What is the structure of DNA?

The DNA exists as a double helix. It consist of two intertwined polynucleotide chains that run anti-parallel (in opposite directions) to each other, resembling a ladder twisted around itself. The DNA polymer is made up of repeating nucleotide units. Each nucleotide contain a sugar and a phosphate group with one of the four different bases attached to the sugar. The four different bases are: A, C, G and T - so there are four different nucleotides.
The sides of the ''ladder'' (the long strands of DNA) consist of alternating sugar and phopshate sections. Attached to each sugar is one of the four bases. The two polynucleotide strands are held by bonds between the bases. This forms the ''rungs'' of the ''ladder''. Bases pair up in a particular way, called ''complementary base pairing'' which means that C is always linked to a G on the opposite strand and a T is linked to an A.