(a) The small blue spheres in the double helical structure of DNA represent the sugars and the phosphates. There are four different kinds of nucleotides in DNA. Each nucleotide has a deoxyribose sugar, a phosphate group, and a nitrogenous base. In each nucleotide the sugars and phosphates are identical. The sugars and phosphates form a chainlike along the sides of a ladder.
(b) The brown slabs represent paired bases. There are four different bases. They are adenine, thymine, guanine, and cytosine. These bases face the center and each base is bonded to the base facing it in the opposite strand by hydrogen. Adenine in one strand is always paired with thymine in the other. Guanine is always paired with cytosine. The sequence of adenine, thymine, guanine, and cytosine on one strand represents the coded information carried by the DNA molecule.
(c) Yes, I agree with the analogy that DNA is structured like ladder. The ladder has two sides and rungs. So also, the DNA is a double helical structure. Two long molecular strands of nucleotides wound each other in a double helix. The sugars and phosphate groups form a chain, like the sides of a ladder. The bases face the center. Each base is hydrogen bonded to the base facing it in the opposite strand. It is just like the rungs of the ladder.