Your Body Manual THE HUMAN GENOME.
Why do some people have brown or blue eyes? What determines how you age--or what diseases you may develop? To find out, Q&A on your body's manual--the human genome.
Q. What's a gene.
A gene is a set of chemical instructions inherited from your parents that determines almost every one of your physical traits--like your hair texture and eye color, bone size and body type, and many other traits. You inherit a complete set of your genes (genome) from each parent.
Scientists now think that genes not only determine your physical appearance but also what diseases you may be susceptible to, and may influence fundamental personality traits like shyness or depression, and whether you have an inherited tendency tobecome addicted to nicotine, alcohol, or drugs. "But we still don't know how or if genes work in different ways at different points in our lives--or how much our environment impacts us," says Greg Kaebnick, a bioethicist (biologist concerned with ethics) at the Hastings Center in Garrison, N.Y.
Q. What does a gene actually do?
Typically, each gene contains the "recipe" for one protein. Proteins are essential substances made by all cells. Some proteins control chemical processes like cell division or repair. Others are body-building materials, forming skin, muscles, and hair, for example. Scientists now know that multiple genes often interact to carry out a single function--at least 20 genes work together to grow your skin, for instance.
Q. Where are genes found in your body?
The nucleus, (control center) of your body's 100 trillion cells contains a complete set of your estimated 35,000 to 120,000 genes--scientists still don't know how many exactly. Inside a cell's nucleus, each gene is embedded in two coiled strands of DNA (deoxyribonucleic acid).
One complete unit of DNA consists of 3.15 billion "letters" made up of only four substances or bases called nucleotides. The bases work like letters that combine to spell out "words" or genes. The four bases are adenine (A), thymine (T), guanine (G), and cytosine (C). They act like rungs of a ladder that fuse the two strands of DNA together. A is always paired with T, G with C (left).
Scientists are struggling to find where genes begin and end in a strand of DNA. Why? The genome is like a vast string of unpunctuated letters--long enough to fill 13 sets of the Encyclopedia Britannica. The lack of punctuation makes genes hard to identify.
Q. What's the Human Genome Project and why should I care?
Since 1990, competing scientists at over 20 laboratories in six nations have raced to locate and sequence the 3.15 billion base letters that make up the human genome. Last June, researchers announced they had mapped 97 percent of the base letters. But it will take decades for researchers to compute the total number and function of genes in your body.
So why does the Human Genome Project matter? "It will lead us to a total understanding of not only human beings--but all of life," says John Sulston of the Sanger Centre in Cambridge, England, where one third of the genome was sequenced.
First and foremost, the Human Genome Project will revolutionize the practice of medicine, allowing doctors to possibly prevent and treat diseases caused by faulty genes--such as Huntington's disease, a nerve-cell disorder that may be the result of a single gene's mutation, or alteration. Human genome research may lead to cures for life-threatening diseases like cancer and AIDS. So far, scientists have identified 7,000 genes, including one linked to obesity and another that stops the formation of cancerous tumors.
Q. What is gene therapy?
Many genetic disorders, like cancer and heart disease, are caused by a faulty or missing gene inherited from one or both parents. One promising solution: gene therapy--a medical procedure that injects a healthy gene into defective cells. In 1990, U.S. doctors first attempted to use gene therapy to save children suffering from a devastating illness called SCID (severe immune deficiency disease).
Since the disease destroys the body's ability to fight off infection, SCID children are confined to life in a germ-free bubble. While initial gene therapy failed, French scientists recently announced an improved gene treatment that has successfully reversed the disease.
Last March, researchers at the University of Michigan inserted a defective gene linked to skin cancer into a mouse embryo. After three months, cancerous moles similar to those seen in humans covered the mouse's skin. The experiment will help scientists determine how a damaged gene causes a normal skin cell to turn deadly.
Scientists are rapidly developing gene therapy treatments for other illnesses, but it will take years before the revolutionary technique becomes standard medical practice. Since genes often work in concert, "gene therapy may turn out to be vastly more complicated than people imagine," says Kaebnick.
Q. Will this research allow people to create "designer babies"?
Scientists have already genetically engineered "designer animals" to perform certain functions. For example, goat embryos (fertilized eggs) have been injected with human DNA. This makes them produce special milk proteins that are used to develop medications to treat life-threatening human diseases like AIDS and hemophilia, a bleeding disorder.
Transferring healthy genes into human embryos carrying defective genes may prevent the development of genetic diseases. But the technology might also be used for controversial ends--like genetically enhancing intelligence, physical appearance, or even musical ability.
The prospect of engineering a "perfect" baby raises serious ethical concerns, says Thomas Murray, president of the Hastings Center. What do you think? Should genetic engineering be performed on humans?
Human DNA is 98 percent identical to that of chimpanzees.
If all the DNA in the human body were stretched out end to end, it would reach the sun and back more than 600 times.
DNA differs by only 0.2 percent in every human being on Earth--except for identical twins, who share identical genes.
Social Studies: Have students research and report on one aspect of DNA use in various areas of society, such as crime detection.
Did You Know?
* To assemble the 3.5 billion letters of the human genome in correct order, supercomputers at Celera made 480,000,000,000,000,000,000 calculations.
* During a routine medical checkup in 2010, your blood or cheek cells could provide a DNA sample. Your DNA would then be screened to determine your risk for a variety of diseases and to discover which medications might prevent or treat them.
* In 1953, James Watson and Francis Crick first unraveled the structure of DNA.
National Science Education Standards Grades 5-8: reproduction and heredity * personal health * science and technology in society
Grades 9-12: the cell * molecular basis of heredity * matter, energy and organization in living systems * science as a human endeavor
"Now the Hard Part: Putting the Genome to Work," The New York Times, 6/27/00, p. F1 Websites related to human genome sequencing.. www.celera.com www.er.doe.gov/production/ober/hug_top.html www.nhgri.nih.gov/ Education: www.ornl.gov/hgmis/Ethical Issues: www.nhgri.nih.gov/ELSI/ www.unesco.org/ibc/ulc/themes/index.html