What are two main types of posttranscriptional modifications that take place in the mrna of eukaryotes? the excision of the introns and the addition of a 7-mg cap to the 3' end. the addition of a poly-t sequence at the 5' end of the gene and the addition of a poly-u tail at the 3' end. the addition of a 7-mg cap at the 5' end of the transcript and the addition of a poly-a sequence at the 3' end of the message. the addition of a poly-a sequence at the 5' end and the addition of a 7-mg cap at the 3' end of the rna transcript.