The sequence below represents a mature mRNA molecule in a eukaryotic cell.5′-GCGGACAUGCGGCCACGAUAGACUCCAAAAAAA-3′If translation of this mRNA molecule were to occur, which of the following most likely corresponds to the anticodons of the first and last tRNAs that would be brought into the ribosome, respectively?

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11ecba2d_103a_db93_a3bf_6161946c0b48_MD0008_00 Codons are consecutive sequences of three nucleotides in DNA or RNA that each code for either an amino acid or a stop signal that halts translation. Together, these codons comprise the genetic code. In eukaryotic cells, RNA polymerase in the nucleus transcribes DNA genes into pre-mRNA. Pre-mRNA is processed into mature mRNA, which is transported out of the nucleus where it can then be translated by the ribosome. Translation of mRNA occurs in the 5′ to 3′ direction and is facilitated by tRNAs. Each tRNA is "charged" with a specific amino acid and contains an anticodon that base pairs with a specific mRNA codon. Translation initiates at the mRNA start codon (5′-AUG-3′), which codes for the amino acid methionine. Translation continues along the reading frame until a stop codon (UAG, UAA, or UGA) is reached. Stop codons do not code for amino acids and are recognized by release factors instead of tRNA. Release factors induce the dissociation of the newly translated peptide from the ribosome. The question presents a mature mRNA molecule that is translated by the ribosome. Because translation of this mRNA initiates at the 5′-AUG-3′ start codon, the first tRNA brought into the ribosome must have the anticodon 3′-UAC-5′. Translation halts at the stop codon (5′-UAG-3′). However, stop codons are recognized by release factors, not tRNA molecules. Therefore, the last tRNA brought into the ribosome would bind the codon directly upstream of the stop codon (5′-CGA-3) and contain the anticodon 3′-GCU-5′. (Choices A and C) Thymine (T) is replaced by uracil (U) in RNA molecules. Therefore, tRNAs used during translation would not contain T nucleotides. (Choice B) Translation of all mRNA molecules begins at the start codon 5′-AUG-3′, which is recognized by the methionine-charged tRNA with the anticodon 3′-UAC-5′. However, translation would be halted at the stop codon (5′-UAG-3′), located upstream of the poly-A tail. Accordingly, the last tRNA would bind the codon directly upstream of the stop codon (5′-CGA-3′) and contain the anticodon 3′-GCU-5′ (not 3′-UUU-5′). Educational objective: Translation of mRNA by the ribosome initiates at the start codon (AUG), which codes for the amino acid methionine. Translation is terminated at the stop codon, which is bound by a release factor that induces dissociation of the translation complex.

The Sequence Below Represents a Mature MRNA Molecule in a Eukaryotic