The three main parts of a nucleotide are:

1. Sugar: The sugar component of the nucleotide is a five-carbon sugar. In DNA, the sugar is deoxyribose, while in RNA, the sugar is ribose. The primary difference between the two sugars is the replacement of a hydroxyl group by hydrogen at the 2’ position in deoxyribose.

2. Nitrogenous base: The nitrogenous base is a nitrogen-containing compound that is either a purine or a pyrimidine. There are five nitrogenous bases found in the DNA and RNA world: adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U). In DNA, the bases are A, C, G, and T, while in RNA, the base uracil (U) takes the place of thymine (T) .

3. Phosphate group: The phosphate group is a molecule made up of phosphorus and oxygen atoms. It is attached to the sugar by a phosphodiester bond, forming the backbone of the DNA. The phosphate group is important in the formation of phosphodiester bonds, which link several nucleotides in a linear fashion.

In summary, a nucleotide is composed of a sugar molecule, a nitrogenous base, and a phosphate group. The sugar can be either deoxyribose or ribose, the nitrogenous base can be a purine or a pyrimidine, and the phosphate group is a molecule made up of phosphorus and oxygen atoms.