What are the types of DNA?

The three main types of DNA include: A-DNA, B-DNA, and Z-DNA. The structure of A-DNA is a right-handed double helix (similar to B-DNA structure). The dehydrated DNA takes an A form that protects DNA during extreme conditions (e.g. desiccation). Protein binding removes solvent from DNA as well, which then the DNA becomes an A form. The main difference between A-DNA and B-DNA is the conformation of the deoxyribose sugar ring. It is in the C2 endo conformation in B-DNA and in the C3 endo conformation in A-DNA. Another difference between the two DNA types is the placement of base pairs in the duplex. In B-DNA, the base pairs are centered over the helical axis, but in A-DNA are near the major groove.

The structure of B-DNA is most commonly a right-handed helix.This is the main type of DNA found in normal conditions in the body and the two strands are linked together by hydrogen bonding between the bases (anti-conformation). Bases fit in the double helical model if pyrimidine is paired with purine on the other strand. The two strands will pair Adenine and Tyrosine and Guanine with Cytosine. These strands cannot be separated without the DNA rotating and there are about 10 base pairs per turn. 

The structure of Z-DNA is a left-handed DNA molecule where the double helix winds to the left in a zig-zag manner in the phosphodiester backbone. Z-DNA is found before the start site of genes and therefore is thought to have a role in gene regulation. Z-DNA can form when DNA is in an alternating purine-pyrimidine sequence, causing a zig zag pattern since guanine and cytosine are in different conformations.