Which one of the following is correct structure for cytosine?

Solution

Chemistry ma’am · 3 steps

Cytosine is a pyrimidine with C=O at C2 and NH₂ at C4. Find the ring that has both.

1. Number the ring. All four options are pyrimidine rings, with nitrogens at positions 1 and 3. Start at the N–H as N1, so the C=O carbon between the two nitrogens is C2, the other nitrogen is N3, and the carbon next to N3 is C4.
2. What cytosine needs. Cytosine is 4-amino-pyrimidin-2-one: a C=O at C2 and an NH₂ at C4, with a double bond from N3 to C4. It has no methyl group and only one C=O.

   Option	At C2	At C4	Extra group	What it is
   (a)	C=O	H	CH₃ at C6	no amino group, not a DNA or RNA base
   (b)	C=O	C=O	CH₃ at C5	thymine
   (c)	C=O	NH₂	none	cytosine
   (d)	C=O	H	NH₂ at C5	amino group in the wrong place

3. So. Only (c) has the C=O at C2 and the NH₂ at C4, beside the ring N=C. That is cytosine.

The idea to keep. Learn the pyrimidines as one family: uracil has C=O at C2 and C4, thymine is uracil with CH₃ at C5, and cytosine has C=O at C2 with NH₂ at C4.

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