Why are certain amino acids called essential amino acids

The human body uses amino acids to produce proteins, perform critical metabolic functions in the formation of other molecules and to produce energy. Some amino acids are synthesized by the human body, but the essential amino acids must be obtained from food.

the backbone of the dna and rna compound is sugar phosphate.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.