(a) Essential amino acids
Definition : Amino acids that cannot be synthesised by the human body and must therefore be supplied through the diet are called essential amino acids.
Key points :
- Out of the twenty amino acids found in proteins, ten are essential.
- Examples : valine, leucine, isoleucine, lysine, phenylalanine, threonine, tryptophan, methionine, histidine, arginine.
- The remaining ten, which the body can make for itself, are called non-essential amino acids (e.g. glycine, alanine, glutamic acid).
(b) Nucleotide
Definition : A nucleotide is the repeating unit of a nucleic acid, made up of three components — a nitrogenous base, a pentose sugar and a phosphate group.
How it is built :
$$\text{Base} + \text{Pentose sugar} \rightarrow \text{Nucleoside} \xrightarrow{H_3PO_4} \text{Nucleotide}$$
The base joins C-1 of the sugar through a glycosidic linkage to give a nucleoside. Phosphoric acid then esterifies the $-OH$ at C-5 of the sugar, giving the nucleotide.
Important distinction :
| Term | Components |
|---|
| Nucleoside | Base + Sugar (two) |
| Nucleotide | Base + Sugar + Phosphate (three) |
Nucleotides are joined to one another through phosphodiester linkages to build up the DNA or RNA chain.
