Base pairings

DNA is double stranded, which means that it has two polynucleotide chains that wind around each other forming a double helix. The two polynecleotide chains are made of nucleotides. These nucleotides consists of three units - a sugar molecule, called deoxyribose, a phosphate group, and one of the four bases. The four bases are adenine (A), thymine (T), guanine (G), cytosine (C). A and G are purines, while C and T are pyrimidines.  The two chains of polynucleotides are successfully held together forming the double helix because of the fact that the bases form base pairs. To be more specific, the base pairs formed are A-T and C-G and they are complemetary base pairs.  Complementary base paring always involvesto une purine and one pyramidine base. The complemtary base pairs form strong hydrogen bonds that are difficult to break, which makes DNA a very stable molecule.

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