Final answer:
DNA molecules are composed of two strands that complement each other. The sides of the DNA ladder are made up of alternating sugar and phosphate groups. The two sides of the ladder are not identical but are complementary, bonded together across pairs of protruding bases.
Step-by-step explanation:
A DNA molecule is made of two strands that complement each other, creating a double-stranded molecule. Each side rail of the DNA ladder is composed of alternating sugar and phosphate groups. The two sides of the ladder are not identical, but are complementary. These two backbones are bonded to each other across pairs of protruding bases, each bonded pair forming one "rung," or cross member.