DNA is a right-handed double helix composed of deoxyribonucleotides. It can take up different forms of structural conformations based on different factors, some are – salt concentration, presence of chemically altered bases, hydration level, sequence of DNA, presence of polyamines in the solution, direction and quantity of supercoiling.
Commonly occurring structural conformations of DNA are – A-DNA, B-DNA and Z-DNA.
The key difference between form B DNA and Z DNA is that the B-DNA is right-handed, while the Z-DNA is left-handed.
B DNA
- Commonly occurring DNA form in normal physiological conditions, this form of DNA is a right-handed double helix
- The two strands of this DNA run in two different directions
- They show an asymmetrical structure, with the alternate presence of major and minor grooves. It is a result of the glycosidic bonds of a base pair not being diametrically opposed to one another
- Between the adjacent deoxyribonucleotides, there is a distance of 0.34 nm and each turn comprises 10.5 base pairs of length 3.4 nm
- The helical width of B-DNA is 2 nm and its backbone comprises sugar phosphates associated continuously through phosphodiester bonds. The core comprises nitrogenous bases
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Z DNA
- Structurally differing, this form of DNA is a left-handed double helix
- The helical width of Z-DNA is 1.8 nm, making it the narrowest compared to the other DNA conformations
- Its distinguishing factor is its backbone appearing as though a zigzag
- Each turns comprises 12 base pairs, 4.56 nm long
- Two adjacent deoxyribonucleotides are 0.37 nm apart with the presence of hydrogen bonds between two strands
Key Difference between B DNA and Z DNA
The table below depicts the differences between B DNA and Z DNA.
| One of the three common conformations of DNA helix, the chain twists up and to right around the front of the helical axis | One of the three common conformations of DNA helix, the chain twists up and to the left around the front of the helical axis |
| Right-handed | Left-handed |
| Common | Less common comparatively |
| Major groove – wide and deep
Minor groove – narrow and deep |
Major groove – narrow and deep
Minor groove – wide and shallow |
| Normal physiological condition | High salt concentration |
| Mononucleotides (1 bp) | Dinucleotides (2 bp) |
| Altering | Not altering |
| 20 Å | 18 Å |
| anti | C: anti, G: syn |
| 10.5 | 12 |
| 34 Å | 45 Å |
| 36° | 60° |
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