Types of DNA

For the most part, DNA is found in the B-DNA form.  This means the DNA is forming a right handed helical structure with 10 base pairs per turn.  There are other types of DNA like A-DNA and Z-DNA which can be found in certain conditions.  A-DNA is right handed and has 11 base pairs per turn.  This can be found in places of low humidity.  Z-DNA is a left handed helical structure with 12 base pairs per turn.  This condition is favored in cytosine and guanine rich sequences in a high salt environment.  In addition, Z-DNA occurs in cytosine methylated regions in a low salt environment.  Another condition that DNA has been found in nature is a triple helix.  In this case, a synthetic piece of DNA inserts itself into the major groove of the naturally occuring DNA.  It is sequence specific and certain protiens have been discovered which recognize the triple DNA helix.