Which of the following describes a nucleotide?

A nucleotide is fundamentally composed of three key components: a nitrogenous base, a five-carbon sugar (ribose in RNA and deoxyribose in DNA), and one or more phosphate groups. The interaction among these components enables the structure and function of nucleic acids, such as DNA and RNA, which are crucial for storing and transmitting genetic information.

The nitrogenous base (which can be adenine, thymine, cytosine, guanine, or uracil) pairs with the sugar (ribose or deoxyribose) and the phosphate group forms the backbone of nucleic acids, contributing to their stability and reactivity. Phosphate groups are not only central to the structure of nucleotides but also play a pivotal role in energy transfer through molecules like ATP (adenosine triphosphate), which is critical for cellular functions.

In contrast, the other options present combinations that do not accurately characterize nucleotides. For example, including protein or calcium deviates from the core components of nucleotides. Thus, the correct description of a nucleotide is indeed a nitrogen base, sugar, and phosphate group.