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How are chromosomes put together?
Every chromosome contains a single molecule of DNA, the skinny, snakelike carrier of hereditary information. If stretched out to its full length, the DNA molecule in a human chromosome would be between 1.7 and 8.5 centimeters (about 0.7 to 3.3 inches) long, depending on the chromosome. But it would be vanishingly thin, less than a millionth of a centimeter across. If such a long, thin molecule just floated around free in the cell, it would be a disaster for the precise hereditary instructions contained in DNA. The molecule would become hopelessly tangled up with itself, a genetic Gordian knot. And because the slightest jostle will rend a molecule that is so long and thin, it would probably break into many pieces. These pieces could then hook back together in the wrong order, scrambling genetic instructions and causing chaos in the cell. But chromosomal proteins prevent that chaos. Like a combination of paper clip, duct tape, and sheepdog, proteins keep the DNA folded into an orderly, compact shape. A chromosome may look simple at first glance, but it is actually quite a complex structure, with the DNA molecule wound around protein spools and fastened into loops, coils, and fibers by other proteins. In a chromosome, protein is the packaging and DNA is the contents of the package. In its most tightly packaged or "condensed" form, a chromosome, which contains several centimeters of DNA, is only a few ten-thousandths of a centimeter long. Generally, however, chromosomes are fully condensed only in preparation for cell division. The rest of the time, some of the loops and coils are unfastened so that the DNA can do its work, communicating hereditary instructions to the rest of the cell. How do new cells get their chromosomes?New cells get their chromosomes from old cells through cell division, or mitosis. In fact, mitosis is the chromosomes' real opportunity to shine. Viewed under a microscope, the chromosomes in a dividing cell seem to perform a series of stylized, dancelike movements, which have fascinated researchers for more than a century.
In preparation for mitosis, a cell makes a copy of each of its chromosomes, building replicas of each DNA molecule and the packaging proteins. Then the chromosomes "condense" and line up in pairs—one old copy and one new copy of the same chromosome—along the diameter of the cell. Once they have all taken their places, the chromosome pairs quite rapidly move apart, as if some powerful chromosome magnet had been placed on each side of the cell. Limbs trailing, the chromosomes move towards opposite ends of the cell, forming two clumps that are separated when the cell divides. If all goes according to plan—and it usually does—each new cell ends up with a full set of chromosomes. The first person to describe this now-famous "dance of the chromosomes" was Walther Flemming, who studied the cells of salamander larvae during the 1880s. Although Flemming described the movements of the chromosomes accurately, he had no idea what this dance meant. He worked at a time when cytology—the study of cells—was a rapidly growing but still infant field. Chromosomes were thought of as mysterious bits of dark material within the cell (in fact, the word "chromosome" is basically Greek for "colored blob"), and no one knew what they were for. For all Flemming or any of his contemporaries knew, the chromosomes were dancing for the sheer joy of being a chromosome. Today, we know they are dancing to distribute their DNA to new generations of cells. How do new organisms get their chromosomes?New organisms get their chromosomes from their parents. For organisms that reproduce asexually, such as bacteria, algae, and sometimes sea anemones, reproduction is as simple as dividing in two. Distributing chromosomes to the next generation merely involves cell division.
But for organisms, like humans, that reproduce sexually, the task of handing down chromosomes to the next generation is more complex. It involves a process called meiosis, or the formation of sex cells (sperm and egg). In meiosis, a cell copies its chromosomes and then divides twice, producing four sex cells. Each sex cell contains half the normal number of chromosomes—only one copy of chromosome 1, one copy of chromosome 2, and so on. In humans, egg and sperm cells each have 23 chromosomes. When sperm and egg unite to form a new organism, the normal number of chromosomes is restored, with each parent contributing half the chromosomes. This process explains why children have a mixture of their parents' characteristics. You have two copies of chromosome 1 in your cells, and one of them is the same as chromosome 1 found in your mother's cells, while the other is the same as chromosome 1 found in your father's cells. How the genes on your mother's and father's chromosomes interact with each other determines which characteristics are passed down to you—your mother's nose, your father's eyes, and so on. The same process also explains why siblings tend to be different mixtures of their parents' characteristics. For example, the chromosome 1 that you inherited from your mother might originally be the one that she, in turn, originally got from her mother, while your sister got the chromosome 1 that originally came from your mother's father. But perhaps you both inherited the same chromosome 1 from your father—the one he got from his father, say. And so on for all 23 pairs of your chromosomes—which is why you have your mother's nose and father's eyes, while your sister has your mother's eyes and your father's nose. . . . . . . . . . . . . . . . . . . . . .
Updated on January 15, 2003
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