Decoding Human DNA

In a milestone for the understanding of human genetics, scientists just announced the results of five years of work in unraveling the secrets of how the genome operates.

The ENCODE project, as it is known, dispensed with the idea that our DNA is largely "junk," repeating sequences with no function, finding instead that at least 80 percent of the genome is important.

The new findings are the latest in a series of increasingly deep looks at the human genome. Here are some of the major milestones scientists have passed along the way.

1. An understanding of heredity, 1866

The realization that traits and certain diseases can be passed from parent to offspring stretches back at least to the ancient Greeks, well before any genome was actually decoded. The Greek physician Hippocrates theorized that "seeds" from different parts of the body were transmitted to newly conceived embryos, a theory known as pangenesis. Charles Darwin would later espouse similar ideas.

What exactly these "seeds" might be was destined to remain a mystery for centuries. But the first person to put heredity to the test was Gregor Mendel, who systematically tracked dominant and recessive traits in his famous pea plants. Mendel published his work on the statistics of genetic dominance in 1866 to little notice. [Genetics by the Numbers: 10 Tantalizing Tales]

2. Chromosomes come to light, 1902

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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Decoding Human DNA