What Are the 3 Parts of a Nucleotide? How Are They Connected?

Nucleotides are the building blocks of the DNA and RNA used as genetic material. Nucleotides also are used for cell signaling and to transport energy throughout cells. You may be asked to name the three parts of a nucleotide and explain how they are connected or bonded to each other. Here's the answer for both DNA and RNA.

Nucleotides in DNA and RNA

Both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are made up of nucleotides which consist of three parts:

1. Nitrogenous Base
   Purines and pyrimidines are the two categories of nitrogenous bases. Adenine and guanine are purines. Cytosine, thymine, and uracil are pyrimidines. In DNA, the bases are adenine (A), thymine (T), guanine (G), and cytosine (C). In RNA, the bases are adenine, guanine, uracil, and cytosine.
2. Pentose Sugar
   In DNA, the sugar is 2'-deoxyribose. In RNA, the sugar is ribose. Both ribose and deoxyribose are 5-carbon sugars. The carbons are numbered sequentially, to help keep track of where groups are attached. The only difference between them is that 2'-deoxyribose has one less oxygen atom attached to the second carbon.
3. Phosphate Group
   A single phosphate group is PO₄^3-. The phosphorus atom is the central atom. One atom of oxygen is connected to the 5-carbon in the sugar and to the phosphorus atom. When phosphate groups link together to form chains, as in ATP (adenosine triphosphate), the link looks like O-P-O-P-O-P-O, with two additional oxygen atoms attached to each phosphorus, one on either side of the atom.

​Although DNA and RNA share some similarities, they are built from slightly different sugars, plus there is a base substitution between them. DNA uses thymine (T), while RNA uses uracil (U). Both thymine and uracil bind to adenine (A).

How Are the Parts of a Nucleotide Connected or Attached?

The base is attached to the primary or first carbon. The number 5 carbon of the sugar is bonded to the phosphate group. A free nucleotide may have one, two, or three phosphate groups attached as a chain to the 5-carbon of the sugar. When nucleotides connect to form DNA or RNA, the phosphate of one nucleotide attaches via a phosphodiester bond to the 3-carbon of the sugar of the next nucleotide, forming the sugar-phosphate backbone of the nucleic acid.