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miRNA
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This researcher who finds that internal mutagenesis leads to abnormal proteins and 3' mutagenesis leads to truncated proteins is most likely studying a tRNA. Changing the anticodon would result in binding to the wrong codon, and changing the 3' CCA would lead to failure of amino acid charging. Transfer RNA (tRNA) are cloverleaf structures that function by recognizing a 3 base pair codon and delivering an appropriate amino acid to the ribosome for that codon. Near the center of tRNAs is the anticodon, a sequence of 3 nucleic acids that recognize codons in mRNA. The first 2 positions provide specificity and the 3rd position has "wobble" meaning that it can recognize many codons that code for the same amino acid. In prokaryotes like bacteria, the 3' aminoacyl end of the tRNA contains a conserved sequence (5'-CCA-3') that allows the tRNA to be "charged" with the appropriate amino acid. In higher organisms, this CCA sequence may be added by a CCA enzyme. Together, these 2 regions allow tRNAs to deliver the correct amino acid to the ribosome for a specific mRNA sequence. Incorrect Answers: Answer 1: Micro RNA (miRNA) are small 22 nucleotide long sequences that function to silence mRNA expression and result in post-transcriptional regulation of protein expression. Mutating miRNA would result in them targeting a different mRNA sequence. Answer 2: Messenger RNA (mRNA) are sequences of nucleotides that code for a specific protein when translated by ribosomes. Mutating an mRNA internally would result in a missense mutation, and mutating an mRNA in the 3' terminus (the poly-A tail) would result in a shorter lifespan for the mRNA. Answer 3: Ribosomal RNA (rRNA) form the catalytic machinery of the ribosome and allow for protein translation. rRNA does not contain codons or anticodons, though some point mutations may destabilize the ribosome as a whole. Answer 4: Small interfering RNA (siRNA) are 20-30 nucleotide long sequences that function in RNA interference by binding specifically to mRNA and catalyzing their destruction. Mutating siRNA would result in them targeting a different mRNA sequence. Bullet Summary: The functional domains of tRNA include an anticodon that binds to RNA and a 3' CCA tail that allows for amino acid charging.
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