C.8.2

=Describe the double helical structure of DNA=

An excellent animation describing the structure of double helix structure of the DNA strand http://207.207.4.198/pub/flash/24/menu.swf View DNA structure. Look at how **two nucleotide strands join together with hydrogen bonds to form DNA**. Specifically look at the atoms involved on each nucleotide strand in hydrogen bonding.

The key to DNA's functioning is its double helical structure with complementary bases on the two strands. James Watson and Francis Crick realized that the backbone was wound into helices, and that the two helices were held together by hydrogen bonds between the bases. The hydrogen bonds are very specific in natural DNA, dictated by the shape and charges of the molecules. A hydrogen bond can only exist between cytosine (C) and guanine (G), and between adenine (A) and thymine (T) as shown below: **This restriction is called complementary base pairing**

he green lines denote where hydrogen bonding occurs between the molecules. Although hydrogen bonding between G and T or C and T may also occur, they are nowhere near as strong as the A:T and G:C hydrogen bonds that occur in natural DNA.

NOTE: in DNA replication, the hydrogen bonds between the nitrogenous bases are the forces broken.