"You reap what you sow"—why does planting melon never yield beans? This hereditary phenomenon is determined by genes—segments of DNA carrying genetic information, housed within chromosomes in the cell nucleus.

In 1868, Swiss scientist Friedrich Miescher examined pus from used bandages—rich in white blood cells—and chemically extracted a substance containing abundant phosphorus and nitrogen. Realizing it differed from known cellular components, he named it "nuclein" —the first isolation of DNA. In 1889, biochemist Richard Altmann purified nucleic acid from yeast and animal tissues, renaming it "nucleic acid."

In 1879, German biochemist Albrecht Kossel systematically studied nuclein structure, discovering it was a protein-nucleic acid complex. He identified two nucleic acids—then called "thymus nucleic acid" and "yeast nucleic acid"—now known as DNA and RNA. Kossel received the 1910 Nobel Prize in Physiology or Medicine.

In 1953, James Watson and Francis Crick discovered DNA's double helix structure—a twisting ladder of sugar-phosphate backbones connected by base pairs: A (adenine) pairs with T (thymine) , and G (guanine) pairs with C (cytosine) . The sequence of these four bases (A, T, C, G) constitutes the genetic code—the "book of life." Each three adjacent bases form a codon, which is transcribed through RNA to guide protein synthesis.

In 1977, Frederick Sanger developed the chain-termination method—the first DNA sequencing technique—and sequenced the first genome: bacteriophage X174 (5,375 bases). In 1990, the Human Genome Project—the "moon landing of life sciences"—was launched. By April 2003, it had completed sequencing of 2.9 billion base pairs of the human genome, mapping all human genetic information and gene locations on chromosomes.