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

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