In the watson-crick model of a double helix, the "steps" of the "spiral staircase" are composed of:

a- sugars
b- nitrogen bases
c- phosphates

Explanation:

DNA’s famous double helix is often depicted as a spiral staircase, with two long strands twisted around each other and steps composed of four chemical building blocks called bases. Each of these bases contain rings of carbon, along with various configurations of nitrogen, oxygen, and hydrogen. The arrangement of these atoms allow G to pair with C and A to pair with T, like interlocking gears in an elegant machine.

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