Difference Between B And Z DNA

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A, B and Z DNA
Structure
These three types of DNA all possess the similar property of a double helical structure formed from a sugar-phosphate backbone and containing the 4 bases, adenine, thymine, guanine and cytosine, complementarily paired with hydrogen bonding. A and G are purines and have a double ring whereas T and C are pyrimidines and have only a single ring. Within the helix a pyrimidine is always paired with a purine such that G is always paired with C and T with A. The difference between A, B and Z DNA arises when you look at the conformation of the helix.
A-DNA, like B-DNA, is a right handed double helix with a major and minor groove, but has a more compact structure with tighter gaps between the grooves. The base pairs are also
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The functions of the other forms of DNA are less clear. The formation of them is dependent upon either binding of a protein or under conditions which favour that structure.
A-DNA has been identified as a protective mechanism in spores of the bacterium Bacillus subtilis. The increased stability compared to B-DNA makes it less vulnerable to damage by UV radiation, leading to resistance in these bacteria to UV. A similar mechanism has also been found in bacteria, which combats the effects of extreme dehydration. Under these conditions, DNA is naturally driven into the A form, thus protecting it from further damage. Switching to the A conformation has also been highlighted as a means of creating a more stable structure for transcription. A HIV reverse transcriptase was shown to switch the DNA to the A form before proofreading, in order to improve the accuracy and reduce errors. Other functions of A-DNA have been shown in protein binding, such as to CAP and
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Supercoiling is the process by which the helix is over or under wound and has important effects on the properties of DNA. In particular, supercoiling is thought to favour the transition from B to Z-DNA. Sequences required to form Z-DNA are often found near replication sites, and it also has the ability to form within a strand of B-DNA. This hints at a role in DNA replication. In addition to this, a number of proteins such as ADAR1 have been shown to contain a specific Z-DNA binding site. This allows localisation of proteins to transcriptional initiation sites using a structural alteration to the DNA. In this way, the change to Z-DNA acts as a transcriptional control mechanism, required for the transcription of certain

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