Tsuneko Okazaki (born Tsuneko Hara, 7 June 1933, died 22 September 2026) was a Japanese molecular biologist whose name is learned by biology students all over the world. Working with her husband, Reiji Okazaki, she showed that one of the two new strands of DNA is built in short pieces that are later joined together. Those pieces became known as Okazaki fragments, and their discovery answered one of the central puzzles of molecular biology: how a cell copies both strands of the double helix when its copying enzymes can only work in one direction.
She was born in Moriyama Ward, Nagoya, the second daughter of a physician, Takima Hara, and grew up with an older sister and two younger brothers, one of whom, Makoto Hara, became a doctor and mountaineer. She was in the sixth grade of elementary school when Japan was defeated in the Second World War, and she later described herself as part of the first generation of Japanese women to be educated alongside men in high schools and universities. After graduating from Aichi Prefectural Asahigaoka Senior High School, she entered the biology department of Nagoya University's School of Science in 1952, although her father had hoped she would become a doctor.
In 1956 she earned her bachelor's degree, entered graduate school and married Reiji Okazaki, a researcher three years older than her from Hiroshima whom she had met in the same developmental biology group. The couple worked in a laboratory housed in a barrack, read foreign journals at the American Cultural Center and bought a fraction collector with their own pocket money. Their work on nucleotides in sea urchin and frog eggs led to the discovery of a new sugar-linked nucleotide, thymidine-diphosphate rhamnose. She received her master's degree in 1958.
That discovery earned the couple Fulbright grants. In September 1960 they sailed for eleven days on the Hikawa Maru to Seattle, then spent fifteen months in the laboratory of Jack L. Strominger at Washington University in St. Louis and a further period with the Nobel laureate Arthur Kornberg at Stanford University, where they learned microscale techniques and began working with DNA polymerase.
They returned to Nagoya in March 1963, the month in which John Cairns showed that both strands of DNA are copied together at a moving replication fork. Because DNA polymerase could only extend chains in one direction, the couple set out to test whether the second, or lagging, strand might be made discontinuously in short pieces. Tsuneko received her doctorate in 1963, had her first child the same year and became an assistant at Nagoya University in 1965. Using pulse labelling with radioactive thymidine in bacteria and bacteriophages, the couple found the short, newly made DNA chains in the late 1960s. They published their results in the Proceedings of the National Academy of Sciences in February 1968, and in June that year Reiji presented them at the Cold Spring Harbor Symposium, where Rollin Hotchkiss coined the term "Okazaki pieces."
A remaining question was how each fragment was started, and the couple suspected short RNA primers. Their work was overtaken by tragedy: Reiji, who had been in Hiroshima as a teenager when the atomic bomb fell, developed leukaemia and died on 1 August 1975, aged 44. Tsuneko, who had given birth to their second child in 1973, carried on with the help of her students and encouragement from Kornberg, and her group went on to prove that Okazaki fragments are initiated from RNA primers and to describe the steps by which the lagging strand is completed.
She became an associate professor in 1976 and a full professor at Nagoya University in 1983. For decades she was described as the university's first female professor, although after her death the university's governing body announced that another woman had been appointed professor in the Faculty of Letters in 1977. As a working mother at a time when childcare was scarce, she joined a citizens' campaign for better nursery provision and was remembered by colleagues as a strong supporter of women in science.
From 1989 she turned to artificial chromosomes, and her group built a human artificial chromosome system from a yeast artificial chromosome carrying human centromere DNA, reported in Nature Biotechnology in 1998. After retiring from Nagoya University in 1997 she continued as a professor and then visiting professor at the Institute for Comprehensive Medical Science of Fujita Health University until 2012, directed the Stockholm office of the Japan Society for the Promotion of Science from 2004 to 2007, near her daughter who had married in Sweden, and led the start-up Chromo Research Inc. from 2008. In 2016 she was named a Distinguished Professor of Nagoya University and Distinguished Professor Emeritus of Fujita Health University.
Her honours included the Chunichi Culture Award (1986), the L'Oreal-Helena Rubinstein award (now the L'Oreal-UNESCO For Women in Science award) in 2000 as its first Japanese recipient, the Medal with Purple Ribbon (2000), the Order of the Sacred Treasure, Gold Rays with Neck Ribbon (2008), designation as a Person of Cultural Merit (2015) and the Order of Culture (2021). In 2015 Nagoya University created the Tsuneko and Reiji Okazaki Award for early career biologists.
Tsuneko Okazaki died of gastrointestinal bleeding at a hospital in Nagoya on 22 September 2026, aged 93. Her funeral was held privately by close relatives, and Nagoya University and Fujita Health University announced her death on 9 October 2026. On 20 October 2026 the Japanese government posthumously conferred on her the court rank of Junior Third Rank, effective from the date of her death.
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