Transcription and RNA Processing

ID #2397

Are the 5' cap and 3' poly-A tail attached to the mature mRNA after splicing of the non-coding sequences taken place? or are they put at the pre-mRNA?

The 5' cap, poly-A tail and splicing of introns all occur "post-transcriptionally," which only means after RNAP II has transcribed. (Transcription does not need to be completely finished for some mRNA processing to occur). The 5' cap is added during transcription. As soon as the 5' end is available, machinery will covalently link the 7-methyl guanosine to the 5' cap. The same is true with intron removal. snRNPs will begin removing introns as soon as the RNAP II is out of the way and the intron junctions are accessible. The poly-A tail is added post-transcriptionally and it occurs concurrent with termination of transcription. 

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