Chromosome

The human genome (which represents the total amount of DNA in a typical human cell) has approximately 3 × 10⁹ base pairs. If these nucleotide pairs were letters, the genome book would number over a million pages. There are 23 pairs of chromosomes, for a total number of 46 chromosomes in a dipoid cell, or a cell having all the genetic material. In a haploid cell, there is only half the genetic material. For example, sex cells (the sperm or the egg) are haploid, while many other cells in the body are diploid. One of the chromosomes in the set of 23 are X or Y (sex chromosomes), while the rest are assigned numbers 1 through 22. In a diplod cell, males have both an X and a Y chromosome, while females have two X chromosomes. During fertilization, the sex cell of the father combines with the sex cell of the mother to form a new cell, the zygote, which eventually develops into an embryo. If the one of the sex cells has the full complement of chromosomes (diploidy), then the zygote would have an extra set of chromosomes. This is called triploidy and represents an anomaly that usually results in a miscarriage. Sex cells are formed in a special kind of cell division called meiosis. During meiosis, two rounds of cell division ensure that the sex cells receive the haploid number of chromosomes.

Other species have different numbers of chromosomes in their nuclei. Mosquitos, for instance, have 6 chromosomes. Lilies have 24 chromosomes, earthworms have 36 chromosomes, chimps have 48 chromosomes, and horses have 64 chromosomes. The largest number of chromosomes are found in the Adders tongue fern, which has more than 1,000 chromosomes. Most species have, on average, 10–50 chromosomes.