What Makes You “You”
BEFORE the Human Genome Project began, scientists had learned much about our genetic makeup. That is why terms such as “genes,” “chromosomes,” and “DNA” frequently appear in news reports as the press announces discovery after discovery of what researchers believe make us what we are. The Human Genome Project now attempts to build on these basics and to read our whole genetic code.
Before we consider how scientists go about this, please read the box “Your Blueprint,” on page 6 of this magazine.
Locating the Genes
As mentioned in the preceding article, the first aim of the Human Genome Project has been to discover where our genes are located on our chromosomes. One gene hunter likens this to “searching for a burnt-out light bulb in a house with no address in an unknown street in an anonymous city in a foreign country.” Time magazine claims that the task is “as difficult as locating a phone number without an address or a last name.” How, then, do scientists tackle this challenge?
Researchers study families to locate the genes that determine characteristics that are inherited with well-known traits and susceptibilities. For example, they have traced genes for color blindness, hemophilia, and cleft palate to areas on one of our chromosomes. These genetic-linkage maps, as they are called, are very rough—they indicate the gene’s location only to within about five-million pairs of bases.
For more precision, scientists intend to compile a physical map. In one method, they break copies of the DNA into randomly sized pieces that they then survey for special marker sequences. Of course, the more pieces there are, the more difficult it is to sort them. If you compare each DNA fragment to a book on a clearly marked library shelf, then locating a gene resembles “finding a quote in a single book rather than having to search through a whole library,” explains New Scientist magazine. These physical maps narrow the search to within 500,000 base pairs. Toward the end of 1993, a team of scientists led by Dr. Daniel Cohen at the Center for the Study of Human Polymorphism in Paris, France, produced what Time magazine called “the first full-fledged—if still rough—map of the human genome.”
The project’s next goal is to list the exact sequence of the chemical components of each of our 100,000 genes, as well as the other parts of the genome. But as scientists develop their DNA-reading skills, they find the genome to be more complex than they envisaged.
Reading the Genome
Genes account for a mere 2 to 5 percent of our genome. The rest is often termed “junk DNA.” Some researchers once thought these so-called useless sequences accidentally developed during evolution. Now they believe that some of these nongene regions regulate the structure of DNA and contain instructions the chromosomes need in order to copy themselves during cell division.
Researchers have long been interested in what switches a gene on and off. New Scientist reports that there could be as many as 10,000 of our genes that code for the production of proteins called transcription factors. Several of these apparently join together and then fit into a groove in the DNA like a key in a lock. Once in place they either spark the nearby gene into action or suppress its function.
Then, there are so-called stuttering genes that contain multiple repeats of parts of the chemical code. One of these normally contains between 11 and 34 repeats of the CAG triplet—a sequence of three nucleotides that identifies a particular amino acid. When it has 37 or more repeats, it provokes a degenerative brain disorder called Huntington’s chorea.
Consider also the effect of the change of a letter in a gene. A wrong letter in the 146-letter sequence of one of the two components of hemoglobin causes sickle-cell anemia. Even so, the body has a proofreading mechanism that checks on the integrity of the DNA when cells divide. One fault in this system reportedly can cause colon cancer. Many other disorders, such as diabetes and heart disease, though not simply the result of a single genetic fault, nevertheless result from the combined action of many faulty genes.
Rewriting the Genome
Doctors look to the Human Genome Project for information that will help them to diagnose and treat man’s ills. Already they have developed tests that reveal abnormalities in certain gene sequences. Some worry that unscrupulous people will use genetic testing to carry out a policy of eugenics. Presently most oppose germ-line therapy, which involves altering the genes in sperm and in egg cells. Even couples who contemplate in vitro fertilization of a genetically normal embryo have to face decisions about what happens to those embryos not selected for reimplantation. Additionally, thinking people voice concerns about the consequences to the unborn of a diagnosis that reveals an apparent genetic fault. Fear that genetic mapping of adults will change the way they are employed, promoted, and even insured worries many. Then there is the vexing question of genetic engineering.
