The genome (inside the cell) contains all of an organism's genetic instructions.
Image courtesy of U.S. Department of Energy Genome Programs

A genome is the full set of instructions needed to make every cell, tissue, and organ in your body. Almost every one of your cells contains a complete copy of these instructions, written in the four-letter language of DNA (A, C, T, and G). The human genome contains 3 billion of these "letters" or bases. This means that if your genome were written out on sheets of paper and stacked as books, the tower of tomes would be almost as high as the Washington Monument!

If you think of the human genome as an encyclopedia, the information it contains is divided into 23 volumes, called chromosomes. Each chromosome contains genes - "sentences" of genetic instructions that tell the cell how to make proteins. We know the human genome contains about 20,500 of these genes, but the meaning of much of the remaining text within it is a mystery.

Surprisingly, the human genome is not static. Throughout life, exposure to certain substances - such as X-rays, sunlight, chemicals, and more - can begin to subtly change the genome in some cells. If a cell acquires a set of genomic changes that allows it to grow out of control, invade surrounding tissue, and spread to other sites in the body, cancer develops. A cancer patient is thought to harbor two distinct human genomes - the version contained in normal cells, and an altered one contained in tumor cells.

But ours is not the only genome on the block. All organisms have genomes - not just humans and animals, but also bacteria, fungi, viruses, and other microorganisms that cause diseases. Studying microbial genomes as well as the genomes of their hosts (including humans) can shed light on the nature of infectious diseases. Moreover, analyzing the genomes of our closer relatives - primates, mammals, and vertebrates - and comparing them to our own genome can help researchers determine what parts of the human genome have remained unchanged over time and are therefore likely to be essential.

In 1990, researchers set out to sequence (determine the order of As, Cs, Ts, and Gs in) the human genome. The effort, known as the Human Genome Project, was an international collaboration that concluded in 2003. However, sequencing the human genome was just a first step - now scientists face the challenge of using the tools and knowledge gained from the Human Genome Project to better understand human health and improve disease diagnosis and treatment.

Want to learn more?

You can learn more about some of the efforts to decipher important information in the human genome and other genomes by reading about the Broad's Genome Biology Program. You can also read about how Broad researchers are applying genomics to the study of infectious diseases like malaria and tuberculosis by visiting the Broad's Infectious Disease Program page and the Genomic Sequencing Center for Infectious Diseases. In addition, scientists involved in the Broad's Cancer Genome Projects are working to document all of the genome-based abnormalities in tumor genomes.

The Human Genome Project website will give you more insights into the public effort to sequence the human genome. You can also watch a NOVA program on "Cracking the Code of Life" to find out more about the race to complete the sequence.

Interested in where the word "genome" comes from? Joshua Lederberg and Alexa T. McCray offer a brief history of the word, as well as a "lexicome" of terms ending in "-ome."