DNA is far from the uniform double helix we are all accustommed to seeing in the diagrams and pictures. DNA obtains different forms in different conditions of the nucleus, and different regions of the genome where the nucleic acid base pair composition is permissive for the DNA to take on a different form. At first glance it may just seem like pretty shapes, but the shapes seem to have functional or pathological significance.
To the far left of the above picture is A-DNA. A-DNA occurs in dehyrdated conditions. So a certain region of the nucleus may have less water in it, or the cell may be dehydrated. A-DNA is favoured by certain Bacteria in their sporing stages as when the spores bud off, they're vulnerable to ultraviolet light. The A-DNA conformation protects the DNA from too much damage from UV light as pyramidine bases are spaced further apart, so they don't form dimers and disrupt the genetic code. A-DNA is a fatter and shorter form of regular B-DNA (which is the most common form found in cells). The bases are thus spaced further apart, and will be less likely to dimerise in UV conditions. The specific bases that are within the DNA double helix are known as being hydrphobic. THis means they don't like water. In A-DNA, they are much more accessible as they are more exposed. This is because they aren't being repelled by water in the low water conditions.
THe middle structure is B-DNA. It is the most common form of DNA found in human DNA. This is the helix that is commonly drawn in diagrams and on signs and symbols. The distance between each base on the 'rungs' of the molecule is around 3.4nm (nanometres). The DNA has a width of 23.7nm.
Both A and B DNA spiral upwards in a right handed screw sense.
On the left: Left Handed Screw sense.
On the right: Right Handed Screw sense.
The structure on the far right is Z-DNA. It has a left handed screw sense. It is much thinner than B-DNA and A-DNA. It occurs in high salt concentrations and regions of DNA with a high alternating pyramdine/purine base composition. The positive ions from the salt associate with the negatively charged DNA phosphate 'backbone'. This means the Backbone isn't being repelled by its' own negative charge as much, so it associates closer together, hence the thin structure. It got its' name from the 'zig-zag' positioning of its bases. It is quite an unstable structure and can lead to chromosome breakage.
(J. Zhua et al, Non B-DNA structure induced genetic instability and evolution, Cell. Mol. Life Sci., 2010)
Above, other forms of DNA.
A: A cruciform complex of DNA
B: Z-DNA (same as the one described before)
C: H-DNA. This is a triple helix of DNA, which is usually formed accidentally when a wayward DNA strand associates with a regular double stranded DNA helix during processing.
These aren't your regular forms of DNA and don't generally help matters if they're formed, as far as I know - but a lot of research is going into these and I'm sure scientists will find some uses for these exotic conformations sometime soon.
There we go. Some extra revision for tomorrows exam.
