Answer;
-Sugar phosphate backbone; pair of bases
In the spiral staircase analogy of DNA structure, each railing represents a sugar phosphate and each step represents a pair of bases.
Step-by-step explanation;
-Double helix is the description of the structure of a DNA molecule. A DNA molecule consists of two strands that wind around each other like a twisted ladder or a spiral staircase. Each strand has a backbone made of alternating groups of sugar (deoxyribose) and phosphate groups.
-DNA has the shape of a double helix, just like a spiral staircase. As a nucleic acid, DNA is composed of nucleotide monomers, consisting of the deoxyribose sugar, a phosphate group, and a nitrogenous base (A, C, G or T). There are two sides to the double helix, called the sugar-phosphate backbone, as they are made from alternating phosphate groups and deoxyribose sugars.