Life shares one genetic code because it was inherited
- The idea
- Predict
- See it
- Why
- Check
The idea
“Every cell reads AAA as lysine because no other code could work.”
Nearly every living thing reads the genetic code the same way, so it seems like no other code could work. Plenty could.
The short version
The mistake: Every cell reads AAA as lysine because no other code could work.
What's actually true: Many codes could work. Nearly all cells share one because they inherited it from a common ancestor.
- A tRNA's anticodon pairs with the codon, and an enzyme called aminoacyl-tRNA synthetase loads that tRNA with its amino acid. Those molecules decide what each codon means.
- So other codes can work. Your own mitochondria read UGA as tryptophan instead of stop.
- Changing a codon's meaning would alter many proteins at once, usually for the worse. So the code passes down almost unchanged, and variant codes like these are rare.
- A trait that function demands, like a streamlined body in sharks and dolphins, can evolve separately. The codon for lysine isn't one.
On the exam: If a trait could easily have been different and species still match, that points to a shared ancestor.