The fundamental units of DNA are known as nucleotides, which consist of three essential components: a sugar molecule, a phosphate group, and a nitrogenous base.

DNA, or deoxyribonucleic acid, serves as the molecular blueprint that carries the genetic instructions necessary for the development, functioning, growth, and reproduction of all known living organisms, as well as many viruses. Each nucleotide, the building block of DNA, is comprised of three parts: a sugar molecule (specifically deoxyribose in the case of DNA), a phosphate group, and a nitrogenous base.

The sugar and phosphate group together form the backbone of the DNA molecule, while the nitrogenous bases are responsible for encoding the genetic information. There are four distinct types of nitrogenous bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). These bases pair specifically to create the rungs of the DNA ladder: adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). This pairing is referred to as complementary base pairing.

The order of these bases determines the genetic code. For instance, the sequence $ATCGTT$ may correspond to a particular protein, while the sequence $TACGAT$ could code for a different protein. This specificity is how DNA encodes the instructions necessary for synthesizing all the proteins within the body.

Additionally, DNA is structured as a double helix, meaning it consists of two intertwined strands of nucleotides. This configuration facilitates accurate replication during cell division, ensuring that each new cell receives a complete set of genetic instructions.

In conclusion, the building blocks of DNA are nucleotides, which are made up of a sugar, a phosphate group, and a nitrogenous base. The specific sequence of these nitrogenous bases encodes the genetic information crucial for life.