(a) What are essential and non-essential amino acids ? Give two examples of each.    (3 marks)

(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.



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.


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