Cover Story

Untangling a ball

Issue: "Cracking the code," April 29, 2000

Human genetic material is organized into 23 pairs of chromosomes, each composed of protein molecules and one long strand of DNA. The chromosomes are in distinct pairs only when the cell is dividing; the rest of the time they form a tangled ball called the genome.

A gene is a section of a DNA strand that "encodes for" or makes the proteins that determine how the cell develops and functions. "Introns," sections of DNA that either apparently do not have a function or whose function is unknown, make up 97 percent of the genome. DNA molecules are made up of nitrogen-containing nucleotides called "bases."

DNA has four types of bases-adenine (A), thymine (T), cytosine (C), and guanine (G)-arranged in pairs in the twisted-ladder shape known as a "double-helix." G is always paired with C and T with A. The Human Genome Project is discovering the order of all three billion of these base pairs in the human genome. A single DNA molecule (one chromosome) is 25 to 125 million base pairs long.

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The sequencing procedure is complicated, but here's a rough sketch: Technicians insert a small section of DNA (it can come from one of about 10-20 anonymous individuals-Baylor works with the DNA of a male) into bacteria which then multiply, producing a supply of the exact same DNA.

Using variations in temperature and chemical processes, this DNA is extracted from the bacteria and then broken down into smaller and smaller pieces. A molecule of florescent dye is attached to each base, with a different color for each type. The fragments are then sent down an array of tiny glass tubes, where a laser beam illuminates the colors on each of thousands of fragments.

Computers then analyze the colors and fragment lengths, looking for overlapping sequences, to partially determine the sequence of the original tiny section of DNA, about 500-700 base pairs long. Human "assemblers" finish the job. With five assemblers, the Baylor operation sequences about two million base-pairs per month. The data from all the Project's sequencing centers go onto an Internet-accessible database that is updated daily.

Les Sillars
Les Sillars

Les directs the journalism program at Patrick Henry College in Purcellville, Va., and is the editor of WORLD's Mailbag section.


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