Pyrimidines have nitrogen atoms at _____ positions.

Options

A. 1', 3', 7', 9'
B. 1', 5', 7', 9'
C. 1', 3'
D. 1', 9'

Answer

To answer the question "Pyrimidines have nitrogen atoms at _____ positions," we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Pyrimidines**: Pyrimidines are one of the two types of nitrogenous bases found in nucleotides, which are the building blocks of nucleic acids like DNA and RNA. The pyrimidines include cytosine, thymine, and uracil.

2. **Structure of Pyrimidines**: Pyrimidines are characterized by a single six-membered ring that contains nitrogen atoms. The general structure consists of two nitrogen atoms and four carbon atoms.

3. **Identifying Nitrogen Positions**: In the structure of pyrimidines, the nitrogen atoms are located at specific positions within the ring. When we number the carbon and nitrogen atoms in the ring, the positions are typically designated as follows:
- Position 1 (N)
- Position 2 (C)
- Position 3 (N)
- Position 4 (C)
- Position 5 (C)
- Position 6 (C)

4. **Conclusion**: From the numbering, it can be observed that the nitrogen atoms are present at positions 1 and 3. Therefore, the correct answer to the question is "1 and 3".

5. **Final Answer**: Pyrimidines have nitrogen atoms at **1 and 3 positions**.