DNA Helix

##DNA DNA is the molecule that holds the instructions for growth and development in every living thing. Its structure is described as a double-stranded helix held together by complementary base pairs. The basic units of DNA are nucleotides. These nucleotides consist of a deoxyribose sugar, phosphate and base. ## Base Paring The nucleotides are identical except for the base, which can be an adenine, thymine, guanine or cytosine. There are chemical cross-links between the two strands in DNA, formed by pairs of bases held together by hydrogen bonds. They always pair up in a particular way, called complementary base pairing: thymine pairs with adenine (T-A) guanine pairs with cytosine (G-C)