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Figure 4.4. The DNA double helix is held together by hydrogen bonds.

will always be paired with a C in the other. Similarly an A will always pair with a T. The two strands are therefore said to be complementary.

Example 4.2

Erwin Chargaff's Puzzling Data

In a key discovery of the 1950s, Erwin Chargaff analyzed the purine and pyrimidine content of DNA isolated from many different organisms and found that the amounts of A and T were always the same, as were the amounts of G and C. Such an identity was inexplicable at the time, but helped James Watson and Francis Crick build their double-helix model in which every A on one strand of the DNA helix has a matching T on the other strand, and every G on one strand has a matching C on the other.

Different Forms of DNA

The original Watson-Crick model of DNA is now called the B-form. In this form, the two strands of DNA form a right-handed helix. If viewed from either end, it turns in a clockwise direction. B-DNA is the predominant form in which DNA is found. Our genome, however, also contains several variations of the B-form double helix. One of these, Z-DNA, so-called because its backbone has a zig-zag shape, forms a left-handed helix and occurs when the DNA sequence is made of alternating purines and pyrimidines. Thus the structure adopted by DNA is a function of its base sequence.

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