Chromosomes, Mitosis, and Meiosis
Cell division is the process by which cells reproduce. A unicellular organism divides and forms two individuals. Multicellular organisms result from divisions of the zygote, a single cell. Even the zygote is produced by the union of two other cells, usually a sperm cell and an egg cell, and each of these cells arose from other cells. Insightful nineteenth-century biologists recognized a pattern: all cells come from preexisting cells. How cells divide is the subject of this chapter.
Information in the cell is contained in genes. Genes code for the proteins that carry out cellular functions. When a cell divides, genes are separated into daughter cells in an orderly, predictable process. This process involves the formation of chromosomes.
When cell division begins, DNA replicates. Then the replicated strands of DNA begin to coil, as shown in Figure 9-1. As coiling proceeds, DNA becomes tightly wrapped around globules of histones, a group of special protein molecules. DNA and histones in this coiled form can be seen as a rod-shaped structure called a chromosome. Note that only the physical arrangement of the DNA strand is changed in the transition from chromatin to chromosome. The chemical makeup of the DNA remains unchanged.
Each chromosome consists of two identical parts, each called a chromatid (KROE-muh-tid). The two chromatids are often called sister chromatids. The point at which each pair of chromatids is attached is called the centromere (SEN-truh-mir.
Chapter 9 is under construction. Please be patient.
Copyright 1993, Holt, Reinhart and Winston, Inc.
Go to cell diagram
Go to study guide questions
Return to Study Guides
