James Watson/ Frances Crick

In 1953, two scientists, James Watson and Francis Crick, were trying to put together a model of DNA. They took a look at Franklin and Wilkin's picture of the X-ray and made their model.

They created a model that has not been changed much since then. Their model showed a double helix with little rungs connecting the two strands. These rungs were the bases of a nucleotide.

They also found that if they paired Thymine with Adenine and Guanine with Cytosine DNA would look uniform. This pairing was also in accordance with Chargaff's rule.

They also found that a hydrogen bond could be formed between the two pairs of bases. In addition, each side is a complete complement of the other.