Why does poly-A,T DNA have a different structure to B-DNA?

[AmberJB]

The full question is:

 

.Short DNA sequences consisting of guanine and cytosine base pairs only (poly-C,G DNA) adopt an anti-parallel, double-stranded helical structure in which the major grove is twice the width of the minor grove. Poly-A,T DNA forms a parallel, double-stranded helical structure in which the major and minor groves are of the same width. DNA containing both C,G and A,T base pairs adopts the same structure as poly-C,G. Discuss.

 

I'm guessing it must have something to do with the way each pair hydrogen bonds, but I'm really not sure how this might go on to affect the secondary structure of the DNA so profoundly. Any hints or links to a good source would be much appreciated!