In most eukaryotic genes, coding regions (exons) are interrupted by noncoding regions (introns). During transcription, the entire gene is copied into a pre-mRNA, which includes exons and introns. During the process of RNA splicing, introns are removed and exons joined to form a contiguous coding sequence. This "mature" mRNA is ready for translation.
The majority of eukaryotic mRNAs contain a tract of A residues at the end. These polyA-tails are not encoded in the DNA. Rather, they are added to the pre-mRNA "post-transcriptionally" (after transcription). The end of pre-mRNA is cut by a specific enzyme