Medical Relevance of Mitosis and Meiosis

The biological significance of mitosis and meiosis lies in ensuring the constancy of chromosome number—and thus the integrity of the genome—from one cell to its progeny and from one generation to the next. The medical relevance of these processes lies in errors of one or the other mechanism of cell division, leading to the formation of an individual or of a cell lineage with an abnormal number of chromosomes and thus an abnormal dosage of genomic material.

As we see in detail in Chapter 5, meiotic nondisjunction, particularly in oogenesis, is the most common mutational mechanism in our species, responsible for chromosomally abnormal fetuses in at least several percent of all recognized pregnancies. Among pregnancies that survive to term, chromosome abnormalities are a leading cause of developmental defects, failure to thrive in the newborn period, and intellectual disability.

Mitotic nondisjunction in somatic cells also contributes to genetic disease. Nondisjunction soon after fertilization, either in the developing embryo or in extraembryonic tissues like the placenta, leads to chromosomal mosaicism that can underlie some medical conditions, such as a proportion of patients with Down syndrome. Further, abnormal chromosome segregation in rapidly dividing tissues, such as in cells of the colon, is frequently a step in the development of chromosomally abnormal tumors, and thus evaluation of chromosome and genome balance is an important diagnostic and prognostic test in many cancers.