... was important to Watson and Crick because it helped them figure out how the double helix was formed. These pictures show a ball and stick model of two DNA nucleotides. Gray balls are carbon atoms, blue balls are nitrogen, red balls are oxygen and the pink ball is phosphorous. The hydrogen atoms are not shown. Adenine Nucleotide (purine) Cytosine Nucleotide (pyrimidine) Base Pairs The nucleotides of DNA line up so that the sugar and phosphate molecules make two long backbones like the handrails of a ladder. To make the rungs of the ladder, two bases join together, between the sugar molecules on the two handrails. The phosphate molecules do not have any "rungs" between them. THERE IS ONLY ONE WAY THE BASES CAN PAIR UP ON THE RUNGS OF THE DNA LADDER. An adenine molecule only pairs with a thymine. A cytosine only pairs with a guanine. They can pair in either ...

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