​​Subtopic 3.3: RNA Processing

The newly synthesized RNA transcripts are processed prior to their use in the cell as mature RNA.

• A 7-methyl guanosine nucleic acid is added to the 5′-end (known as a 5′ cap) of the pre-mRNA as it emerges from RNA polymerase II (Pol II). The cap protects the RNA from being degraded by enzymes and serves as an assembly point for the proteins to begin translation to protein.

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• Removal of introns present in the pre-mRNA and splicing of the remaining exons, in a process called RNA splicing. The continuous series of DNA bases coding for a protein are interrupted by base sequences that are not translated. The translated sequences are referred to as exons (expressed sequences) and the nontranslated sequences as introns (intervening sequences).

Figure 3. RNA processing

• Synthesis of the poly(A) tail. This is a stretch of adenine (A) nucleotides. When a special poly(A) attachment site in the pre-mRNA emerges from Pol II, the transcript is cut there, and the poly(A) tail is attached to the exposed 3′ end.

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​This completes the mRNA molecule, which is now ready for export to the cytosol. (The remainder of the transcript is degraded, and the RNA polymerase leaves the DNA.)

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