Alternative Splicing Humans alternative RNA splicing human 5' untranslated region Alternative splicing exon human cell priority journal reverse transcription polymerase chain reaction transcription elongation 3' untranslated region Base Sequence DNA Primers Exons HEK293 Cells Kinetics Models, Biological Pol II elongation RNA RNA Polymerase II RNA Precursors RNA Splice Sites RNA polymerase II article chromatin structure controlled study intron messenger RNA Air pollution