Pre-mRNA

Pre-mRNA

Pre-messenger RNA (pre-mRNA) is a type of RNA that is the initial nucleic acid transcript from gene expression in eukaryotes. It is synthesized from a DNA template in the cell nucleus by transcription. Pre-mRNA comprises the bulk of heterogeneous nuclear RNA (hnRNA). Before becoming functional messenger RNA (mRNA), which is translated into protein, pre-mRNA undergoes several processing steps. These steps include capping, splicing, and polyadenylation.

Capping

Capping of pre-mRNA occurs immediately after the initiation of transcription. A 7-methylguanosine cap is added to the 5' end of the growing transcript by a guanylyl transferase enzyme. This modification is crucial for the stability, nuclear export, and translation of the mRNA.

Splicing

Splicing is a process where introns (non-coding regions) are removed from the pre-mRNA and exons (coding regions) are joined together. This process is carried out by the spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs). Alternative splicing can result in multiple mRNA variants from a single gene, increasing the diversity of proteins that can be produced.

Polyadenylation

Polyadenylation involves the addition of a poly(A) tail to the 3' end of the pre-mRNA. This tail consists of 50 to 250 adenine nucleotides. Polyadenylation signals the end of transcription, aids in the export of the mRNA from the nucleus, and contributes to the stability and translation efficiency of the mRNA.

Function and Significance

The processing of pre-mRNA is a critical step in gene expression. It ensures that the mRNA is properly modified and ready for translation into protein. The mechanisms of pre-mRNA processing also allow for the regulation of gene expression and the generation of protein diversity through alternative splicing. Defects in pre-mRNA processing can lead to various diseases, including cancer and genetic disorders.