Resource: Genetic Variation

The announcement in February 2001 that scientists had completed a map of the human genome included some surprising information -- information that made some people uncomfortable. The announcement revealed that humans have only thirty thousand genes, many fewer than the one hundred thousand that scientists predicted, and just twice as many genes as fruit flies.

For those who have long considered our species to be evolutionarily advanced, it seemed inconceivable that our parts list could be only slightly longer than -- not to mention identical in many ways to -- that of a lowly invertebrate. Fortunately, the lack of variability between us and them, with regard to the code of our genes, belies a more complex story.

Genes are units of DNA that provide the instructions required to build proteins. They do this in a two-step process in which DNA is transcribed into RNA, which is then translated into a specific type of protein. In many cases, one gene codes for one type of protein. But that's not always the case, as evidenced by our low gene count relative to our high level of complexity. Higher animals have evolved greater complexity, not by acquiring more genes, but by using their genes differently.

As it turns out, about a third of all human genes code for several, sometimes many, different proteins. They do this through a process called "alternative splicing" in the second half of the protein-building process, when RNA is being translated into proteins. RNA is similar to a compound word: alternative, for example, can be read as one word, or it can be split to form two new words -- alter and native -- that have entirely different meanings. Likewise, RNA can create proteins with its entire length, or with shorter sections that independently create their own proteins. This alternative splicing, in which one section of RNA is active at one time and another section is active at a different time, often corresponds to different stages in the growth of cells, as the protein needs of the cells change.

Learn how to extract human DNA from cheek cells in this NOVA classroom activity.

Whose genes are being sequenced in the two human genome projects?

Are these genes representative of the human species? Why or why not?

What is the percentage of genetic difference between any two humans? What does this tell us?

What does the similarity of basic functions between different organisms indicate?