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What Are Three Types of Molecules That Make Up DNA?

The DNA of an organism contains the genetic code that generates its unique characteristics and provides the instructions necessary to sustain life. While 99 percent of all human DNA is the same, it is the 1 percent that makes you different from every other person that has ever lived. Even though the genetic code contained within your DNA is complex and extensive, it's made up of only three types of molecules.

DNA Basics
- DNA contains the entire genetic code of an organism. This DNA is a polymer, meaning it consists of many monomer units called nucleotides. For example, human DNA contains around 3 billion nucleotides. Two strands of DNA are then joined together, creating the iconic double-helix structure. This entire structure is made from three types of molecules: a sugar, phosphate and nitrogen base.
Nucleotides
- A single strand of DNA consists of alternating sugar and phosphate molecules. Each group of sugar and phosphate molecules is also joined to a nitrogen base. Together, these three molecules are called a nucleotide. These nucleotides represent the genetic alphabet, spelling out the directions for building and maintaining the organism. While all nucleotides share the same basic structure, there are four different bases, or genetic letters, within this alphabet. The bases are divided into two groups: purines and pyrimidines.
Backbone Molecules
- The backbone of the DNA strand is formed by the sugar and phosphate molecules. The sugar molecules consist of five carbon atoms and three oxygen atoms. Because of the five carbon atoms, they are referred to as pentose sugar. The pentose molecule in DNA is deoxyribose, while in RNA it is ribose. The phosphate molecule consists of four oxygen atoms surrounding a single phosphorus atom.
Nitrogen Bases
- Purine bases represent the larger of the nitrogen bases. These nitrogen-based molecules consist of two flat rings fused together. The rings are formed from nine atoms, including five carbon atoms and four nitrogen atoms. Pyrimidine bases represent the smaller of the nitrogen bases. These nitrogen-based molecules consist of one flat ring. Each pyrimidine ring consists of only six atoms, including four carbon atoms and two nitrogen atoms. Adenine and guanine are purine bases, while cytosine and thymine are pyrimidine bases.
The Double-Helix
- Two strands of DNA, called polynucleotide chains, are joined together by their nitrogen bases. A purine base always bonds with a pyrimidine base. Furthermore, adenine always pairs with thymine, and cytosine bonds together with guanine; this produces a double-stranded macromolecule, called a double-helix. The double-helix shape resembles a spiral staircase. The alternating sugar and phosphate molecules represent the rails, while the nitrogen-based molecules form the interior steps.

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