POSTTRANSCRIPTIONAL MODIFICATION IN EUKARYOTES
The basic features of transcription are the same in prokaryotic and eukaryotic cells, but eukaryotic genes and their mRNA molecules are more complex than those of bacteria. Eukaryotic mRNA molecules undergo posttranscriptional modifications, which apparently protect eukaryotic mRNA molecules from degradation and to give them longer lifetimes than bacterial mRNA.

A DNA sequence containing both exons (coding regions) and introns (noncoding regions) is transcribed by RNA polymerase to make the primary transcript, or mRNA precursor. As it is synthesized, the pre-mRNA is capped by the addition of a modified base to its 5' end (5' capping) and a poly-A tail (100 to 250 nucleotides long) is added to the 3' end. Introns are later removed from the mRNA precursor. Exons are spliced together to produce a continuous protein-coding sequence. Finally, mature mRNA is transported through the nuclear envelope and into the cytosol to be translated.