The minimum legth of a codon could be two. If it was only one of the 6 nitrogeneous bases in a codon, we'd only have 6 possible amino acids. If we have, though, a combination of two amino acids out of the 6 nitrogeneous bases, we would have then 36 possible combinations (6 possibilities for the first position × 6 possibilities for the second position) that would allow the existence of the 20 different amino acids. In the human case, for example, with only 4 nitrogeneous bases, a combination of two amino acids would be insufficient (4×4=16) for the 20 amino acids.
Natural selection occurs when some of those traits help some individuals survive and reproduce more than others. That causes their genes to become more common in the population over time, and it's the way species evolve to adapt to changes in their environment.
When the terminal (third) phosphate is cut loose, ATP becomes ADP (Adenosine diphosphate; di= two), and the stored energy is released for some biological process to utilize.
