Genetic Code and Codon Translation (1B)
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MCAT Biological and Biochemical Foundations of Living Systems › Genetic Code and Codon Translation (1B)
In a bacterial expression study, a single-base substitution changed codon 12 in an enzyme mRNA from 5'-GAA-3' to 5'-GAG-3'. Protein yield and enzymatic activity were unchanged across replicates. A lab note lists the relevant code: GAA → Glu, GAG → Glu. Based on the information, which conclusion is most consistent with codon redundancy during translation?
The substitution is synonymous and preserves the encoded amino acid, so primary structure is unchanged at position 12
The substitution forces the ribosome to read the mRNA in a different reading frame downstream of codon 12
The substitution changes the anticodon sequence in the tRNA gene, altering tRNA charging specificity
The substitution introduces a premature stop codon, truncating the enzyme at residue 12
Explanation
This question tests understanding of the genetic code and its role in translation, specifically codon redundancy. The genetic code is degenerate, meaning multiple codons can encode the same amino acid, often differing only in the third base due to wobble pairing. In this bacterial expression study, the single-base substitution changes codon 12 from GAA to GAG, both of which encode glutamate. Therefore, the substitution is synonymous, preserving the encoded amino acid and leaving the primary structure unchanged at position 12, as stated in choice A. Choice B fails because neither codon is a stop codon, reflecting a misconception that any base change introduces premature termination. To apply similar reasoning in future contexts, always reference a codon table to determine if a mutation alters the amino acid. Additionally, consider that while synonymous mutations do not change the protein sequence, they may subtly affect translation efficiency depending on codon usage.
A mitochondrial gene is sequenced from a patient sample. A variant changes a codon in the middle of the open reading frame from 5'-UAU-3' to 5'-UAC-3'. The peptide sequence around this site is unchanged by mass spectrometry. A note in the sequencing report states: UAU → Tyr; UAC → Tyr. Based on the information, which conclusion is most consistent with codon redundancy?
The variant necessarily changes the reading frame because it alters the second base of the codon
The variant prevents translation initiation because UAC is recognized only as a start codon
The variant is synonymous at the protein level because both codons specify tyrosine
The variant must change the amino acid because every amino acid is encoded by exactly one codon
Explanation
This question tests understanding of the genetic code and its role in translation, specifically codon redundancy. Redundancy means synonymous codons encode the same amino acid, preserving protein sequence despite nucleotide changes. In this mitochondrial gene, the variant changes UAU to UAC, both encoding tyrosine. The variant is thus synonymous, explaining the unchanged peptide sequence by mass spectrometry, per choice A. Choice B fails by assuming a reading frame shift, ignoring that point mutations do not alter frame. To reason similarly, confirm both codons map to the same amino acid. Note that mitochondrial codes may vary slightly from standard.
A bacterial gene was engineered so that a single codon in the coding region changed from 5'-UAU-3' to 5'-UAA-3'. In vitro translation produced a truncated polypeptide, and adding more aminoacyl-tRNAs did not restore full-length product. The team suspects the mutation altered a key genetic code signal rather than tRNA availability. Which effect is most consistent with the described mutation?
Insertion of one nucleotide caused a reading-frame shift that can be corrected by increasing tRNA concentration
A silent mutation occurred because UAU and UAA both encode tyrosine due to redundancy
A nonsense mutation introduced a stop codon, causing premature termination at that position
A missense mutation replaced tyrosine with phenylalanine, shortening the protein by destabilizing it post-translationally
Explanation
This question tests knowledge of nonsense mutations and their effects on translation termination. The genetic code includes three stop codons (UAA, UAG, UGA) that signal translation termination, and UAU codes for tyrosine. In this bacterial system, the mutation from UAU to UAA introduces a premature stop codon, causing the ribosome to terminate translation early and produce a truncated protein. The correct answer C properly identifies this as a nonsense mutation causing premature termination. Answer D incorrectly claims that both UAU and UAA encode tyrosine, when UAA is actually a stop codon, while answer B suggests post-translational effects rather than the direct translational consequence of a stop codon. When analyzing codon changes, always check if the new codon is a stop signal (UAA, UAG, or UGA) as these will terminate translation regardless of tRNA availability.
Researchers observed that a single tRNA species in a mitochondria-like translation system could support incorporation of alanine at both 5'-GCU-3' and 5'-GCC-3' codons, even though only one alanine tRNA was detected by sequencing. The anticodon was reported as 3'-CGI-5' (I = inosine). The group proposes that flexible pairing at the third codon position explains the observation. Which mechanism explains the observed translation process?
