34.2

34.2 The Genetic Code

Overview of the Genetic Code

  • The genetic code describes the relationship between mRNA codons and the amino acids they specify.

  • Each codon consists of three nucleotides, and there are 61 codons that correspond to 20 different amino acids.

  • Example: Codon UUU corresponds to the amino acid phenylalanine (Phe).

  • The code is termed "redundant" as multiple codons can encode the same amino acid (e.g., UUU and UUC both code for phenylalanine).

  • The standard genetic code is used by almost all organisms, with slight variations noted in some.

Codons and Their Functions

  • Initiation Codon: The codon that signals the start of protein synthesis, AUG, which specifies methionine (Met).

    • Met is retained in proteins when present at the amino end; if not, it can be cleaved off after synthesis.

  • Stop Codons: UAA, UAG, and UGA are signals that translation should terminate. They do not specify any amino acids and act as punctuation marks in the genetic sequence.

Historical Context of Deciphering the Genetic Code

  • The genetic code was deciphered in the 1960s, with significant contributions from Har Gobind Khorana.

    • Khorana and colleagues utilized synthetic RNAs to explore the relationship between codons and amino acids, receiving the Nobel Prize for their work in 1968.

Methodology of Khorana's Experiments

Experiment 1: Poly(U) mRNA

  • Setup: Synthetic mRNA composed entirely of uracil (poly(U)) was tested.

  • Result: The produced polypeptide was polyphenylalanine (Phe–Phe–Phe …).

  • Conclusion: The codon UUU corresponds to phenylalanine.

Experiment 2: Alternating U and C mRNA

  • Setup: Synthetic mRNA with alternating U and C was utilized.

  • Result: Resulted in alternating serine (Ser) and leucine (Leu) polypeptides.

  • Conclusion: The researchers could not definitively assign UCU to Ser or CUC to Leu; further experiments were required for correct identification.

Experiment 3: Repeating UCA mRNA

  • Setup: Synthetic mRNA repeating UCA was examined.

  • Result: Generated three different polypeptides (polyserine, polyhistidine, polyisoleucine).

  • Conclusion: The experiment showcased the ambiguity in assigning codon to amino acid correspondence, necessitating additional studies beyond this experiment.

Implications and Evolutionary Significance

  • The universality of the genetic code highlights a common ancestry across diverse life forms, including bacteria and humans.

  • The genetic code demonstrates strong patterns, particularly in redundancy related to the third codon position.

    • Example: Amino acids specified by two codons commonly differ in their third positions between pyrimidines (U, C) and purines (A, G).

  • This genetic code similarity across organisms serves as evidence for evolutionary relationships, alongside other factors like fossil records and structural similarities among species.