Living organisms have a number of carbon compounds in which heterocyclic rings can be found. Some of these are nitrogeneous bases_____

Answer

To answer the question regarding the nitrogenous bases that contain heterocyclic rings found in living organisms, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Heterocyclic Rings**:
- Heterocyclic rings are cyclic compounds that contain at least one atom in the ring that is not carbon. In the case of nitrogenous bases, these rings often contain nitrogen atoms.

2. **Identifying Nitrogenous Bases**:
- Nitrogenous bases are organic molecules that contain nitrogen and are fundamental components of nucleic acids (DNA and RNA). They can be categorized into two groups based on their structure: purines and pyrimidines.

3. **Listing Purines**:
- Purines are characterized by having a double-ring structure. The two main purines found in living organisms are:
- **Adenine (A)**
- **Guanine (G)**

4. **Listing Pyrimidines**:
- Pyrimidines have a single-ring structure. The nitrogenous bases classified as pyrimidines include:
- **Cytosine (C)**
- **Thymine (T)** (found only in DNA)
- **Uracil (U)** (found only in RNA)

5. **Conclusion**:
- Therefore, the nitrogenous bases that contain heterocyclic rings in living organisms are:
- **Purines**: Adenine and Guanine
- **Pyrimidines**: Cytosine, Thymine, and Uracil

### Final Answer:
The nitrogenous bases found in living organisms that contain heterocyclic rings are:
- **Purines**: Adenine (A), Guanine (G)
- **Pyrimidines**: Cytosine (C), Thymine (T), Uracil (U)

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