Nucleotides are the building blocks of nucleic acids like DNA and RNA. They are composed of three main parts:

1. A nitrogenous base: This is a ring-shaped molecule containing nitrogen. There are five main types of nitrogenous bases found in DNA and RNA:

* Adenine (A)

* Guanine (G)

* Cytosine (C)

* Thymine (T) - found only in DNA

* Uracil (U) - found only in RNA

2. A pentose sugar: This is a five-carbon sugar. In DNA, the sugar is deoxyribose, while in RNA, it's ribose.

3. A phosphate group: This is a molecule containing phosphorus and oxygen.

How they are put together:

The nitrogenous base attaches to the 1' carbon of the pentose sugar. The phosphate group attaches to the 5' carbon of the pentose sugar.

Key takeaway: The combination of a specific nitrogenous base, pentose sugar, and phosphate group defines a particular nucleotide. For example, adenine + deoxyribose + phosphate group forms a deoxyadenosine monophosphate (dAMP) nucleotide, a building block of DNA.