Daniel Nathans, 71, a Johns Hopkins University molecular biologist who shared the 1978 Nobel Prize for Medicine for research that enabled scientists to break apart and rearrange the molecules that shape life, died of leukemia Nov. 16 at his home in Baltimore.

This work, said Stockholm's Karolina Institute in announcing the award, increased the body of knowledge that "should help in the prevention and treatment of malformations, hereditary disease and cancer." It has since become the basis for much of today's genetic research.

Dr. Nathans, who was a professor of molecular biology and genetics, a former interim president of Johns Hopkins and chairman of the department of microbiology, shared the Nobel Prize with a Johns Hopkins colleague, Hamilton O. Smith, and a Swiss researcher, Werner Arber.

Specifically, Dr. Nathans was cited for the use of a restriction enzyme discovered by Smith as a "biochemical scissors" that enabled researchers to break apart a monkey cancer virus known as SV40 and thus to map the SV40 genes.

"In order to have any hope of understanding the complicated genetic picture" of cancer, birth defects and human development, "we have to break it up one piece at a time," Dr. Nathans told a news conference when the 1978 Nobel award was announced.

Describing this research in his Nobel lecture, Dr. Nathans said, "In time, it should be possible to make out the basic regulatory mechanisms used by plant and animal cells and eventually to understand some of the complex genetic programs that govern the growth, development and specialized functions of higher organisms, including man."

In the two decades since 1978 Nobel Prizes were awarded, restriction enzymes have enabled researchers to assemble genes in new combinations, opening a new arena of genetic engineering and paving the way for the development of such new products as synthetic human insulin, growth hormones and interferon, Johns Hopkins said in a statement announcing Dr. Nathan's death.

A native of Wilmington, Del., Dr. Nathans was the youngest of eight children of Sarah and Samuel Nathans, both of whom were Russian Jewish immigrants. He attended Wilmington's public schools, working part time after school and on weekends. In 1950, he graduated from the University of Delaware.

After college he received a medical degree from Washington University Medical School in St. Louis, then did his medical residency at Columbia-Presbyterian Medical Center in New York. He was a clinical associate at the National Cancer Institute in Bethesda in the mid-1950s, then an investigator at Rockefeller Institute for Medical Research. He joined the Johns Hopkins University faculty in 1962.

It was in 1969 while studying the monkey virus SV40 that Dr. Nathans was alerted by his colleague Smith about the restriction enzyme that could cut a piece of DNA, which is the material containing the "blueprint" of life. Dr. Nathans was able to discover how to cut the SV40 DNA into 11 fragments and to determine where genes began and ended. This helped him locate a gene in the virus that caused the production of a tumor-making protein.

After the publicity spotlight of winning a Nobel Prize faded, Dr. Nathans returned to his scientific research and teaching. In 1979, he was elected to the National Academy of Sciences, and in 1985 to the American Philosophical Society. From 1990 to 1993, he served on the Presidential Council of Advisors on Science and Technology. In 1993, he received the National Medal of Science, which is the nation's highest scientific award.

From June 1995 until August 1996, Dr. Nathans was interim president of Johns Hopkins while a search committee sought a permanent appointee. "It was an interesting job," he said in what was described as a characteristic understatement.

His avocations included reading history and literature, walking, swimming and listening to chamber music.

Survivors include his wife, Joanne Gomberg Nathans, a lawyer who had served in the department of legislative reference for the City of Baltimore; three sons, Eli, Jeremy and Ben; and six grandchildren.