All flashcards
Flashcard 1: Which ribosomal site holds the growing polypeptide attached to tRNA?
Answer: 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.
Flashcard 2: What is a codon in the context of the genetic code?
Answer: 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.
Flashcard 3: What is an anticodon, and where is it found during translation?
Answer: 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.
Flashcard 4: Which codon is the canonical start codon for translation initiation?
Answer: AUG. AUG serves as the initiation signal, recruiting the first tRNA and setting the reading frame for polypeptide synthesis in most organisms.
Flashcard 5: What amino acid is encoded by the start codon AUG in the standard genetic code?
Answer: Methionine (Met). In the standard genetic code, AUG uniquely codes for methionine, which is incorporated at the start of nearly all proteins.
Flashcard 6: Which three codons are stop codons in the standard genetic code?
Answer: UAA, UAG, UGA. These codons signal translation termination by lacking corresponding tRNAs and recruiting release factors to disassemble the ribosomal complex.
Flashcard 7: What does it mean that the genetic code is degenerate?
Answer: 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.
Flashcard 8: What does it mean that the genetic code is unambiguous?
Answer: 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.
Flashcard 9: What does it mean that the genetic code is (nearly) universal?
Answer: 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.
Flashcard 10: What is the wobble hypothesis describing in codon–anticodon pairing?
Answer: 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.
Flashcard 11: Which mRNA direction does the ribosome read to translate a protein?
Answer: It reads mRNA 5′→3′. Ribosomes process mRNA in the 5′→3′ direction to align with the antiparallel synthesis of the polypeptide chain.
Flashcard 12: In what direction is a polypeptide synthesized during translation?
Answer: 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.
Flashcard 13: Which ribosomal site is the entry site for an incoming aminoacyl-tRNA?
Answer: A site (aminoacyl site). The A site accommodates the charged tRNA, allowing codon-anticodon recognition before peptide bond formation during elongation.
Flashcard 14: Which ribosomal site contains the deacylated tRNA just before it exits?
Answer: E site (exit site). The E site temporarily holds the uncharged tRNA after peptide transfer, facilitating its release from the ribosome.
Flashcard 15: Which enzyme charges a tRNA with its correct amino acid?
Answer: Aminoacyl-tRNA synthetase. Aminoacyl-tRNA synthetases ensure fidelity by specifically attaching the correct amino acid to its cognate tRNA using ATP hydrolysis.
Flashcard 16: What type of bond is formed between amino acids during elongation?
Answer: 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.
Flashcard 17: Identify the mRNA codon that base-pairs with the tRNA anticodon 3′−UAC−5′.
Answer: 5′−AUG−3′. Anticodons pair antiparallel to codons, so 3′−UAC−5′ complements 5′−AUG−3′ via Watson-Crick base-pairing rules.
Flashcard 18: Identify the tRNA anticodon that base-pairs with the mRNA codon 5′−GAA−3′.
Answer: 3′−CUU−5′. Codon-anticodon pairing is antiparallel, making 3′−CUU−5′ the complementary sequence to 5′−GAA−3′ for accurate translation.
Flashcard 19: Which amino acid is encoded by the mRNA codon UGG in the standard genetic code?
Answer: Tryptophan (Trp). In the standard genetic code, UGG is the sole codon for tryptophan, ensuring its specific incorporation during protein synthesis.
Flashcard 20: Which amino acid is encoded by the mRNA codons UUU and UUC in the standard genetic code?
Answer: Phenylalanine (Phe). UUU and UUC exemplify code degeneracy, both specifying phenylalanine due to third-position wobble in codon recognition.
Flashcard 21: Which amino acid is encoded by the mRNA codons CCU, CCC, CCA, and CCG?
Answer: Proline (Pro). These codons demonstrate degeneracy, all coding for proline via flexible base-pairing at the third position.
Flashcard 22: If a mutation changes an mRNA codon from UAU to UAA, what mutation type is this?
Answer: Nonsense mutation (creates a stop codon). Changing a sense codon to a stop codon results in premature termination, characteristic of nonsense mutations.
Flashcard 23: If a mutation changes an mRNA codon from GAA to GAG, what mutation type is this?
Answer: Silent (synonymous) mutation. Both codons encode glutamic acid, making the change silent due to the degenerate nature of the genetic code.
Flashcard 24: What is the reading frame in translation?
Answer: 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.
Flashcard 25: If one nucleotide is inserted into a coding region, what is the typical translation consequence?
Answer: 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.