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Parallel DNA helix can exist and this has been observed experimentally. However these structures are stabilized by Hoogsteen type base pairing [1,2] and not the usual Watson-Crick type pairing because the parallel conformation does not allow the latter (See the figure below). This elongates the hydrogen bonds and also causes a loss of one hydrogen bond ...


This is more chemistry than Biology. Both strands of DNA are 5' to 3' direction. Why so? because the direction is determined in terms of the direction in which purine or pyrimidine bases are added. The four bases that exist in DNA are Adenine, Guanine, Thymine and Cytosine. They are referenced under Purine and Pyrimidine links posted above. This molecule ...

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