Chromosomes are another testament to our bodies’ amazing engineering. Because human DNA is too long to fit into a cell’s nucleus—the entire strand of DNA consists of 20,000 to 25,000 genes and stretches up to 6 feet in length—the genes are efficiently packaged into a microscopic structure, the chromosome. Like thread around a spool, DNA coils tightly around itself in a spiral ladder shape called a double helix, and sequences of DNA line up to form a chromosome. Each cell contains 46 of these chromosomes, one set of 23 from the mother and one set of 23 from the father.

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Each chromosome has a cinched waist called a centromere that divides it into two sections, or arms. The shorter arm is known as the p arm, while the longer one is the q arm. At the ends of each arm are protective stretches of DNA called telomeres. Like the tips on shoelaces, telomeres prevent chromosomes from unraveling.

Because cells reproduce to replace themselves, chromosomes must divide to ensure that each new cell receives a full set of genes. Any changes to their number or structure can lead to serious health problems. Some cancers, such as a type of leukemia, are caused by defective genes, while extra chromosomes can lead to genetic disorders, such as Down syndrome.

To study a person’s chromosome set, scientists often treat cells with chemicals to define and stain the chromosomes. Then they take a picture of the stained chromosomes, called a karyotype. This process of staining gave the structure its name: Chromosome is derived from the Greek words for “color” (chroma) and “body” (soma).

ADDITIONAL FACTS

  1. Scientists first discovered chromosomes in the late 1800s but did not understand their function. In the early 1900s, the American geneticist Thomas Hunt Morgan (1866–1945) ascertained the link between chromosomes and inherited traits through studies of fruit flies.
  2. When not dividing, chromosomes are not visible under the microscope; they remain curled up in the nucleus of the cell.
  3. In some cells, telomeres lose a tiny part of their DNA each time the cell replicates; when the telomere becomes completely depleted, cell division cycles may stop, which is part of the aging process.