Post transcription processing

Post-Transcriptional Processing Post-transcriptional processing is the modification of newly formed RNA (primary transcript or hnRNA) before it becomes mature RNA. In eukaryotic cells, this process mainly occurs inside the nucleus. Main Steps of Post-Transcriptional Processing 1. Cleavage Cleavage means cutting the primary RNA transcript at specific sites. The long RNA molecule is cut to remove extra sequences. In mRNA processing, cleavage occurs at the 3′ end before the addition of the poly-A tail. In some RNAs, large precursor molecules are cleaved into smaller functional RNAs. Importance Produces proper RNA ends Helps in maturation of RNA 2. Splicing Splicing is the removal of non-coding regions called introns and joining of coding regions called exons. Process Introns are removed by a complex called the spliceosome. Exons are joined together to form mature mRNA. Types Normal splicing – introns removed Alternative splicing – different exon combinations produce different proteins Importance Produces functional mRNA Increases protein diversity Illustration \text{Pre-mRNA} \rightarrow \text{Exons joined after intron removal} 3. Editing RNA editing is the alteration of nucleotide sequence after transcription. Examples Addition, deletion, or substitution of nucleotides Common changes: Adenosine → Inosine (A→I) Cytidine → Uridine (C→U) Importance Changes protein sequence Increases genetic diversity Regulates gene expression 4. Nucleotide Modification Chemical modifications are added to RNA nucleotides. Common Modifications in mRNA (a) 5′ Capping Addition of 7-methyl guanosine cap at the 5′ end Functions Protects mRNA from degradation Helps ribosome binding during translation (b) Polyadenylation Addition of poly-A tail at the 3′ end Functions Increases stability of mRNA Helps transport from nucleus to cytoplasm Illustration mRNA = 5'\text{ Cap} + \text{Coding Region} + \text{Poly-A Tail} Summary Table Process Main Function Cleavage Cuts RNA transcript at specific sites Splicing Removes introns and joins exons Editing Alters nucleotide sequence Nucleotide Modification Adds cap and poly-A tail for stability