Question

Why do purine:purine and pyrimidine:pyrimidine complements not exist?

a.Because purines form 3 Hydrogen bonds and pyrimidines only 2

b.Because the anti-parallel arrangement of DNA strands prevents them.

c.If only purine:pyrimidine pairs are permitted the helix will remain uniform in width from end to end.

d.Because DNA polymerase cannot initiate strand synthesis on its own.

Answer 1

C. If only purine:pyrimidine pairs are permitted the helix will remain uniform in width from end to end.

If there were purine : purine bonds, then both the bases are too large and as a result they will not be able to fit in the strand of the DNA. Pyrimidine : pyrimidine binds will not occur since these are very small. Of these complementary bonds will not be formed, there would be non uniformity in the width of the helix, being broader at purines and narrower at pyrimidines, which is not possible.