複選題22. Pre-mRNA is subjective to processing after transcription. Which of the following statement(s) is (are) correct? (A) The 5' cap is added to nascent RNA shortly after transcription initiation. Capping could promote transcription elongation by stimulating phosphorylation of RNA polymerase C-terminal domain (B) Alternative splicing is one of the processing mechanisms. It reduces the number of final protein products encoded by a given region of genomic DNA. (C) Polyadenylation is a process of adding 100-250 adenines at the end of transcripts. It helps to stabilize mature (processed) mRNAs (D) RNA splicing involves two transesterification reactions, which produce excised lariant intron and spliced exons. (E) Splicing involves protein machineries called spliceosome to make the cut and ligation.