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Post-Transcriptional RNA Processing
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 and it would take longer for translation machinery to recognize RNA molecules.
In this section, we will discuss the three processes that make up these post- transcriptional modifications: GTP capping, addition of the poly A tail, and splicing. The GTP capping reaction replaces the triphosphate group at the 5' end of the RNA chain with a special nucleotide that is referred to as the GTP 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.
GTP Capping
Post-transcriptional processing of the 5' end of the RNA product of DNA transcription comes in the form of a process called the GTP cap. At the end of transcription, the 5' end of the RNA transcript contains a free triphosphate group since it was the first incorporated nucleotide in the chain. The capping process replaces the triphosphate group with another structure called the "cap". The cap is added by the enzyme guanyl transferase. This enzyme catalyzes the reaction between the 5' end of the RNA transcript and a guanine triphosphate (GTP) molecule. Once in place, the cap plays a role in the ribosomal recognition of messenger RNA during translation into a protein. Prokaryotes do not have a similar cap because they use other signals for recognition by the ribosome.
The Poly-A Tail
Post-transcriptional RNA processing at the opposite end of the transcript comes in the form of a string of adenine bases attached to the end of the synthesized RNA chain. This string of adenine is called the "poly-A tail". The addition of the adenines is catalyzed by the enzyme poly-A polymerase, which recognizes the sequence AAUAAA as a signal for the addition. The poly-A tail is found on most, but not all, eukaryotic RNA transcripts.
