Watson and Crick initially proposed that the structure of DNA was a helix with the bases bonded in a complementary manner, such that A was bonded to T and C was bonded to G. However, this model did not fit with the known facts about DNA at the time. One problem with this model was that it did not account for the observed symmetry of the DNA molecule. Another problem was that it did not explain how the DNA molecule could replicate itself, as the bonded base pairs would need to separate in order for replication to occur.