The procedure outlined above for mapping a disease gene on a chromosome is quite involved. Because of this, a major international scientific collaboration to sequence the whole human genome was set up. Sequencing the 3.15 billion base pairs of the human genome was accomplished in 2001. With this information in hand, finding and cloning the gene for a disease should be greatly facilitated. Indeed, after sequencing the DNA fragment associated with the disease gene, powerful software can scan the whole human DNA sequence and precisely position that fragment. By so doing, researchers can determine the sequence of the disease gene located next to the fragment used to probe the DNA.
DNA sequencing is largely automated today. First, DNA is isolated from an organism, broken into small fragments, and cloned in a plas-mid vector. The sequences of these fragments are determined using chemicals that react with the four bases, and the sequences of the fragments are arranged in a linear fashion using powerful computer software. Bacterial genomes that consist of a few million base pairs can be assembled in a few months. Of course, it takes longer to sequence larger genomes. An interesting finding of the Human Genome Project was that most of the human DNA does not contain genes. Indeed, over 95 percent of human DNA does not code for proteins. This also holds true for other animals whose DNA has been sequenced. The function of noncoding DNA is not entirely clear. Much of it consists of simple sequences repeated thousands of times. It goes without saying that spotting human genes in a mass of DNA is not an easy task. This work is still in progress. To date, it is estimated that humans have about 35,000 genes, compared to 16,000 in the fruit fly and 19,000 in the simple nematode worm Caenorhabditis elegans.
Other genome projects aim at sequencing the fruit fly, mouse, rat, and chimpanzee genomes. Examples of plant genomes sequenced or with sequencing in progress are rice and Arabidopsis, a small plant with a small genome. Genomes of medical importance, such as that of the malaria parasite have been sequenced. Comparison of genes sequenced in humans and fruit flies have identified hundreds of genes that are so similar between them that scientists can use fruit flies to investigate genes implicated in human genetic diseases. Mice and rats are often used as model systems for the study of human diseases. Therefore, knowing their DNA sequence will facilitate the study of disease genes in animal models. Knowledge of the chimpanzee genome may tell us what genes make us different from our closest cousins.
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