Wobble pairing allows inosine in the anticodon to pair with multiple bases at the codon’s third position, enabling recognition of both codons
Aminoacyl-tRNA synthetase changes alanine into a different amino acid depending on whether the codon is GCU or GCC
Strict Watson–Crick pairing at all three positions forces the same tRNA to recognize only one codon
The ribosome edits the mRNA sequence during elongation so that both codons become identical before tRNA binding
Explanation
This question tests understanding of wobble base pairing and how it enables genetic code degeneracy during translation. The wobble hypothesis explains that non-Watson-Crick pairing can occur at the third codon position, allowing a single tRNA to recognize multiple codons for the same amino acid. In this mitochondrial-like system, the tRNA with anticodon 3'-CGI-5' (where I is inosine) can pair with both GCU and GCC codons because inosine can form stable base pairs with multiple nucleotides (U, C, or A) at the wobble position. The correct answer B accurately describes this wobble pairing mechanism. Answer A incorrectly suggests strict Watson-Crick pairing at all positions, which would prevent one tRNA from recognizing multiple codons, while answer C proposes an impossible mRNA editing mechanism during elongation. To identify wobble pairing scenarios, look for inosine in anticodons or situations where one tRNA recognizes multiple synonymous codons differing only at the third position.
In a reconstituted translation experiment, an mRNA segment contained the codons 5'-AUG-3' followed by 5'-UUU-3'. When the initiator tRNA was omitted, no peptide product was detected, even though all elongator tRNAs and amino acids were present. When the initiator tRNA was restored, translation proceeded normally. The researchers propose that a specific codon–tRNA interaction is required to establish the reading frame. Which mechanism explains the observed translation process?
The start codon is recognized by aminoacyl-tRNA synthetases rather than by tRNA anticodons, so initiator tRNA omission should not matter
Initiation occurs only after the ribosome synthesizes the first peptide bond, so initiator tRNA is unnecessary
Initiation requires pairing of an initiator tRNA with the start codon to position the ribosome and set the reading frame for elongation
Elongator tRNAs can initiate at any codon, so omission of initiator tRNA should not affect peptide synthesis
Explanation
This question tests understanding of translation initiation and the special role of initiator tRNA in establishing the reading frame. Translation initiation requires specific recognition of the start codon (AUG) by initiator tRNA (carrying N-formylmethionine in bacteria or methionine in eukaryotes) to properly position the ribosome and establish the correct reading frame for subsequent elongation. Without initiator tRNA, the ribosome cannot properly assemble at the start codon or begin synthesis, even if all elongator tRNAs are present. The correct answer B explains this requirement for initiator tRNA-start codon pairing to set the reading frame. Answer A incorrectly suggests elongator tRNAs can substitute for initiator tRNA, ignoring their distinct structural features and ribosomal binding properties, while answer C reverses the order of events in translation initiation. To understand translation initiation, remember that the initiator tRNA-AUG interaction is essential for ribosome positioning and cannot be replaced by elongator tRNAs.
A point mutation was introduced into an mRNA such that the coding sequence changed from 5'-AUG UCU GGC-3' to 5'-AUG UCC GGC-3'. In a purified translation system, the resulting polypeptide had the same length and identical amino acid composition compared with wild type. The researchers conclude the mutation did not change the encoded amino acid at that position. Which effect is most consistent with the described mutation?
A missense mutation occurred because UCU encodes glycine while UCC encodes serine
A nonsense mutation occurred because UCC is a stop codon, but translation continued due to wobble
A frameshift mutation occurred because a base substitution changes the reading frame downstream
A synonymous mutation occurred because UCU and UCC encode the same amino acid, preserving the peptide sequence
Explanation
This question tests recognition of synonymous mutations in the genetic code during translation. The genetic code shows that both UCU and UCC encode serine, making this a synonymous (silent) mutation that preserves the amino acid sequence. In the given mRNA sequence, the change from UCU to UCC maintains serine at the second position, resulting in an identical polypeptide (Met-Ser-Gly). The correct answer D properly identifies this as a synonymous mutation preserving the peptide sequence. Answer C incorrectly assigns different amino acids to these codons (both encode serine, not glycine), while answer B wrongly suggests a frameshift from a simple substitution mutation. To verify synonymous mutations, use the genetic code table to confirm both the original and mutant codons encode the same amino acid, and remember that such mutations can still affect translation kinetics despite producing identical proteins.
A synthetic biology team designed an mRNA with repeated glycine codons. When they replaced several 5'-GGU-3' codons with 5'-GGA-3' codons, the amino acid sequence remained glycine at those positions, but the ribosome pause frequency increased at the modified region. tRNA quantification showed that the tRNA decoding 5'-GGA-3' was less abundant than the tRNA decoding 5'-GGU-3'. Based on the information, which conclusion is most consistent with codon redundancy?
