21. If a purine were substituted for a pyrimidine at a single position in one strand of a DNA double helix, what would happen?
22. The DNA double helix looks like a twisted ladder. What makes up each rung of the ladder? What holds the rungs together at the sides?
23. Is there mostly empty space between the atoms in a DNA double helix?
24. In a DNA double helix, why doesn’t an A or T form two hydrogen bonds (out of the three possible) with G or C?