Meiosis is a specialized process of cell division resulting in the production of four separate cells, each of which has the haploid number of chromosomes. The total process involves two sequential divisions during which one diploid cell reduces to four haploid cells. Since the chromosomes act as carriers for genetic information, genes separate into different sets during meiosis.

Specific chromosome combinations determine the sex of many kinds of organisms. In humans, females have two X chromosomes, while males have an X and a Y chromosome. At puberty, hormones influence the development of secondary sex characteristics and the functioning of gonads.

Sexual reproduction involves the production of gametes by meiosis in the ovaries and testes. In males, each cell that undergoes spermatogenesis results in four sperm; in females, each cell that undergoes oogenesis results in one oocyte.

To maintain good health, people must receive nutrient molecules that can enter the cells and function in the metabolic processes. The proper quantity and quality of nutrients are essential to good health. An important measure of the amount of energy required to sustain a human at rest is the basal metabolic rate. To meet this and all additional requirements, the United States has established the RDAs, RecommendedDietary Allowances, for each nutrient. Should there be metabolic or psychological problems associated with a person's normal metabolism, a variety of disorders may occur, including obesity, anorexia nervosa,bulimia, kwashiorkor, and vitamin-deficiency diseases.