Codons & Cistrons
DNA and RNA each have 4 nucleobases. Combining base pairs comes up short in expressing enough variations to encode 20 different amino acids: 42=16. But 43=64. So, encoding 20 amino acids requires an arrangement capable of more combinations than base pairs alone can provide.
Hence, genetic coding is not as simple as matched base pairs on the rung-by-rung DNA ladder. Instead, amino acid templates were originally presumed as specified by nucleotide triplets, termed codons, which run along the length of a DNA ladder; not base pairs.
It was once thought that genes would be codon sequences regularly arranged in some discernible order, and that a single gene was a unit of heredity for a trait. Instead, as little as 10% of human DNA has a known coding function. Genetic instructions turned out to be enormously more complicated than codon sequences.
The term codon, while facilely descriptive of the way that genetic data is stored, became considered as insufficient. Codons are not the physical equivalent to genetic function once thought.
Genetics turned out to be infinitely more intricate than early optimism justified. Sometimes the definition of a word is clarified with further understanding, as it was with nucleic acid. Other times, as in the case of codon, where the original definition turns out to be partly inaccurate, a new word is concocted to cover the deficiency.
Though codon is still commonly used, presumed precision of coding sequence to gene is now termed a cistron. A cistron is a hypothetical localized segment of DNA with all the template information required for producing a single protein. The term cistron emphasizes that a gene provides for a specific trait.
The terms gene and cistron are synonymous; 2 names for a baroque complex for producing bioproducts – particularly proteins – needed by cells. These words speciously congeal what Nature adroitly conceals.