Final answer:
The sugar-phosphate backbone of DNA is analogous to the spiral staircase’s banister (side rails), providing structural support and holding the nitrogenous base pairs in place.
Step-by-step explanation:
In the structure of DNA, the component analogous to the banister (side rails) of a spiral staircase is the sugar-phosphate backbone. The backbone is composed of alternating sugar and phosphate groups and lies on the outside of each strand of DNA. This backbone supports the DNA structure and holds the nitrogenous bases in place, like the banister of a staircase holding the steps. The base pairs, which include adenine pairing with thymine and cytosine pairing with guanine, are analogous to the actual steps of the staircase, positioned between the sugar-phosphate backbones.