Okazaki Tsuneko

Okazaki Tsuneko (1933—), Molecular Biologist

by Miwae Yamazaki

Working in the United States and Japan as part of a husband-wife research team, Tsuneko Okazaki helped discover the "Okazaki fragments" that enable DNA to replicate. On the heels of this triumph, however, came the ultimate tragedy of losing her partner in work and in life. How she overcame this loss and numerous other obstacles to make her mark on the world of molecular biology is one of science's truly inspiring stories.
All living organisms require a kind of blueprint consisting of genetic information. The human body is made up of about 60 trillion cells, and each one of these cells contains within it the genetic blueprint that makes us human—the human genome. The aim of my research over the years has been to determine how that information is replicated.

In Search of Unchanging Truths

I was born on June 7, 1933, in Nagoya. I grew up surrounded by my mother, father (a surgeon), older sister, and two younger brothers. World War II broke out in East Asia when I was in second grade. By the time I was in fifth and sixth grade, it was almost impossible to concentrate on one's studies. Japanese cities were being bombed to ashes, and there was a serious shortage of both food and labor. As a result, many children were busy working in the fields or performing other chores.
In the summer of 1945, when I was in sixth grade, the war came to an end. I will never forget seeing the textbooks we had been using heavily censored with black ink, as ordered by the administrative headquarters of the Allied Occupation forces. We were told that much of what we had been taught up until then was untrue, and our faith in the state seemed to collapse overnight.
As a junior high school student, I was fond of science and mathematics. When I was in high school, my father let me look through a microscope and watch in amazement as an antibiotic prevented bacteria from multiplying. My father himself was deeply impressed to see the effects of the antibiotics that began to be imported soon after the war ended; cholera and tuberculosis patients who until then would most likely have died were recovering before his eyes. That was what first stimulated my interest in biology.
In 1952, I enrolled in the Nagoya University School of Science. As a result of the educational reforms carried out after the war, junior high school and high school had been made coeducational, and the national universities, including Nagoya's, had opened their gates to women. It seems to me that the confusion and insecurity that seized me when I sensed all our values being overturned led me to search for unchanging truths. And in those days, when even school buildings were in short supply, I felt fortunate just to be able to study in peace.

The Code of Life

As I studied, I found myself particularly drawn by developmental biology, or embryology, which focuses on the chemical, physiological, and morphological processes between the fertilization of an egg and the birth of an individual organism. I wanted to understand what sort of substances controlled the formation of tissues and organs to create an individual organism, and how the genetic information on which this process is based is passed from parents to offspring. At that time, however, Japanese universities were ill-equipped to teach anything beyond Gregor Mendel's work on the inherited traits of pea plants.
All genetic information needed for the development of each individual organism is contained in the molecules of deoxyribonucleic acid—commonly known as DNA—which are chemical codes for the creation of various proteins. DNA contains four chemical components referred to as bases—adenine (A), guanine (G), thymine (T), and cytosine (C) —which, lined up in various combinations, create the codes for the 20 different amino acids. These amino acids, in turn, link together in chains of anywhere from several dozen to several hundred to make proteins. Depending on how the four bases of the DNA molecule are arranged, the combination of amino acids changes, resulting in different proteins with different properties and functions. The information thus encoded in the genome even includes instructions on faithfully and reliably passing that information (the whole genome) to the cells and individuals of the next generation. The study of life and its underlying mechanisms at the level of molecules is called molecular biology, and when I was a student the field was still very new.
In 1953, James Watson and Francis Crick discovered that the four bases that form the two chains of the DNA molecule link together in pairs—A with T, and G with C—forming a beautiful structure called a double helix. It was this discovery that marked the true beginning of molecular biology as a field in its own right. I continued on to graduate school at Nagoya University, hoping to learn more about how DNA replicates.

