RNA (Ribonucleic Acid) is a single-stranded nucleic acid that plays a crucial role in protein synthesis and gene expression. It is composed of nucleotides, which consist of a sugar molecule (ribose), a phosphate group, and a nitrogenous base. There are four nitrogenous bases present in RNA, namely adenine (A), guanine (G), cytosine (C), and uracil (U). Let's discuss each of these bases in detail:

Adenine (A):
- Adenine is a purine base that pairs with uracil (U) in RNA through hydrogen bonds.
- It forms two hydrogen bonds with uracil, contributing to the stability of the RNA molecule.
- Adenine is also present in DNA, where it pairs with thymine (T) instead of uracil.

Guanine (G):
- Guanine is also a purine base, and it pairs with cytosine (C) in RNA through three hydrogen bonds.
- This base pairing mechanism helps in maintaining the structural integrity of the RNA molecule.
- Guanine is also found in DNA, where it pairs with cytosine (C) through three hydrogen bonds as well.

Cytosine (C):
- Cytosine is a pyrimidine base that pairs with guanine (G) in RNA through three hydrogen bonds.
- This base pairing is crucial for the stability and structural integrity of the RNA molecule.
- Cytosine is also present in DNA, where it pairs with guanine (G) through three hydrogen bonds.

Uracil (U):
- Uracil is a pyrimidine base that replaces thymine (T) in RNA.
- It pairs with adenine (A) through two hydrogen bonds, forming a stable base pair.
- Uracil is not found in DNA; instead, thymine is present in DNA, which pairs with adenine through two hydrogen bonds.

Therefore, all four nucleotide bases (adenine, guanine, cytosine, and uracil) are present in RNA. These bases play a crucial role in the structure, function, and genetic information encoding of RNA molecules.