A geneticist is performing an experiment to alter protein structures by incorporating modified amino acids into their polypeptide sequences. During the process, she incubates dermal fibroblasts in a medium containing fluorescently labeled lysine residues. After several hours, she finds that aminoacyl tRNA synthetase in the fibroblasts "loads" lysine residues onto tRNA molecules containing the anticodon UUU. This residue most likely attaches to tRNA at which of the following sites in the image shown below?

Question

A geneticist is performing an experiment to alter protein structures by incorporating modified amino acids into their polypeptide sequences.  During the process, she incubates dermal fibroblasts in a medium containing fluorescently labeled lysine residues.  After several hours, she finds that aminoacyl tRNA synthetase in the fibroblasts "loads" lysine residues onto tRNA molecules containing the anticodon UUU.  This residue most likely attaches to tRNA at which of the following sites in the image shown below?

Quizplus · Accepted Answer

11ec160b_3773_f3d0_9249_3f614fa6d384_MD0001_00 The image above shows the "cloverleaf" secondary structure of transfer RNA (tRNA), a small, noncoding subtype of RNA that is responsible for transporting amino acids to the site of protein synthesis and introducing them into the growing polypeptide chain at the correct locations. The acceptor stem of tRNA is created through the base pairing of the 5'-terminal nucleotides with the 3'-terminal nucleotides. It contains the CCA tail, which is added to the 3' end of tRNA as a post-transcriptional modification and serves as the amino acid binding site. Aminoacyl tRNA synthetase is the enzyme responsible for "loading" the appropriate amino acid to the 3' terminal hydroxyl group of the CCA tail. The acceptor stem helps mediate correct tRNA recognition by the proper aminoacyl tRNA synthetase. (Choice B) The T loop contains the TΨC sequence, which is necessary for the binding of tRNA to ribosomes. The TΨC sequence refers to the presence of the chemically modified bases ribothymidine and pseudouridine, and cytidine. (Choice C) The variable loop contains a variable number of bases that lie between the T and anticodon loops. The variable loop is not present in all tRNAs. (Choice D) The anticodon loop contains sequences that are complementary to the mRNA codon. During translation, the tRNA anticodon binds to the mRNA codon and assures placement of the proper amino acid in the growing polypeptide chain. (Choice E) The D loop contains numerous dihydrouridine residues, which are modified bases often present in tRNA. The D loop (along with the acceptor stem and anticodon loop) facilitates correct tRNA recognition by the proper aminoacyl tRNA synthetase. (Choice F) The 5' end of tRNA contains a terminal phosphate group that does not participate in amino acid or mRNA binding. Educational objective: The 3' CCA tail of tRNA serves as the amino acid binding site. Aminoacyl tRNA synthetase is the enzyme responsible for "loading" the appropriate amino acid to the 3' terminal hydroxyl group of the CCA tail. __________ References: CCA addition to tRNA: implications for tRNA quality control. (http://www.ncbi.nlm.nih.gov/pubmed/20101632)

A Geneticist Is Performing an Experiment to Alter Protein Structures