The study by Puck and Marcus investigates the effects of X-ray irradiation on single HeLa cells, a human cervical carcinoma cell line. They developed a method to grow single cells into macroscopic colonies, allowing for precise measurement of reproductive capacity similar to techniques used in microorganisms. The research focuses on the survival and growth of HeLa cells after exposure to various doses of X-rays. Key findings include: 1. **Growth Rate Reduction**: Irradiation not only kills cells but also slows down the growth of survivors, as evidenced by smaller and fewer colonies. 2. **Multiple Hit Mechanism**: The survival curve shows a shoulder at around 75 r, indicating a multiple hit mechanism where each additional dose reduces the survivors by a factor of 37%. 3. **Cell Fate**: Cells killed by radiation can still divide a limited number of times, forming microcolonies or giant cells. Giant cells are highly metabolically active but do not reproduce. 4. **Genetic Defect**: Evidence suggests that the lethal effect is due to a radiation-induced genetic defect, possibly chromosomal, rather than a simple single gene mutation. 5. **Radiosensitivity**: The HeLa cell is extremely sensitive to radiation, with a D0 value of 96 r, much higher than that of microorganisms. The study highlights the significant radiosensitivity of mammalian cells and the potential impact of this sensitivity on whole-body irradiation in mammals. The findings also suggest that the formation of giant cells may contribute to the delayed onset of symptoms in radiation syndrome.The study by Puck and Marcus investigates the effects of X-ray irradiation on single HeLa cells, a human cervical carcinoma cell line. They developed a method to grow single cells into macroscopic colonies, allowing for precise measurement of reproductive capacity similar to techniques used in microorganisms. The research focuses on the survival and growth of HeLa cells after exposure to various doses of X-rays. Key findings include: 1. **Growth Rate Reduction**: Irradiation not only kills cells but also slows down the growth of survivors, as evidenced by smaller and fewer colonies. 2. **Multiple Hit Mechanism**: The survival curve shows a shoulder at around 75 r, indicating a multiple hit mechanism where each additional dose reduces the survivors by a factor of 37%. 3. **Cell Fate**: Cells killed by radiation can still divide a limited number of times, forming microcolonies or giant cells. Giant cells are highly metabolically active but do not reproduce. 4. **Genetic Defect**: Evidence suggests that the lethal effect is due to a radiation-induced genetic defect, possibly chromosomal, rather than a simple single gene mutation. 5. **Radiosensitivity**: The HeLa cell is extremely sensitive to radiation, with a D0 value of 96 r, much higher than that of microorganisms. The study highlights the significant radiosensitivity of mammalian cells and the potential impact of this sensitivity on whole-body irradiation in mammals. The findings also suggest that the formation of giant cells may contribute to the delayed onset of symptoms in radiation syndrome.