What is DNA?

DNA is a large molecule that encodes hereditary information. It is the structure that encodes all the information for "building" a living organism. For this reason it is often called the "blueprint" of life. Every living cell, from bacterial cells to human cells have DNA. The structure of DNA is a double helix with 2 antiparallel strands composed of 4 different types of nucleotides. A (adenine), T (thymine), C (cytosine) and G (guanine). A always pairs with T (pairing with 2 hydrogen bonds) and C always pairs with G (pairing with 3 hydrogen bonds).

The structure of DNA with hydrogen bonding bases

DNA is arranged into structures called chromosomes and the chromosomes within a single cell are collectively called the genome.

Diagram showing how genes are found within chromosomes
For single celled organisms like bacteria, this genome usually consists of only one chromosome. In more complex organisms, like humans, the genome consists of 46 chromosomes (2 copies of the 22 autosomal chromsomes plus either 2 X chromosomes or 1 X and 1 Y chromosome.
A karyotype of the normal set of human chromosomes

Chromosome can basically be thought of as long stretches of DNA. The genome is typically divided up into several of these structures called chromsomes. It is important to note that the complete genome is present in every cell. Not every cell is the same however (heart cells, lung cells, skin cells, etc.) since a different set of genes is expressed in different cell types. In humans chromosomes are composed of both DNA and proteins called histones which mainly serve to package the DNA in a more compact arrangement.

The Building Blocks of DNA

The structure of DNA was determined by James Watson and Francis Crick and published in the journal Nature in 1953. DNA is made up of building blocks called nucleotides. Each nucleotide consists of a pentose sugar (deoxyribose in the case of DNA), phosphate, and one of four nitrogenous bases (adenine, thymine, guanine or cytosine).