Why is a cap added to MRNA, but not to 1RNA or RRNA? Each of the three types of RNA are transcribed by different RNA polymerases. Only RNA polymerase II, involved in mRNA synthesis, contains a domain capable of interacting with enzymes that form the cap. 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. Neither tRNA nor FRNA contain introns. Capping occurs simultaneously with intron removal because the same enzymes and proteins involved in both of these processes must bind to RNA at the same time. 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. How do stop codons on MRNA function to end protein synthesis? Since a stop codon is composed of more than three nucleotide bases, a stop codon cannot enter the ribosome, which forces protein synthesis to end. The polypeptide chain breaks away from the ribosome when the amino acid, methlonine, is added to the chain in response to a stop codon. A stop codon does not code for an amino acid, so protein synthesis ends when no new amino acid is added to the polypeptide chain. A series of three stop codons in a row adds the three amino acids that signal the terminal end of the growing polypeptide chain.
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