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.