DNA transcription occurs in a cell's nucleus. The RNA that is synthesized in this process is then transferred to the cell's cytoplasm where it is translated into a protein. In prokaryotes, the RNA that is synthesized during DNA transcription is ready for translation into a protein. Eukaryotic RNA from DNA transcription, however, is not immediately ready for translation.
Post-transcriptional modifications OF RNA accomplish two things: 1) Modifications help the RNA molecule to be recognized by molecules that mediate RNA translation into proteins; 2) During post-transcriptional processing, portions of the RNA chain that are not supposed to be translated into proteins are cut out of the sequence. In this way, post-transcriptional processing helps increase the efficiency of protein synthesis by allowing only specific protein- coding RNA to go on to be translated. Without post-transcriptional processing, protein synthesis could be significantly slowed, since it would take longer for translation machinery to recognize RNA molecules and significantly more RNA would have to be unnecessarily translated to achieve the same results.
In this section, we will discuss the three processes that make up these post- transcriptional modifications: 5' capping, addition of the poly A tail, and splicing. The 5' capping reaction replaces the triphosphate group at the 5' end of the RNA chain with a special nucleotide that is referred to as the 5' cap. It is thought to help with mRNA recognition by the ribosome during translation. A modification also takes place at the opposite end of the RNA transcript. To the 3' end of the RNA chain 30-500 adenines are added in what is called a poly A tail.
Note: For the most part, post-transcriptional processing is not covered in courses such as AP Biology. For the AP, it is enough to know that mRNA is modified before leaving the cytoplasm and engaging in protein synthesis, and that these modifications include the editing out of introns, a 5' cap, and a poly A tail. In other words, this summary is probably enough to cover post- transcriptional processing for the AP test. But for the bold, there is this SparkNote...