The increased pausing is most consistent with synonymous codons being translated at different rates due to differences in available tRNAs
The increased pausing is most consistent with a change in amino acid identity from glycine to glutamate
The increased pausing is most consistent with GGA being a stop codon that triggers termination and reinitiation
The increased pausing is most consistent with the ribosome requiring perfect matching at the third base and rejecting all GGA codons
Explanation
This question tests understanding of how synonymous codon usage affects translation kinetics through tRNA availability. Both GGU and GGA encode glycine due to genetic code degeneracy, but they are decoded by different tRNAs that may vary in cellular abundance. When the less abundant tRNA (decoding GGA) is required, the ribosome must wait longer for the correct aminoacyl-tRNA to arrive, causing increased pausing without changing the amino acid sequence. The correct answer B correctly identifies that synonymous codons can be translated at different rates based on tRNA availability. Answer A incorrectly suggests an amino acid change when both codons encode glycine, while answer D wrongly claims GGA is a stop codon. To analyze translation efficiency, remember that even synonymous codons can have dramatically different translation rates depending on the matching tRNA concentrations in the cell.
A virology group introduced a point mutation into a viral coding sequence that changed 5'-UGG-3' to 5'-UGA-3'. In infected cells, the major translation product was truncated, but a low level of full-length protein was still detected. The group notes that the host has near-cognate tRNAs that occasionally insert an amino acid at stop codons (readthrough). Which effect is most consistent with the described mutation?
A missense mutation changed tryptophan to glycine, and full-length protein reflects compensatory splicing
A synonymous mutation occurred because UGG and UGA both encode tryptophan, but yield differs due to codon bias
A nonsense mutation introduced a stop codon; occasional readthrough can produce low amounts of full-length protein
A frameshift mutation occurred because a single-base substitution shifts the reading frame, and readthrough restores it
Explanation
This question tests knowledge of nonsense mutations and stop codon readthrough during translation. The mutation from UGG (tryptophan) to UGA creates a stop codon, which normally terminates translation to produce a truncated protein. However, some near-cognate tRNAs can occasionally decode stop codons through imperfect base pairing (readthrough), allowing translation to continue and produce full-length protein at low levels. The correct answer B properly identifies this as a nonsense mutation with readthrough explaining the full-length protein. Answer C incorrectly claims UGA encodes tryptophan when it's actually a stop codon, while answer D mischaracterizes a simple substitution as causing a frameshift. When analyzing mutations to or from UGA, UAG, or UAA, recognize these as stop codons that terminate translation, but remember that readthrough can occur at low frequency in certain cellular contexts.
During analysis of a mutant bacterial strain, a single-base deletion was found early in a coding region. Ribosome profiling showed normal initiation at the start codon but widespread changes in ribosome density downstream, and the detected protein product was markedly shorter and had an altered C-terminus compared with wild type. The mutation did not create a new promoter or affect mRNA abundance. Which effect is most consistent with the described mutation?
A missense mutation changed one amino acid but preserved the downstream reading frame and protein length
A wobble interaction at the first codon position allowed the ribosome to maintain the original reading frame after the deletion
A synonymous substitution changed one codon to another for the same amino acid, primarily affecting translation speed
A frameshift altered the reading frame, changing downstream codons and increasing the likelihood of encountering a premature stop codon
Explanation
This question tests understanding of frameshift mutations and their consequences for translation and the genetic code. A single-base deletion disrupts the reading frame, causing all downstream codons to be read incorrectly from the deletion point onward, typically resulting in a completely different amino acid sequence and often encountering a premature stop codon. In this bacterial strain, the frameshift explains both the altered ribosome density pattern (reflecting changed codon usage) and the shorter protein with altered C-terminus (due to encountering an out-of-frame stop codon). The correct answer B accurately describes the frameshift effect and its consequences. Answer A incorrectly describes a synonymous substitution which wouldn't cause the observed changes, while answer D misapplies the concept of wobble pairing which cannot compensate for frameshifts. When analyzing deletion or insertion mutations, remember that any change not divisible by three will shift the reading frame and dramatically alter all downstream translation.
A researcher observes that one tRNA species can decode the codons 5'-CUU-3', 5'-CUC-3', and 5'-CUA-3' (all Leu), but not 5'-CUG-3'. The anticodon is 3'-GAI-5' (I = inosine) at the wobble position. Which mechanism best explains this pattern, consistent with wobble pairing rules?
The ribosome changes CUG into CUA via RNA editing during elongation, enabling decoding
Wobble occurs at the first base of the codon, so third-base differences should not matter
Inosine at the anticodon wobble position can pair with U, C, or A in the codon third position, but not G
Inosine pairs only with G, so it should decode CUG but not CUU/CUC/CUA
Explanation
This question tests understanding of the genetic code and its role in translation, specifically wobble pairing rules. Wobble allows inosine in the anticodon to pair with U, C, or A, but not G, in the codon's third position. In this observation, the tRNA with 3'-GAI-5' decodes CUU, CUC, CUA but not CUG (all leucine). Inosine pairs with U, C, A but not G, explaining the pattern, as in choice D. Choice B fails by stating inosine pairs only with G, inverting the actual wobble rules. For application, list possible pairings for modified bases like inosine. Predict tRNA decoding range based on anticodon composition.