34 Mitosis: Eukaryotic Cell Division
Eukaryotes use two major types of cell division: mitosis and meiosis. Mitosis is used to produce new identical somatic (body) cells for growth and healing, while meiosis is used to produce sex cells (eggs and sperm). Meiosis will be discussed in a later chapter.
The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells via mitosis. The length of the cell cycle is highly variable even within the cells of an individual organism. In humans, the frequency of cell turnover ranges from a few hours in early embryonic development to an average of two to five days for epithelial cells, or to an entire human lifetime spent without dividing in specialized cells such as cortical neurons or cardiac muscle cells. There is also variation in the time that a cell spends in each phase of the cell cycle. When fast-dividing mammalian cells are grown in culture (outside the body under optimal growing conditions), the length of the cycle is approximately 24 hours. The timing of events in the cell cycle is controlled by mechanisms that are both internal and external to the cell.
Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produce two genetically identical cells. The cell cycle has two major phases: interphase and the mitotic phase (Figure 1). During interphase, the cell grows and DNA is replicated. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated and the cell divides.
Interphase
During interphase, the cell undergoes normal cellular processes while also preparing for cell division. For a cell to move from interphase to the mitotic phase, many internal and external conditions must be met. During interphase, the cell is very active biochemically. It is getting ready to divide by accumulating the required molecules and sufficient energy reserves. One very important process that happens during interphase is DNA replication. By the end of interphase, there are two identical copies of the DNA. Each chromosome is replicated and the two identical copies remain attached to each other at the centromere (Figure 2).
The centrosome is also duplicated during interphase. Centrioles help organize cell division. Centrioles are not present in the centrosomes of other eukaryotic species, such as plants and most fungi. Spindle fibers connect the centrosomes to the centromere of each chromosome. The spindle fibers will direct movement of the chromosomes during the rest of the process.
The Mitotic Phase
To make two daughter cells, the contents of the nucleus and the cytoplasm must be divided. The mitotic phase is a multistep process during which the copied chromosomes are lined up in the center of the cell, then pulled apart to opposite ends of the cell. The cell is then divided into two new identical daughter cells. The first portion of the mito