But the painstaking work of cross-breeding pea plants wouldn't languish for long. In 1869, Swiss physician Johannes Friedrich Miescher became the first scientist to isolate nucleic acids, the active ingredient of DNA. Over the next several decades, scientists peering deeper into the cell discovered mitosis and meiosis, the two types of cell division, and chromosomes, the long strands of DNA and protein in cell nuclei.
In 1903, early geneticist Walter Sutton put two and two together, discovering through his work on grasshopper chromosomes that these mysterious filaments occur in pairs and separate during meiosis, providing a vehicle for mom and dad to pass on their genetic material. "I may finally call attention to the probability that the associations of paternal and maternal chromosomes in pairs and their subsequent separation … may constitute the physical basis of the Mendelian law of heredity," Sutton wrote in the journal The Biological Bulletin in 1902. He followed up with a more comprehensive paper, "The Chromosomes in Heredity" in 1903. (German biologist Theodor Boveri came to similar conclusions about chromosomes at the same time Sutton was working on his chromosome discovery.)
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