What are the different conformations of DNA, and where are they found?

The different conformations of DNA are B-DNA, A-DNA, Z-DNA, and they are found in various cellular environments.

B-DNA is the most common conformation and is often what people think of when they imagine the structure of DNA. It is a right-handed double helix, with about 10.5 base pairs per turn. The sugar-phosphate backbone is on the outside, and the bases are stacked on the inside. B-DNA is most stable under physiological conditions, which is why it is the most common form in cells.

A-DNA is another right-handed helix, but it is shorter and wider than B-DNA. It has 11 base pairs per turn, and the bases are tilted relative to the helical axis. A-DNA is less common than B-DNA, but it can form in dehydrated samples of DNA, such as those used in crystallography experiments. It can also occur in hybrid DNA-RNA double helices and in DNA-protein complexes.

Z-DNA is a left-handed double helix, a mirror image of the right-handed forms. It has 12 base pairs per turn, and the sugar-phosphate backbone zigzags back and forth, hence the name 'Z-DNA'. Z-DNA is the least common conformation, but it can form in sequences with alternating purines and pyrimidines, especially under high salt concentrations or when DNA is negatively supercoiled. Some proteins can bind specifically to Z-DNA, suggesting it may play a role in gene regulation.

In summary, the conformation that DNA adopts can depend on the sequence of the bases, the level of hydration, the salt concentration, and the presence of DNA-binding proteins. These different conformations allow DNA to interact with other molecules in the cell in different ways, which can affect how genes are expressed.