It has been suggested that the present-day triplet genetic code evolved from a doublet code when there were fewer amino acids available for primitive protein synthesis.

As determined by comparisons of ancient and recently evolved proteins, cysteine, tyrosine, and phenylalanine appear to be late-arriving amino acids. In addition, they are considered to have been absent in the abiotic Earth. All three of these amino acids have only two codons each, while many others, earlier in origin, have more. Is this mere coincidence, or might there be some underlying explanation?

Key Concepts

Genetic Code Evolution

The genetic code is a set of rules that dictates how sequences of nucleotides in DNA correspond to specific amino acids in proteins. The evolution from a doublet to a triplet code suggests that early life forms utilized simpler coding systems due to a limited number of amino acids. This transition likely allowed for greater complexity and diversity in protein synthesis as more amino acids became available.

Amino Acid Codons

Amino acids are the building blocks of proteins, and each is encoded by one or more codons, which are sequences of three nucleotides in mRNA. The presence of only two codons for cysteine, tyrosine, and phenylalanine indicates that these amino acids may have been added to the genetic code later in evolutionary history. Understanding the distribution of codons helps elucidate the evolutionary timeline of protein synthesis.

Abiotic Earth and Amino Acid Availability

The abiotic Earth refers to the planet's early conditions before the emergence of life, characterized by a lack of biological processes. The absence of certain amino acids during this period suggests that the initial protein synthesis relied on a limited set of building blocks. This context is crucial for understanding how the genetic code may have adapted as environmental conditions changed and more amino acids became available.