Why are certain amnio acids called a essential amnio acid?

a. because they are available in abundance
b. because they are not manufactured by the body
c. because they can form bonds with any molecule
d. because they are the most stable amongst all the amino acids
e. because they are needed for the formation of all the biomolecules

Assume that you are interested in separating short (200-400 bp) dna molecules from a pool of longer molecules in the 10,000-20,000 nucleotide range. you have two recipes for making your polyacrylamide gels: one recipe uses 1.5 percent agarose and would be considered a “hard gel,” while the other uses 0.5 percent agarose and would be considered a loose gel. which gel should you use to separate the short (200-400 bp) dna molecules from a pool of longer molecules in the 10,000-20,000 nucleotide range?