What is a Genome?

Scientists throw the term “genome” around all the time but to the average person (and to most doctors and nurses), the definition of a genome is not quite clear.

So, what is a genome? It’s an organism’s complete set of DNA, including all of its genes. Each genome contains all of the genomic information needed to build and support a living organism. In humans, a copy of the entire genome is made up of more than 3 billion DNA base pairs; these are essentially four different chemicals that makeup DNA. That’s it—four chemicals (adenine, thymine, cytosine and guanine, aka A, T, C, and G) lined up in certain combinations encode the instructions to make a human being. What’s even more fascinating is that only about 1% of the 3 billion base pairs make up the approximately 20,000 genes most responsible for creating humans. These are the genes that produce the symphony of proteins that control everything from what we look like to how we respond to food or medications. The other 99% of the genome is made up of non-coding DNA which contains genetic material that helps genes work correctly or turns them on and off.

An often-used analogy is to compare a genome to a book. You can think of the entire genome as a complete book, from the very first page to the very last. In this analogy, the book (genome) has 23 chapters, representing the 23 chromosomes most of us have. The chapters (chromosomes) are made up of sentences and words that tell the story inside the book. The sentences are like genes and the individual letters of each word in the sentence are the base pairs. Finally, non-coding DNA plays the role of spaces and punctuation between words—the spaces tell you when words start and stop, and punctuation lets you know when to pause, break or move on to the next sentence.

Fun genome facts:

  • Written out, the human genome would stretch 5,592 miles (9,000 km).
  • It would take a typist working eight hours a day half a century to type.
  • It would fill one million pages; that’s 5,000 books stacked 200 feet high; or two hundred telephone directories.1

What is the Human Genome Project?

Our ability to have an understanding of what a genome is comes from the efforts of the Human Genome Project (HGP). The project was an international research effort started in October of 1990 to map out an entire human genome; this included understanding our complete DNA sequence (all 3 billion base pairs) as well as mapping out all of the human genes. In the US, the project was led by researchers at the National Human Genome Research Institute within the National Institute of Health. The effort took 13 years and from it, the HGP has given science an unparalleled resource of in-depth information about the structure, organization, and function of the complete set of human genes (as well as those of other organisms, such as yeast, mice and worms). The HGP also led to the development of better and faster genetic testing technologies and has made an enormous impact in biomedical technology and medical research.

Genomics & Medicine

Today, the Human Genome Project is changing health care in major ways. Over the past five to seven years, there have been significant advances in our understanding of the genetics of both rare and common disorders. This has led to more genetic testing and genetic counseling in individuals—with and without disease—to better assess their risk of developing specific disorders. The testing is not just for the sake of knowing, but in order to improve health, by screening for various conditions and, when possible, preventing them from ever occurring.

For individuals with particular diseases, our understanding of the genome has led to the development of drugs targeting specific genetic causes (this often referred to as precision or personalized medicine.) For instance, for individuals diagnosed with cancer, it is becoming common to order genetic testing on the cancer cells. From the cancer’s genetic profile, it’s possible to get a sense of how aggressive the cancer may be and whether there are specific chemotherapies that the cancer will respond to more favorably. here are hundreds of other applications of genetic information in health care. As our knowledge continues to grow, having a glimpse into your genome can better guide you and your doctors in managing your health.


1. http://www.thehumangenome.co.uk/THE_HUMAN_GENOME/Primer.html