Rosalind Franklin's X-ray crystallography data suggested DNA is double stranded and has a uniform diameter. These observations can be used to rule out base pairing between two of the same nucleotide because . thymine dimers are removed by excision repair the antiparallel orientation of strands would result in one of the pair being upside down an A-A pair is wider than a C-C pair identical nucleotides would not have the appropriate number of hydrogen bonding sites to pair