Question 5
How many different amino acids are there, and why?
The right answer is the one marked above.
There are 20. Every amino acid has the same amino group (NH2), the same carboxyl group (COOH), a hydrogen and a central carbon — the only thing that changes is the R group hanging off that carbon.
Twenty units, combined in different orders and different numbers, build every protein from bacteria to human beings. There are far more than 20 proteins; there are only 20 letters in the alphabet they are written with.