Your Friend, the Chromosome

Sausage-shaped: chromosomes, left and opposite, the focus of Willard's esoteric research, are made up of tightly coiled strands of DNA Adrian T. Sumner

Without chromosomes, your cells would be an untidy, malfunctioning mess. In fact, without chromosomes, you would never have evolved to the pinnacle of supreme intellect that enables you to read and appreciate this fine article.

Each cell in your body contains roughly six feet of string-like DNA, the repository of your genetic information. Strung like beads along the length of your DNA's molecular chains are some 20,000 encoded segments--genes--that form the blueprints for proteins. Through a process called "gene expression," a gene copies its genetic information in the form of a "messenger RNA" molecule. The messenger RNA then wends its way to the cell's protein-making machinery. There it acts as the blueprint for the proteins that catalyze life-giving chemical reactions.

Rather than crumpling this critical genomic blueprint haphazardly into the cell's nucleus, nature has evolved a neat method of winding that DNA into precise spools around special proteins and packing them ever so neatly into the sausage-shaped chromosomes that you see under the microscope. This packing job, which would thrill Martha Stewart, reduces the length of your six feet of DNA by some 10,000-fold.

You have twenty-three pairs of such well-ordered chromosomes--one set each from Mom and Dad. And besides the garden-variety chromosomes, you have two sex chromosomes--a pair of X chromosomes if you are a female, and an X and a Y if you are male.

Each pair of chromosomes connects at a midpoint called the centromere--another example of nature's orderly housekeeping. As each chromosome duplicates itself during cell division, the centromere provides a critical attachment point. Each cell sends out spindle fibers that grab the duplicated chromosomes at the centromere and separate them into the two new "daughter" cells.

In the earliest days of genetics, scientists faithfully, and, it turned out, naively, believed the "central dogma" that each gene provides the blueprint for one protein. The only "genetic code" they thought existed was the one that specified how a given sequence of DNA units provided the code for the structure of a given protein.

However, in the last decade, scientists have come to the unsettling realization that the cell is a far more intricate cryptographer than they had ever dreamed. Besides the protein-making code, the cell also uses other as yet unknown codes to read mysterious "epigenetic"--that is, outside the genetic code--signals from the vast regions of what had seemed like "junk" DNA that scientists had discovered among genes. In fact, it's quite likely that within this "junk" lies the very epigenetic information that enabled us to evolve into humans.