A-form DNA

A-DNA is a right-handed double helix made up of deoxyribonucleotides. It appears when the relative humidity of the environment is less than 75%, which means that it is rarely present in normal physiological condition. The two strands of A-DNA are anti-parallel with each other and not symmetrical. The molecule is asymmetrical is because the glycosidic bonds of a base pair are not diametrically opposite to each other. Therefore, major grooves and minor grooves can be observed in each turn. One turn of the helix consists of 11 base pairs with a length of 2.86nm. The backbone of A-DNA is formed by sugar phosphates that are linked continuous using phosphodiester bonds. All the nitrogenous bases are at the core centre of the helix. Hydrogen bonds between nitrogenous bases allow the molecule to exhibit the double helix structure. The helix width of A-DNA is 2.3nm. Overall, A-DNA is wider than the more commonly found B-DNA.

Click for revision questions

B-form DNA

B-form DNA is a right-handed double helix, which was discovered by Watson and Crick based on the X-ray diffraction patterns. It is the common form of DNA exists under normal physiological condition.  The double strands of B-DNA run in opposite directions. The structure is asymmetrical with major grooves and minor grooves present alternatively. The molecule is asymmetrical is because the glycosidic bonds of a base pair are not diametrically opposite to each other. In one turn, there are 10 base pairs with a length of 3.4nm. The distance between adjacent deoxyribonucleotides is 0.34nm. Same as A-DNA, the backbone of B-DNA is formed by sugar phosphates that are linked continuous using phosphodiester bonds and the core region consists of nitrogenous bases. The two strands are held together by hydrogen bonds between nitrogenous bases. The helix width of B-DNA is 2nm. B-DNA is narrower than A-DNA. 

Click for revision questions

Z-form DNA

Z-form DNA is a left-handed double helix. It has a very different structure when compared with A-DNA and B-DNA. The zigzag appearance of backbone allows it to be distinguished from other forms of DNA. The helix width is 1.8nm, which is the narrowest among the three types. The structure consists of major and minor grooves. One turn of Z-DNA has 12 base pairs and the length is 4.56nm. The distance between two adjacent deoxyribonucleotides is 0.37nm. Same as the other forms of DNA, hydrogen bond is present to hold the two strands together. Z-DNA is difficult to be observed since it is unstable. It may take part in expression regulation of some genes or in genetic recombination. It can be found in bacteria, eukaryotes and viruses. In some viruses, they require Z-DNA binding proteins for pathogenesis.

Click for revision questions

Copyright © 2018. All Rights Reserved. The Chinese University of Hong Kong.