Dr. Ramakrishnan, 57, is the senior scientist and group leader at the Structural Studies Division of the MRC Laboratory of Molecular Biology in Cambridge, England. He said that he wasn’t convinced when he got the morning phone call from the academy.

Two Americans and an Israeli scientist won the 2009 Nobel Prize in Chemistry on Wednesday for atom-by-atom mapping of the protein-making factories within cells.

The Royal Swedish Academy of Sciences said Americans Venkatraman Ramakrishnan, Thomas Steitz and Israeli Ada Yonath’s work on ribosomes has been fundamental to the scientific understanding of life and has helped researchers develop antibiotics.

Dr. Yonath, 70, is the fourth woman to win the Nobel Chemistry prize and the first since 1964, when Dorothy Crowfoot Hodgkin of Britain received the award.

“I’m really, really happy,” Dr. Yonath said. “I thought it was wonderful when the discovery came. It was a series of discoveries ... We still don’t know every, everything, but we progressed a lot.”

This year’s three laureates, who will share the 10 million kronor ($1.4 million) award, generated three-dimensional models that show how different antibiotics bind to ribosomes.

“These models are now used by scientists in order to develop new antibiotics, directly assisting the saving of lives and decreasing humanity’s suffering,” the academy said in its announcement.

They used a method called X-ray crystallography to pinpoint the positions of the hundreds of thousands of atoms that make up the ribosome.

“This knowledge can be put to a practical and immediate use; many of today’s antibiotics cure various diseases by blocking the function of bacterial ribosomes,” the citation said. “Without functional ribosomes, bacteria cannot survive. This is why ribosomes are such an important target for new antibiotics.”

Building on Darwin’s theory

Their work builds on Charles Darwin’s theory of evolution and, more directly, on the work done by James Watson, Francis Crick and Maurice Wilkins, who won the 1962 Nobel Prize in medicine for mapping DNA’s double helix, the citation said.

In 2006, Roger D. Kornberg won the Nobel Prize in chemistry for X-ray structures that showed how information is copied to messenger RNA molecules, which carry information from DNA to the ribosomes.

“Now, one of the last pieces of the puzzles has been added — understanding how proteins are made,” said Professor Gunnar von Heijne of the Swedish Academy of Sciences, the chairman of the Nobel Committee for Chemistry. “This discovery is important not only for science as such, but also gives us tools to develop new antibiotics.”

Dr. Ramakrishnan, 57, who was born in Chidambaram, Tamil Nadu, India, is the senior scientist and group leader at the Structural Studies Division of the MRC Laboratory of Molecular Biology in Cambridge, England.

Dr. Ramakrishnan said that he wasn’t convinced when he got the morning phone call from the academy.

“Well, you know, I thought it was an elaborate joke. I have friends who play practical jokes,” Dr. Ramakrishnan told The Associated Press by telephone from his lab in Cambridge. “I complimented him on his Swedish accent.”

Dr. Steitz, a 69-year-old born in Milwaukee, is a professor of molecular biophysics and biochemistry at Yale University and attached to the Howard Hughes Medical Institute.

Dr. Yonath is a professor of structural biology at the Weizmann Institute of Science in Rehovot, Israel, and the ninth Israeli to win a Nobel prize. She told Israel Radio she didn’t think her gender played a role in the decision.

“It’s true that a woman hasn’t won since 1964. But I don’t know what that means — does it mean that I’m the best woman since then? I don’t think that gender played a role here,” she said.

She had to end the interview abruptly because Israeli President Shimon Peres, a Nobel Peace prize laureate, was on the other line.

Alfred Nobel, a Swedish industrialist who invented dynamite, established the Nobel Prizes in his will in 1895. The first awards were handed out six years later.

Each prize comes with a 10 million kronor ($1.4 million) purse, a diploma, a gold medal and an invitation to the prize ceremony in Stockholm on Dec. 10. The Peace Prize is handed out in Oslo.

On Monday, three American scientists shared the Nobel Prize in Medicine for discovering a key mechanism in the genetic operations of cells, an insight that has inspired new lines of research into cancer.

The Physics prize on Tuesday was split between a Hong Kong-based scientist who helped develop fibre-optic cable and two Canadian and American researchers who invented the “eye” in digital cameras — technology that has revolutionised communications and science.

The Literature and Peace prize winners will be announced later this week and the Economics announcement is set for Monday.