Answer:
Nitrogenous bases
Step-by-step explanation:
DNA is a nucleic acid molecule that acts as the genetic material of the organisms. The structure of DNA is made of two strands of nucleotides that are linked together and form a helical structure. The helical structure is known as the double-helical model with a ladder-like appearance.
The backbone of the ladder is composed of the sugar and phosphate groups at alternate positions whereas the staircase of the ladder are made of the different type of nitrogenous bases (A, G, T, C) joined by the hydrogen bond.
Thus, Nitrogenous bases is the correct answer.