Genetic Code and Codon Translation (1B) - MCAT Biological and Biochemical Foundations of Living Systems

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.

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.

AUG. AUG serves as the initiation signal, recruiting the first tRNA and setting the reading frame for polypeptide synthesis in most organisms.

Methionine (Met). In the standard genetic code, AUG uniquely codes for methionine, which is incorporated at the start of nearly all proteins.

UAA, UAG, UGA. These codons signal translation termination by lacking corresponding tRNAs and recruiting release factors to disassemble the ribosomal complex.

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.

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.

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.

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.

It reads mRNA $5' \rightarrow 3'$. Ribosomes process mRNA in the $5' \rightarrow 3'$ direction to align with the antiparallel synthesis of the polypeptide chain.

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.

A site (aminoacyl site). The A site accommodates the charged tRNA, allowing codon-anticodon recognition before peptide bond formation during elongation.

E site (exit site). The E site temporarily holds the uncharged tRNA after peptide transfer, facilitating its release from the ribosome.

Aminoacyl-tRNA synthetase. Aminoacyl-tRNA synthetases ensure fidelity by specifically attaching the correct amino acid to its cognate tRNA using ATP hydrolysis.

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.

$5'-AUG-3'$. Anticodons pair antiparallel to codons, so $3'-UAC-5'$ complements $5'-AUG-3'$ via Watson-Crick base-pairing rules.

$3'-CUU-5'$. Codon-anticodon pairing is antiparallel, making $3'-CUU-5'$ the complementary sequence to $5'-GAA-3'$ for accurate translation.

Tryptophan (Trp). In the standard genetic code, UGG is the sole codon for tryptophan, ensuring its specific incorporation during protein synthesis.

Phenylalanine (Phe). UUU and UUC exemplify code degeneracy, both specifying phenylalanine due to third-position wobble in codon recognition.

Proline (Pro). These codons demonstrate degeneracy, all coding for proline via flexible base-pairing at the third position.

Nonsense mutation (creates a stop codon). Changing a sense codon to a stop codon results in premature termination, characteristic of nonsense mutations.

Silent (synonymous) mutation. Both codons encode glutamic acid, making the change silent due to the degenerate nature of the genetic code.

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.

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.