| Alternate forms of DNA may help to regulate gene expression
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| Figure 30.2 Watson-Crick base pairing of nucleotides in DNA. The AT base pairs form two hydrogen bonds and the GC base pairs form three hydrogen bonds. Thus, GC-rich regions are more stable than AT-rich regions. |
| Although the majority of DNA within a cell exists in the form described above (the B-form), alternative forms of DNA also exist. When the relative humidity of B-form DNA falls to less than 75%, the B-form undergoes a reversible transition into the A-form of DNA. In the A-form of DNA, the nucleotide base pairs are tilted 20°relative to the helical axis and the helix diameter is increased, compared to the B form (Fig. 30.3). When the DNA strands consist of polypurine and polypyrimidine tracks, the DNA helix shows different properties. Polypurine regions are A-like, while polypyrimidine regions are B-like. These regions do not efficiently bind histones and are therefore unable to form nucleosomes (see below),
resulting in nucleosome-free (exposed) regions of DNA.
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| Another unique form of DNA exists when the sequence of nucleotides consists of alternating purine/pyrimidine stretches. This form, termed Z-DNA, is also favored at high ionic concentrations. In Z-DNA, the base pairs flip 180° relative to the sugar nucleotide bond. This results in a novel conformation of the base pairs relative to sugar- phosphate backbones, yielding a form of DNA with a zigzag configuration (hence the name Z-DNA) along the sugar phosphate backbone. Surprisingly, this change in conformation leads to the formation of a left-handed DNA helix. While the Z-DNA form is favored at high ionic concentrations, it can also be induced at normal ionic concentrations by DNA methylation. The positioning of these alternate forms of DNA throughout the genome participates in the regulation of gene expression.
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