After being transcribed, pre-mRNA must be processed. During processing, certain features are added to the nascent mRNA.
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Before mRNA can be used by ribosomes as a template for building proteins, it must first be processed. Key steps are the addition of a methylated cap and a polyadenylated tail. Involved in processing are: RNA polymerase, cleavage factors, and poly A polymerase.
Processing of mRNA, begins with transcription. Soon after RNA polymerase begins transcription, a methylated cap is added to the 5′ end. Transcription then continues to completion.
Following completion, RNA polymerase releases the capped strand of pre-mRNA. Specific nucleotide sequences in the mRNA are bound by cleavage factors. The 3′ end of the mRNA is next moved into the correct configuration for cleavage. Stabilizing factors are then added to the complex.
Poly A polymerase now binds to the mRNA and cleaves the 3′ end. The complex begins to dissociate, and the cleaved 3′ end quickly degrades. Poly A polymerase now synthesizes the polyadenylated tail by adding adenine residues to the cleavage site. Additional proteins then bind to the tail, increasing the rate at which it grows.
When the tail reaches its full length, the poly A polymerase is signaled to stop adding residues, and the polyadenylation process is completed. The processed mRNA is now ready to undergo splicing in preparation for translation.
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