removed from the pre- mRNA by the activity of a complex called the spliceosome. • The spliceosome is made up of proteins and small RNAs that are associated to form protein-RNA enzymes called small nuclear ribonucleoproteins or snRNPs (pronounced SNURPS). • The splicing machinery must be able to recognize splice junctions (i.e., the end of each exon and the start of the next) in order to correctly cut out the introns and join the exons to make the mature, spliced mRNA. • The base sequence at the 5‘ end (donor site) of an intron is GU while the sequence at the 3’ end (acceptor site) of an intron is AG. • There is also a third important sequence within the intron, called a branch point, that is important for splicing.
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   There aretwo main steps in splicing: • In the first step, the pre-mRNA is cut at the 5' splice site (the junction of the 5' exon and the intron). • The 5' end of the intron then is joined to the branch point within the intron. This generates the lariat-shaped molecule characteristic of the splicing process. • In the second step, the 3' splice site is cut, and the two exons are joined together, and the intron is released. •
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  Alternate Splicing • Manypre-mRNAs have a large number of exons that can be spliced together in different combinations to generate different mature mRNAs. This is called alternative splicing. • It allows the production of many different proteins using relatively few genes, since a single RNA can, by combining different exons during splicing, create many different protein coding messages. • It is a regulated process during gene expression. • Alternative splicing occurs in a eukaryotes, where it increases the biodiversity of proteins that can be encoded by the genome. • During alternative splicing, cis-acting regulatory elements in the mRNA sequence determine which exons are retained and which exons are spliced out. • 75% of 30,000 human genes undergo alternative splicing to encode two or more splice isoforms, that encode different proteins.
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  THANK YOU