Name the only heterocyclic nitrogen base, which is present in DNA but not is RNA.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a specific type of nitrogen base that is found in DNA but not in RNA.

2. **Identifying Nitrogen Bases**: In nucleic acids, there are two types of nitrogenous bases: purines and pyrimidines. The common nitrogen bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T). In RNA, the bases are adenine (A), guanine (G), cytosine (C), and uracil (U).

3. **Comparing DNA and RNA Bases**: By comparing the bases, we see that thymine (T) is present in DNA, while RNA contains uracil (U) instead of thymine.

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