In eukaryotes, the transcription process is catalyzed by one of three RNA polymerases: RNA polymerase I, RNA polymerase II, and RNA polymerase III. These three RNA polymerases produce different RNA products, each with characters unique from the others. But regardless of which enzyme homolog is used, transcription still happens in the nucleus, though they might vary in the actual site.
Why is a cap added to mRNA, but not to tRNA or rRNA?
A. RNA polymerase II transcribes mRNA, whereas RNA polymerase I transcribes rRNA, and RNA polymerase III transcribes tRNA. The domain that assists other enzymes in adding the cap is found in RNA polymerase II only.
B. The double stranded regions on tRNA and mRNA result in complex folding and a three-dimensional shape of each molecule. The structure of tRNA and rRNA makes the 5’ end of the molecule inaccessible to the enzymes that add the cap.
C. Transcription and processing of mRNA occur in the nucleus, where cap binding proteins are found. These proteins, which add and modify the cap, are not found in the cytoplasm, where tRNA and rRNA are transcribed and processed.
D. Only mRNA contains introns. The capping process requires RNA to interact with the proteins that remove introns. Therefore, the capping and splicing processes occur at the same time.
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