Chapter 4 - Growth, Development, and Reproduction

Abstract

All organisms package their genetic information in tiny cellular structures called chromosomes . In humans, and other eukaryotes (organisms composed of one or more cells containing a visible nucleus and organelles) which reproduce sexually, chromosomes are paired, one inherited from each parent. Human cells contain 23 pairs of chromosomes for a total of 46. These diploid cells—cells that contain two copies of every chromosome—transmit their chromosomes during two kinds of cell division: mitosis and meiosis . In mitotic cell division, which occurs in all cells of the body, each “ daughter cell ” receives the identical chromosome complement of its parent cell. In meiosis, which occurs only in germ cells (reproductive cells), the diploid number (46) is halved, producing haploid gametes (cells that will fuse during fertilization), each with 23 chromosomes. Inherited variation between any two people is largely a function of two events that occur during meiosis : random chromosome separation and crossing over between members of each chromosome pair. Reproduction proceeds through the union of gametes , an oocyte (egg) from the female, a sperm from the male. The fertilized egg, or zygote , is diploid , as are all other body cells. Prenatal and postnatal development occurs through mitosis, which is how the single-cell zygote develops into a 40-billion cell newborn and a 100-trillion cell adult.