“Not satisfied with reading the book of life,” comments The Economist, “they want to write in it as well.” One way doctors may be able to do this is by using retroviruses. A virus can be thought of as a group of genes in a chemical bag. Starting with a virus that affects humans, scientists remove the genes the virus needs to reproduce itself and replace these with a healthy version of the patient’s faulty genes. Once injected into the body, the virus penetrates the target cells and replaces the faulty genes with the healthy ones it carries.
Based on the discovery of a gene that can give protection against skin cancer, scientists recently reported a simple treatment. Since only 1 person in 20 carries this gene, the aim is to include it in a cream that will insert this gene into skin cells. There the gene triggers production of an enzyme that doctors believe breaks down cancer-causing toxins that attack the body.
Marvelous as these procedures are, strict controls limit the use of genetic engineering as scientists battle public anxieties over its possible consequences.
Much remains to be discovered about the intricacies of the human genome. Indeed, “there is no single human genome,” notes geneticist Christopher Wills. “There are five billion of them, one for virtually every human being on the planet.” Your genome reveals much about you. But does it tell all?
Does Your Genome Reveal All?
Some believe that genes are little dictators that make us behave as we do. In fact, recent press reports have announced the discovery of genes that some believe are responsible for schizophrenia, alcoholism, and even homosexuality. Many scientists advise caution on these possible links. For example, author Christopher Wills writes that in some cases gene variants simply “predispose their carriers towards alcoholism.” According to The Times of London, molecular geneticist Dean Hamer expressed the view that human sexuality was much too complex to be determined by one gene. Indeed, the 1994 Britannica Book of the Year reports: “No specific gene was identified as predisposing to homosexuality, however, and the work done thus far would have to be confirmed by others.” Moreover, Scientific American magazine notes: “Behavioral traits are extraordinarily difficult to define, and practically every claim of a genetic basis can also be explained as an environmental effect.”
Interestingly, in the BBC television series Cracking the Code, geneticist Dr. David Suzuki expressed the belief that “our personal circumstances, our religion, even our sex can change the way our genes affect us. . . . The way genes affect us depends upon our circumstances.” Consequently, he warns: “If you read in the newspapers that scientists have discovered a gene for alcoholism, or criminality, or intelligence, or whatever, take it with a pinch of salt. To tell how a particular gene affects someone, the scientists would need to know everything about that person’s environment as well, and even that might not be enough.”
Indeed not, for yet another factor can influence what you are. The following article considers what this is and how it can affect you for your good.
[Box/Diagram on page 6, 7]
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• Your body is made of some 100 trillion cells, most of which contain the complete blueprint of you. (Your red blood cells, however, have no nucleus and therefore do not contain the blueprint.)
• Your cells are complex structures, rather like cities with industries, energy-storage depots, and definite routes leading in and out. Direction comes from the cell’s nucleus.
• Your cell’s nucleus, home of your blueprint, can be compared to city hall, where the local government authority generally keeps plans of buildings constructed in the area. To build them, someone has to order materials, line up tools and equipment for the job, and organize the builders.
• Your chromosomes spell out your blueprint. These 23 pairs of tightly spiraling DNA molecules are present in each cell. If all chromosomes in all your body cells were unraveled and joined together, they would stretch to the moon and back some 8,000 times!
• Your DNA has sides joined by pairs of chemical components called bases, like the rungs of a ladder but a ladder twisted into a spiral. The base adenine (A) always joins with thymine (T), cytosine (C) with guanine (G). Split the ladderlike DNA as you would open a zipper, and you reveal the genetic code spelled out in those four letters, A, C, G, and T.
• Your ribosomes, like mobile factories, attach themselves to read the RNA’s (ribonucleic acid) coded message. As they do so, they string together different compounds called amino acids, which form the proteins that make you “you.”
• Your genes are sections of DNA that provide templates by which to make the body’s building blocks, proteins. These genes determine your susceptibility to some diseases. To read your genes, chemical tools called enzymes unzip a stretch of DNA. Other enzymes then “read” their way along the gene, constructing as it goes a complementary series of bases at the rate of 25 a second.
[Box/Picture on page 8]
Extract some of the DNA from human tissues and break it into fragments. Insert the fragments into a gel, pass an electric current through, and then soak the resulting blots onto a thin film of nylon. Add a radioactive gene probe, and photograph. The result is a DNA fingerprint.