Question

explain how the composition and distribution of codons among living organisms provide evidence from a common ancestor

Answer 1

Answer :

All living things share many of the basic molecular components that function within the cell.

• All living cells use chains (polymers) of nucleotides, amino acids, and simple sugars to perform the basic functions of life.

• We all use the same set of nucleosides (roughly ATCGU) out of almost 100 that occur naturally plus many more that can be synthesized.

• We all use the same set of 22 amino acids out of hundreds that occur naturally plus many more than can be synthesized.

• We all use DNA and RNA with the same left-right orientation (chirality).

• We all use the same set of “codons” – length three sequences that translate

RNA to amino acids – except for a handful of simple variants that are well explained as mutations.

The genetic code is nearly universal, and the arrangement of the codons in the standard codon table is highly non-random. The three main concepts on the origin and evolution of the code are the stereochemical theory, according to which codon assignments are dictated by physico-chemical affinity between amino acids and the cognate codons (anticodons); the coevolution theory, which posits that the code structure coevolved with amino acid biosynthesis pathways; and the error minimization theory under which selection to minimize the adverse effect of point mutations and translation errors was the principal factor of the code’s evolution. These theories are not mutually exclusive and are also compatible with the frozen accident hypothesis, i.e., the notion that the standard code might have no special properties but was fixed simply because all extant life forms share a common ancestor, with subsequent changes to the code, mostly, precluded by the deleterious effect of codon reassignment. Mathematical analysis of the structure and possible evolutionary trajectories of the code shows that it is highly robust to translational misreading but there are numerous more robust codes, so the standard code potentially could evolve from a random code via a short sequence of codon series reassignments. Thus, much of the evolution that led to the standard code could be a combination of frozen accident with selection for error minimization although contributions from coevolution of the code with metabolic pathways and weak affinities between amino acids and nucleotide triplets cannot be ruled out. However, such scenarios for the code evolution are based on formal schemes whose relevance to the actual primordial evolution is uncertain. A real understanding of the code origin and evolution is likely to be attainable only in conjunction with a credible scenario for the evolution of the coding principle itself and the translation system.