The polyadenylation signal (AAUAAA) is recognized by specific proteins like CPSF and poly-A polymerase.
Source: Proudfoot NJ. Ending the message: poly(A) signals then and now. Genes Dev. 2011.
3. Splicing
Non-coding regions called introns are removed.
Coding segments (exons) are joined together.
Spliceosome, a complex of snRNAs and proteins, mediates this process.
Splicing increases transcript diversity via .
Example:
pre-mRNA: Exon 1 - Intron - Exon 2 - Intron - Exon 3
mature mRNA: Exon 1 - Exon 2 - Exon 3
Source: Matera AG, Wang Z. A day in the life of the spliceosome. Nat Rev Mol Cell Biol. 2014.
VIII. Export of Mature mRNA
Once processed, mature mRNA must exit the . Specialized export receptors recognize:
The 5' cap
The poly-A tail
The splicing marks (exon junction complexes)
Export occurs through nuclear pore complexes (NPCs) and is tightly regulated to ensure only fully processed transcripts enter the cytoplasm.
IX. Regulatory Implications
The precision of and processing plays a critical role in gene expression. Disruption at any step, from initiation to mRNA splicing, can lead to disease. For example:
Mutations in splicing factors can result in spinal muscular atrophy.
Defective capping or polyadenylation can lead to mRNA degradation.
Aberrant promoter recognition by factors is implicated in cancer.
Understanding these steps provides the foundation for numerous therapeutic approaches, including antisense oligonucleotides, RNA interference, and -based al control.
is not a mere mechanical copying of DNA into RNA, it is a highly regulated, multistep process central to life itself. From the precise orchestration of factors at the promoter to the post-al modifications that ready an mRNA for translation, this chapter has provided a comprehensive overview of how genetic information flows from DNA to functional RNA molecules.
Next, we will explore translation, the final leg of the central dogma, and see how the information in mRNA is used to build proteins that sustain and regulate life.