Adenine and Guanine are purines. Cytosine and Thymine are pyrimidines. Purines are larger. A purine and a pyrimidine must pair together, so that the DNA is a certian width. If two pyrimidines were paired together, the DNA would be too narrow, if two purines were paired together the DNA would be too wide. This is way Adenine pairs with Thymine and Guanine pairs with Cytosine.
DNA is constructed by a phosphate backbone, which are held together by phosodiester bonds. DNA starts with a phosphate attached to a deoxyribose sugar, which is attached to one for the four base pairs. DNA is double stranded, which forms a twist of the strands, which is called a double helix.
Rosalind Franklin took the first x-ray of DNA, this x-ray is known as photo 51. From this photo Watson and Crick were able to make the first model of DNA as a double helix. From the photo they were also able to tell how far apart the nucleotides are from eachother. They figured out that DNA makes one complete twist every 10 nucleotides. There are also .34 nm between each base pair and 3.4 nm is one full twist. DNA is also antiparell, which runs 3' to 5'. Watson and Crick later received a noble prize for their work with DNA structure. Rosalind Franklind died before she was recognized and never received a noble prize.
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