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Genetic Code and Codon Translation (1B) - Bio & Biochem

  1. Which three codons are stop codons in the standard genetic code?
    UAA, UAG, UGA. These codons signal translation termination by lacking corresponding tRNAs and recruiting release factors to disassemble the ribosomal complex.
  2. Which ribosomal site is the entry site for an incoming aminoacyl-tRNA?
    A site (aminoacyl site). The A site accommodates the charged tRNA, allowing codon-anticodon recognition before peptide bond formation during elongation.
  3. What amino acid is encoded by the start codon AUG in the standard genetic code?
    Methionine (Met). In the standard genetic code, AUG uniquely codes for methionine, which is incorporated at the start of nearly all proteins.
  4. What type of bond is formed between amino acids during elongation?
    Peptide (amide) bond. Peptide bonds link the carboxyl group of one amino acid to the amino group of another, forming the backbone of the polypeptide chain.
  5. Which amino acid is encoded by the mRNA codons UUU and UUC in the standard genetic code?
    Phenylalanine (Phe). UUU and UUC exemplify code degeneracy, both specifying phenylalanine due to third-position wobble in codon recognition.
  6. Identify the tRNA anticodon that base-pairs with the mRNA codon 5′−GAA−3′5'-GAA-3'.
    3′−CUU−5′3'-CUU-5'. Codon-anticodon pairing is antiparallel, making 3′−CUU−5′3'-CUU-5' the complementary sequence to 5′−GAA−3′5'-GAA-3' for accurate translation.
  7. What does it mean that the genetic code is unambiguous?
    Each codon specifies only one amino acid (or stop). Unambiguity ensures precise translation, as each codon corresponds to only one specific outcome in protein synthesis.
  8. What is the wobble hypothesis describing in codon–anticodon pairing?
    Flexible base-pairing at the 3rd codon position reduces needed tRNAs. The wobble hypothesis explains how non-standard base-pairing at the third position allows fewer tRNA types to recognize multiple codons for the same amino acid.
  9. What is the reading frame in translation?
    The grouping of mRNA nucleotides into successive, nonoverlapping codons. The reading frame establishes the correct triplet grouping from the start codon, ensuring accurate amino acid sequence translation.
  10. Which ribosomal site contains the deacylated tRNA just before it exits?
    E site (exit site). The E site temporarily holds the uncharged tRNA after peptide transfer, facilitating its release from the ribosome.
  11. Identify the mRNA codon that base-pairs with the tRNA anticodon 3′−UAC−5′3'-UAC-5'.
    5′−AUG−3′5'-AUG-3'. Anticodons pair antiparallel to codons, so 3′−UAC−5′3'-UAC-5' complements 5′−AUG−3′5'-AUG-3' via Watson-Crick base-pairing rules.
  12. What does it mean that the genetic code is (nearly) universal?
    Codon meanings are conserved across most organisms (few exceptions). The near-universality reflects evolutionary conservation, allowing genetic information to be interpreted similarly across diverse species.
  13. What is a codon in the context of the genetic code?
    A 3-nucleotide mRNA sequence that specifies an amino acid or stop. Codons are the fundamental units of the genetic code, each consisting of three nucleotides that correspond to a specific amino acid or termination signal during protein synthesis.
  14. If one nucleotide is inserted into a coding region, what is the typical translation consequence?
    Frameshift mutation altering downstream codons and often causing early stop. Single nucleotide insertions disrupt the reading frame, leading to altered codons and often premature stops in the protein sequence.
  15. In what direction is a polypeptide synthesized during translation?
    From the N-terminus to the C-terminus. Polypeptide synthesis proceeds from N- to C-terminus, mirroring the directional addition of amino acids in the ribosomal active site.
  16. Which enzyme charges a tRNA with its correct amino acid?
    Aminoacyl-tRNA synthetase. Aminoacyl-tRNA synthetases ensure fidelity by specifically attaching the correct amino acid to its cognate tRNA using ATP hydrolysis.
  17. Which amino acid is encoded by the mRNA codon UGG in the standard genetic code?
    Tryptophan (Trp). In the standard genetic code, UGG is the sole codon for tryptophan, ensuring its specific incorporation during protein synthesis.
  18. If a mutation changes an mRNA codon from UAU to UAA, what mutation type is this?
    Nonsense mutation (creates a stop codon). Changing a sense codon to a stop codon results in premature termination, characteristic of nonsense mutations.
  19. If a mutation changes an mRNA codon from GAA to GAG, what mutation type is this?
    Silent (synonymous) mutation. Both codons encode glutamic acid, making the change silent due to the degenerate nature of the genetic code.
  20. What is an anticodon, and where is it found during translation?
    A 3-nucleotide sequence on tRNA that base-pairs with an mRNA codon. Anticodons enable specific base-pairing between tRNA and mRNA, ensuring accurate amino acid delivery during the translation process.
  21. Which amino acid is encoded by the mRNA codons CCU, CCC, CCA, and CCG?
    Proline (Pro). These codons demonstrate degeneracy, all coding for proline via flexible base-pairing at the third position.
  22. Which mRNA direction does the ribosome read to translate a protein?
    It reads mRNA 5′→3′5' \rightarrow 3'. Ribosomes process mRNA in the 5′→3′5' \rightarrow 3' direction to align with the antiparallel synthesis of the polypeptide chain.
  23. What does it mean that the genetic code is degenerate?
    Most amino acids are encoded by more than one codon. Degeneracy allows for redundancy in the code, providing robustness against mutations while maintaining the same amino acid sequence.
  24. Which ribosomal site holds the growing polypeptide attached to tRNA?
    P site (peptidyl site). The P site anchors the tRNA with the nascent polypeptide, positioning it for peptide bond formation with the incoming amino acid.
  25. Which codon is the canonical start codon for translation initiation?
    AUG. AUG serves as the initiation signal, recruiting the first tRNA and setting the reading frame for polypeptide synthesis in most organisms.

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