Answer :
Observations that codons for amino acids one biosynthetic step apart differ by one base and that the codon organization minimizes translational errors both support the gradual assignment of codons to amino acids with the evolution of biosynthesis pathways. The correct answer is that both observations indicate gradual codon assignment alongside pathway evolution.Hence option 3 is the correct answer.
The question posed asks what observations support the idea that codons were assigned to amino acids gradually over time, a concept that aligns with the evolution of pathways for amino acid biosynthesis. Codons are sequences of three nucleotides in mRNA that correspond to specific amino acids or stop signals during protein synthesis. The assignment of codons to amino acids is a highly organized process, as understood from extensive research into the genetic code.
To address the initial question, observation (1) reflects that codons for amino acids that are one biosynthetic step apart often differ by only one base. This suggests a gradual process where minor modifications can lead to new codon assignments as the biosynthetic pathways for amino acids evolved. Additionally, observation 3 mentions that codon assignments in the genetic code are not random but appear to be influenced by the similarities in the chemical properties of amino acids and their biosynthetic relationships.
Coevolutionary theory proposes that the genetic code evolved by expanding to incorporate more complex amino acids in parallel with the development of their respective biosynthetic pathways. This implies that each codon block could be divided to encode multiple amino acids upon the arrival of new metabolic pathways, influenced by the biochemical relationships between precursor and product amino acids. On the other hand, the error minimization theory of code evolution posits that the code's structure is determined by selection for robustness to translational errors, minimizing the impact of mutations that could lead to the incorporation of incorrect amino acids and adversely affect protein function.
Both theories imply that codon assignment to amino acids was a gradual process. However, while the coevolutionary theory is more directly associated with biochemical pathways, the error minimization theory considers robustness as a crucial factor. It is key to acknowledge that both theories do not exist in isolation but rather can complement each other when considering the complexity of genetic code evolution.
In conclusion, the correct answer to the given question is that both observations support the gradual assignment of codons to amino acids alongside the evolution of amino acid biosynthesis pathways. Codons for closely related amino acids in biosynthetic pathways tend to differ by only one base, and the overall organization of codon assignments suggests evolutionary pressure to minimize translational errors.Hence option 3 is the correct answer.
