A-DNA
• X-ray diffraction studies on dehydrated DNA fibersrevealed A form, called A-DNA.
• A-DNA is found in conditions of low humidity andhigh salt concentration.
• Right-handed double helix like B-DNA
B-DNA
• Physiologically most common form.
• Right handed double helix
• B-DNA is found in conditions of high humidity andlow salt concentration.
• Highly flexible.
Z-DNA
• Phosphodiester back bone assume a zigzag form
• Left-handed double helix
• Seen in the 5′ end of chromosomes.
• Longer and thinner than B-DNA
• 12 hp per turn
• Particularly seen in sequence of alternating purineand pyrimidine
• Particularly in d(GC)0 sequence
• Methylation of Guanine and Cytosine residuesstabilizes Z-form.
• Sequence that are not strictly alternating purine andpyrimidine also form Z-DNA on methylation.
• Z-DNA influences gene expression and regulation.