(a) What are essential and non-essential amino acids? Give two examples of each. 
(b) How are nucleosides different from nucleotides?

(A) Non-essential amino acids : The amino acids which can be synthesised in the body, are known as non-essential amino acids. Example : Glycine, Alanine etc. 

Essential amino acids : The amino acids which cannot be synthesised in the body and must be obtained through diet are known as essential amino acids. Example : Valine, Leucine etc. 

(B) Nucleoside : A nucleoside contains only two basic components of nucleic acids i.e. a pentose sugar and a nitrogenous base. During their formation 1-position of the pyrimidine or 9-position of the purine moitey is linked to C1 of the sugar (ribose or deoxyribose) by a β-linkage.

Nucleotides : A nucleotide contains all the three basic components of nucleic acids, i.e. a phosphoric acid group, a pentose sugar and a nitrogenous base. These are formed by esterification of C5‘ – OH of the sugar of the nucleoside with phosphoric acid.