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DNA is a polynucleotide with each nucleotide made of a nitrogen base and a phosphate group linked to a sugar molecule called deoxyribose. The phosphate group is attached to the 5th carbon of the sugar ring. The hydroxyl group at the 3rd carbon of the sugar ring forms a phosphodiester bond with the phosphate group of another nucleotide, thus linking the nucleotides, and giving a 5′ to 3′ directionality or polarity to the DNA strand. The alternating sugar and phosphate groups forms the “backbone” of DNA. The type of nucleotide determines the DNA sequence and thereby the precise genetic code. There are four types of nucleotides, Adenine (A), Guanine (G), Cytosine (C) and Thymine (T). A and G are two-ring nitrogen bases called purines whereas C and T are single-ring nitrogen bases called pyrimidines. Purines pair with pyrimidines through hydrogen bonds and A’ always pairs with ‘T’ and ‘G’ with ‘C. The complementary base pairing gives DNA its three dimensional double helical structure with one strand complementary to the other.