Throughout the body - in cells...

Our bodies are formed from between 50 and 100 trillion cells (a trillion is a thousand billion, or a thousand, thousand million). These cells are organized into tissues, such as skin, muscle, and bone. Each cell contains all of the organism's genetic instructions stored as DNA. However, each cell uses only the instructions from part of the DNA. For example, a muscle cell uses the DNA that specifies the muscle apparatus, whereas a nerve cell uses DNA that specifies the nervous system. It is as if each cell reads only that part of a book of instructions that it needs.

Within the cell - in chromosomes...

Each very long DNA molecule is tightly wound and packaged as a chromosome. Humans have two sets of 23 chromosomes in every cell, one set inherited from each parent. A human cell therefore contains 46 of these chromosomal DNA molecules.

Within each chromosome - in genes...

Each DNA molecule that forms a chromosome can be viewed as a set of shorter DNA sequences. These are the units of DNA function, called genes, each of which guides the production of one particular component of an organism. A set of human chromosomes contains one copy of each of the roughly 30,000 genes in the human "genome" - the term used to refer to the complete genetic instructions for an organism.