Solving the Puzzle of DNA Replication

Each of us receives two complete genomes, or sets of chromosomes, from our parents, one from our mother and one from our father. In the case of human beings, it takes 60 trillion cell divisions for the fertilized egg to develop into a fully formed organism, and each time, the two genomes are copied with perfect accuracy. This copying process is carried out very quickly and accurately within each tiny cell. In the 1950s, the mechanism of the process was still a riddle.
In 1956, I married Reiji Okazaki, who was three years my senior at the university. That was also the year that the American scientist Arthur Kornberg succeeded in extracting from the bacterium Escherichia coli an enzyme that synthesizes a new DNA molecule from the old. It was a major discovery for which he was later awarded the Nobel Prize in physiology and medicine.
My husband Reiji submitted an application to do research at Stanford University under Dr. Kornberg, the world's leading expert in DNA synthesis. Fortunately, I was also admitted to Stanford. Beginning in late 1961, the two of us spent a very productive year and three months carrying out research under Dr. Kornberg.`
There was a problem when it came to explaining DNA synthesis with the enzyme Kornberg had discovered, DNA polymerase. The DNA molecule consists of two complementary, oppositely oriented chains, or strands, which are joined to make a kind of ladder—the double helix. When DNA replicates, the two strands of the double helix begin to separate, or "unzip," and as they do, a new complementary "daughter strand" grows along each original "parental strand." This yields two DNA molecules, each containing a parental strand and a daughter strand, and each identical to the original molecule (referred to as the template). But with Kornberg's enzyme, it was clear that new strands only grew in one chemical direction. This worked fine for one of the parental strands; growth of the complementary daughter strand would proceed from one end of the unzipping DNA molecule in the direction of the "replication fork" until the entire molecule was unzipped and a new molecule synthesized. But since the other parental strand had the opposite chemical orientation, how could a new chain grow along it to synthesize the second DNA molecule? Some scientists suspected that a different enzyme, as yet undiscovered, synthesized the second new strand. After my husband and I returned to Japan in 1963, we concentrated on unraveling this mystery.
Because the natural process of DNA replication takes place in an instant, we struggled to develop techniques that would allow us to analyze the process in a test tube. Meanwhile, the birth of our first child not long after our return to Japan obliged me to juggle research and child rearing. Eventually, however, we discovered that in the earliest stage of replication, small fragments of DNA are formed. This led us to hypothesize that the problematical daughter strand was synthesized through a repeated process in which small fragments grew, one after another, and were later joined. This is how it works: As the template DNA unzips and exposes a segment of the parental strand, a complementary segment is synthesized, growing in the opposite direction from the replication fork. As more of the template un­zips, another segment is formed, and so forth, until the entire template is unzipped. In this way, a daughter strand is synthesized on each of the two parental strands with the help of just the one DNA­synthesizing enzyme, creating two DNA molecules. This was hailed as a major finding. The small DNA fragments were given the name "Okazaki fragments," which is how they appear in biochemistry textbooks around the world today.

On My Own

After this important discovery, we decided to try to isolate the "primer" that triggers the synthesis of the Okazaki fragments. Because the substance breaks down quickly, and the small fragments of DNA link up neatly leaving no clues behind, analysis was extremely difficult.
Then in 1975, in the midst of this research, tragedy struck. My husband Reiji became ill and died. Our son was 12 years old, and our daughter was only two and a half. I had lost my research partner of 20 years, and I doubted whether I had the courage to continue on my own.
In the midst of my grief, I received a letter from Dr. Kornberg. He urged me not to give up what I did best and reminded me that through my research I could support myself and my children while staying connected with the rest of the scientific community. I resolved to continue, and three years later, to my great joy, I finally succeeded in separating and analyzing the substance that triggers the formation of the Okazaki fragments.
As a woman scientist, I faced serious obstacles, but with the help and support of many people, including the graduate students who worked with me, I was able to continue the research we began with my husband. I also owe a great deal to those who looked after my children over the years. Were it not for all these people, I would never have been able to continue my research these many years. And had I not continued my research, I would never have met all the wonderful people who have helped me along the way.



The Committee for the Encouragement of Future Scientists. Blazing a Path: Japanese Women’s Contributions to Modern Science. Tokyo: 2001.