DNA STRUCTURE

DNA Is Composed of Polynucleotide Chains:

Schematic model of the double
helix

         The most importnt featur of DNA is that it is usually composed of two polynucleotde chains twistd around each other in the form of a double helix. The upper part presents the structure of the double helix shown in a schematc form. Note that if inverted 180°, the double helix looks superficially the same, due to the complementary nature of the two DNA strands. The space-filling model of the double helix, in the lower part, shows the components of the DNA molecule and their relative positions in the helical structure. The backbone of each strand of the helix is composed of alternating sugar and phosphate residues; the bases project inward but are accessible through the major and minor grooves.