Cell Division Health Article

Common diseases and disorders

In humans, errors in chromosome division occur frequently during meiosis. Although these errors can take place either during the formation of the egg or the sperm, most errors occur during meiosis in the female for reasons that are not yet clearly understood. If mistakes occur during meiosis, eggs and sperm can be formed with either too many or too few chromosome. Fertilization then results in a fertilized egg than has less than or more than 46 chromosomes, a situation with major health consequences. For example, roughly 20% of all clinically recognized pregnancies result in miscarriage. Half of these are due to an extra or missing chromosome(s) in the developing embryo. Among live births, one in 150 infants has some type of chromosome abnormality. One of the more common is Down syndrome. Most cases of Down syndrome are due to an error in meiosis that results in an extra chromosome (extra chromosome 21) being present in the fertilized egg. This condition is called trisomy 21. The individual who develops from this egg will have the clinical features of Down syndrome including mental retardation. Trisomy 21, as well as other similar chromosome errors, occurs more often in the pregnancies of women as they get older. For example, older women have a higher risk for miscarriages associated with chromosome errors. They also have a higher risk of giving birth to an infant with trisomy 21 Down syndrome or a similar chromosome abnormality. For this reason, women in their mid-30s or older are usually referred to a geneticist or genetic counselor to learn about prenatal testing options such as amniocentesis and chorionic villus sampling (CVS).

KEY TERMS

Amniocentesis—A procedure performed around the fourth month of pregnancy in which a needle is inserted through a woman's abdomen into her uterus to draw out a small sample of the amniotic fluid from around the baby. Fetal cells in the fluid can be used to check the chromosome make-up of the baby.

Chorionic villus sampling (CVS)—A procedure used for prenatal diagnosis at eight to 10 weeks gestation. Under ultrasound guidance a needle is inserted either through the mother's vagina or abdominal wall and a sample of cells is collected from around the early embryo. These cells can be used to study the chromosomes of the fetus.

Chromosomes—Structures in the nucleus of a cell that contain a thread of DNA containing the genetic information (genes). Humans have 46 chromosomes in 23 pairs.

Cytoplasm—The portion of the cell that surrounds the nucleus.

DNA—Deoxyribonucleic acid, the molecule that encodes the genes.

Genetic counselor—An individual, usually with an advanced degree and board certification, who specializes in assessing genetic risk and informing patients about these risks and the options for dealing with them.

Geneticist—A individual with an advanced degree (MS, MD, PhD) in genetics. Human geneticists and medical geneticists specialize in genetic issues pertaining to humans. Many geneticists are certified by specialty boards.

Nucleus—The membrane-bound body within a cell that contains the chromosomes.

BOOKS

Carlson, Bruce M. Human Embryology and Developmental Biology. 2nd ed. St. Louis: Mosby, 1999.

Endow, Sharyn A., and David M. Glover, eds. Dynamics of Cell Division. New York: Oxford University Press, 1998.

Jorde, Lynn B., et al. Medical Genetics. 2nd ed. New York: Mosby, 1999.

Tortora, Gerard, and Sandra Reynolds Grabowski. Principles of Anatomy and Physiology. 9th ed. New York: HarperCollins, 2000.

OTHER

The Biology Project. <http://www.biology.arizona.edu>.

Mitosis Animation. <http://galileo.physiology.uiowwa.edu/animations/mitosis.htm>.

Sallie Boineau Freeman, PhD