DNA molecule contains 4 bases whose structure are shown below. One of the structures is not correct, identify the incorrect base structure.

Solution

Chemistry ma’am · 3 steps

Check where each methyl group sits: only thymine has one, and it is on the six-membered pyrimidine ring.

1. Know the four bases. DNA has two purines, adenine and guanine, which are fused 6 + 5 rings. It also has two pyrimidines, cytosine and thymine, which are single six-membered rings. Compare each drawing with the real structure.

   Option	Base drawn	What the real base has	Verdict
   (a)	Thymine	Pyrimidine, C=O at C-2 and C-4, CH₃ at C-5	correct
   (b)	Cytosine	Pyrimidine, C=O at C-2, NH₂ at C-4, no methyl	correct
   (c)	Adenine	Purine, NH₂ at C-6, no oxygen, N-9 carries the H	correct
   (d)	Guanine	Purine, C=O at C-6, NH₂ at C-2, only H at C-8	wrong: extra CH₃ at C-8

2. Spot the error. The drawing in (d) has the right carbonyl and amino groups, but a CH₃ group sits on C-8 of the five-membered ring. Real guanine has only a hydrogen there, so no DNA base carries this group.
3. So. The incorrect structure is (d), guanine drawn with an extra methyl group.

The idea to keep. Learn the bases by their substituents. A has only NH₂, G has C=O + NH₂, C has C=O + NH₂, and T has two C=O + CH₃. Thymine is the only base with a methyl group.

Answer from the NTA final answer key. Solution by PrepWiser.