95 Cell Division
Learning Objectives
By the end of this section, you will be able to do the following:
- Describe the structure of prokaryotic and eukaryotic genomes
- Describe the mechanisms of chromosome compaction
The continuity of life is the ability of organisms to reproduce their own kind. Thus, cell division from one cell to another has its foundation in the cell cycle. The cell cycle is an orderly sequence of events that describes the stages of a cell’s life from the division of a single parent cell to the production of two new genetically identical daughter cells.
Genomic DNA
Before discussing the steps a cell must undertake to replicate and divide its DNA, a deeper understanding of the structure and function of a cell’s genetic information is necessary. DNA is the mainstay of the genetic material present in a cell or organism. A cell’s DNA comprises the complete set of genes packaged as a double-stranded DNA molecule, commonly called its genome. Prokaryotes (domains Archaea and Bacteria) are single-celled organisms that lack a nucleus. They have a single piece of circular DNA in the nucleoid area of the cell. In prokaryotes, which lack a nucleus, the genome is composed of a single, double-stranded DNA molecule in the form of a loop or circle found in a region or area called the nucleoid (Figure 10.2). This single, double-stranded DNA molecule must not be confused with separate smaller loops of DNA called plasmids that can be found in some of these cells but are not essential for normal growth. Bacteria can exchange these plasmids with other bacteria, sometimes receiving beneficial new genes that the recipient can add to their chromosomal DNA. Antibiotic resistance is one such trait that often spreads through a bacterial colony through plasmid exchange from resistant donors to recipient cells. Naturally occurring plasmids are usually very small, containing a few useful additional genes during certain conditions. However, their study has enabled bacterial transformation involving the design of artificial or synthetic plasmids, which are widely used in recombinant DNA technology as vectors in molecular cloning.
In eukaryotes, a nucleus is present and encloses the genome, which consists of several double-stranded linear DNA molecules (Figure 10.3). Each species of eukaryotes has a characteristic number of chromosomes in the nuclei of its cells. Human body (somatic) cells have 46 chromosomes, while human gametes (sperm or eggs) have 23 chromosomes each. The number of chromosomes is not the same among all species. For example, it is 48 in chimpanzees and 18 in cabbage plants. A typical body cell contains two matched or homologous sets of chromosomes (one set from each biological parent)—a configuration known as diploid. (Note: The letter n is used to represent a single set of chromosomes; therefore, a diploid organism is designated 2n) Human cells that contain one set of chromosomes are called gametes, or sex cells; these are egg and sperm, a