ABSTRACT
Deoxyribonucleic acid is the molecule that stores the complex genetic information necessary for normal growth, development, and cell function, and is the site for the mutations responsible for genetic diseases. In humans, DNA is found in the chromosomes of the nucleus and in mitochondria. They are large polymers, with a linear backbone of alternating sugar and phosphate residues. The sugar in DNA is deoxyribose, a 5-carbon sugar, and successive sugar residues are linked by covalent phosphodiester bonds. Covalently attached to carbon atom number 1 of each sugar residue is a nitrogenous base. Four types of base are found: adenine (A), cytosine (C), guanine (G), and thymine (T) and they consist of heterocyclic rings of carbon and nitrogen atoms. They can be divided into two classes: purines (A and G) have two joined heterocyclic rings; pyrimidines (C and T) have a single such ring. RNA is an abbreviation for ribo nucleic acid. The composition of RNA molecules is similar to that of DNA molecules, but differs in that they contain ribose sugar residues in place of deoxyribose and uracil (U) instead of thymine.
