what is right and left handed dna?

Portions of proteins can arrange in regularly repeating structures such as α-helices: the vast majority of these helices are right-handed (the left-handed versions are very rare). Similarly, DNA (double) helices can arrange into right-handed (A-DNA and B-DNA) or left-handed (Z-DNA) helices. If you have a helix in front of you, imagine a staircase inside it, the actual helix being the handrail: if you go up the stairs, which hand would you put on the handrail? If the handrail is on your right, that’s a right-hand helix, otherwise it’s a left-handed helix. Remember that rotating a helix 180˚ does not change whether it is right- or left-handed.