Gene linked to heart attacks is discovered.

2004 Korean scientists report the cloning of human embryos.

Recent DNA Research

Watson and Crick's model of the DNA double helix, established in 1953, has served as a foundation for the ever-growing body of DNA research. In the relatively brief time since the structure of DNA was first determined, several groundbreaking discoveries—many of them very recent—have expanded greatly our knowledge of DNA.

One area in which many developments have been made in recent years is genetic technology. One of the most important of these was the development of the polymerase chain reaction (PCR) in 1983. PCR allows researchers to quickly make billions of copies of a specific segment of DNA. The ability to copy DNA in mass quantities has made the study of DNA much easier. Another important technological development is the DNA chip, patented in 1992. The DNA chip can be used to track gene expression in organisms. Other recent technological developments include fluorescence in-situ hybridization (FISH), a method used to identify specific parts of a chromosome, and spectral karyotyping (SKY), a form of FISH used to study complex changes in genetic material.

An enormous amount of research has been done in recent years to map the genomes of various organisms, including humans. The Human Genome Project was a cooperative, 13-year effort to map the human genome. The Human Genome Project was funded by the United States Government, contributions from several other countries, and private corporations. By the completion of sequencing in 2003, the U.S. government had spent over 437 million dollars. The project determined the sequence of the 3 billion base pairs that make up human DNA. Remarkably, all goals of the project were completed on time, and at a significantly lower cost than projected. As a result of the Human Genome Project, more than 1,400 genes related to disease have been identified.

Since 1995, scientists have sequenced the genomes of more than 150 organisms. In 1998, the first full sequencing of a genome in a multicellular organism—the roundworm—was completed. In 2002, sequencing of the mouse genome was completed. The mouse genome has its own version of nearly every human gene.

Along with all this recent DNA research come many difficult ethical questions. In 1997, the first successful cloning of an organism from differentiated cells resulted in the birth of Dolly the sheep. Since then many animals have been cloned. But Dolly began to suffer early from conditions normally found only in older sheep, raising questions about possible problems of premature aging in clones. Dolly was euthanized in 2003.


1. Name a practical application of the Human Genome Project.

2. Critical Thinking Why might it be useful to study the genomes of other organisms besides humans?

3. Critical Thinking Why might clones such as Dolly the sheep suffer from premature aging? Topic: Genetic Tools Keyword: HM60657


Copyright © by Holt, Rinehart and Winston. All rights reserved.

figure 11-9

The top photo shows a healthy lung. The bottom photo shows carcinomas in a diseased lung. Lung cancer is one of the deadliest forms of cancer; 87 percent of lung cancer patients die within five years of diagnosis.

figure 11-9

The top photo shows a healthy lung. The bottom photo shows carcinomas in a diseased lung. Lung cancer is one of the deadliest forms of cancer; 87 percent of lung cancer patients die within five years of diagnosis.

Risks of Developing Cancer

Whether a person actually develops cancer seems to depend on many factors. Some families exhibit higher-than-average rates of certain cancers, leading researchers to determine that some people have a genetic predisposition to these types of cancer. With regard to cancers caused by mutagens, the number of exposures to the carcinogen and the amount of carcinogen in each exposure are significant factors. Mutations in gametes (egg or sperm cells) are especially important because these mutations are passed along to offspring.

Usually, more than one mutation is needed to produce a cancer cell. Perhaps this helps to explain why the cancer risk increases with the number of exposures to carcinogens and with the age of the individual. The longer an individual lives, the more mutations he or she will accumulate. But according to the National Cancer Institute in 2003, heightened awareness of the causes of cancer, combined with improved detection and treatment of the disease, has resulted in a decline in the number of deaths in the United States caused by the four most common cancers. The death rate for all cancers combined has also stabilized.

Kinds of Cancer

Malignant tumors can be categorized according to the types of tissues affected. Carcinomas (KAHR-suh-NOH-muhz) grow in the skin and the tissues that line the organs of the body. Lung cancer, shown in Figure 11-9, and breast cancer are examples of carcinomas. Sarcomas (sahr-KOH-muhz) grow in bone and muscle tissue. Lymphomas (lim-FOH-muhz) are solid tumors that grow in the tissues of the lymphatic system. Tumors in blood-forming tissues may cause leukemia (loo-KEE-mee-uh), the uncontrolled production of white blood cells. Usually, it takes several years for cancer to develop. However, when a vital organ, such as the liver or pancreas, is involved, the symptoms caused by organ dysfunction due to cancer may develop more rapidly.

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