There are two groups of nitrogenous bases in nucleotides, .the................... (single ring of atoms) and the ...................(double rings of atoms).

Explanation

To answer the question regarding the two groups of nitrogenous bases in nucleotides, we can break down the information as follows:

1. **Identify the two groups of nitrogenous bases**:
- The two groups of nitrogenous bases are classified based on their structure: one group consists of bases with a single ring of atoms, and the other group consists of bases with double rings of atoms.

2. **Define the single-ring nitrogenous bases**:
- The nitrogenous bases that have a single ring structure are called **pyrimidines**. Pyrimidines are characterized by their six-membered ring structure and include bases such as thymine, cytosine, and uracil.

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