The genetic code is nearly the same in all living things because it was inherited from a shared common ancestor. Its conservation across diverse organisms is therefore strong evidence for the universal common ancestry of life.
The Reasoning
The genetic code is the set of rules linking each three-base mRNA codon to a particular amino acid or to a stop signal during translation. For example, the codon UUU specifies phenylalanine in organisms as different as bacteria, plants and humans.
This similarity can be explained step by step:
- An early ancestral population developed a system in which particular codons specified particular amino acids.
- Descendants inherited this translation system, including its codon assignments, tRNAs and ribosomes.
- Altering a codon’s meaning would change that codon’s effect in many genes simultaneously. This could produce many incorrectly structured polypeptides and would usually be harmful.
- Natural selection therefore conserved the established code as lineages diverged and evolved into different species.
The code is more precisely described as nearly universal, because a small number of codons have different meanings in certain organisms or organelles, including some mitochondria. These limited exceptions do not weaken the common-ancestry explanation; they represent later evolutionary modifications of an ancient shared system.
A common misconception is that “the same genetic code” means all organisms have the same DNA sequence. It does not. Organisms have different base sequences and genes, but they generally use the same codon-to-amino-acid assignments to translate genetic information.
Exam Technique
This is SL and HL content in A1.2 Nucleic acids. For an explain question, define the genetic code, state that it is nearly universal, and link its conservation explicitly to inheritance from a common ancestor. Do not merely write that “all organisms have DNA,” because that does not explain why the codon assignments are shared.
