Alternative form of DNA
Watson and Crick’s model of DNA is called as B-DNA or B-form. However , DNA can exist in other forms also (Figure 35.8). During 1950’s DNA studies were based on X-ray diffraction. The re-cent investigations use single-crystal X-ray analysis. The X-ray diffrac-tion achieved resolution of about 5Å, but single crystal X-ray analysis at 1Å, near atomic resolution. Because of this, greater structural detail of DNA is now available.
If water content increases to about 75%, the A form of DNA (A-DNA) occurs. A DNA is more compact with 11 base pairs per turn of the helix and is 23Å in diameter. The A DNA can occur under experimental conditions.
In 1979, Alexander Rich and his colleagues discovered a left handed helix, called Z DNA. It is called because its backbone formed a zigzag structure. The Z DNA looks like B DNA in which each base was rotated180 degrees resulting in a zigzag left handed structure
1. Z DNA has a left handed helical case
2. The phosphate backbone of Z-DNA follows a zigzag course
3. In Z-DNA, the adjacent sequence or residues have opposite ori-entation.
4. In Z DNA one complete twist (a twist through 360o)has 12 base pairs.
5. The angle of twist in Z-DNA is 60o
6. The Z-DNA one complete turn of helix is 45Ålong
7. The diameter of Z-DNA is 18oA
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