Plant Biology: Eukaryote Cell Divison: The Cell Cycle

When compared with prokaryotic cell division, the process isn't as simple in eukaryotes, where linear chromosomes that are contained within a membrane-bound nucleus have to be apportioned equally between two daughter cells. If something goes wrong and they aren't distributed equally, chances are the daughter cells will die for lack of instructions on how properly to conduct the business of life. The eukaryote cell is also filled with organelles and other cytoplasmic materials that must be divided. Small wonder, then, that the process not only is more highly orchestrated, but that it takes much longer to accomplish.

The essentially continuous process of cellular division in body ( somatic) cells has three significant steps: 1.) the actual division of the nucleus, called mitosis; 2.) the division of the cytoplasmic material— cytokinesis—into two daughter cells after the nuclear division; and 3.) the interphase just before and after division. The division to produce sex cells ( gametes) is called meiosis and involves still other complications.

Stages

The entire sequence of repeating events from one mitotic division to the next is referred to as the cell cycle. The cycle has two principal parts: 1.) interphase, divided into G₁ (Gap 1), S (DNA synthesis), and G₂ (Gap 2) and, 2.) the M phase—the combination of mitosis and cytokinesis. Figure 1 diagrams the stages in the cycle, and Table 1 lists the events of significance in each. Figure 2 diagrams the steps in mitosis.

Figure 1

Figure 2

TABLE 1

Steps in the Plant Cell Cycle

The time to complete the cell cycle varies among species, the tissues in which the cells occur, and general environmental conditions. Of the nuclear division stages, prophase is the longest, and the separation and movement of the daughter chromatids in anaphase is the shortest. Relative lengths might be: prophase 1 to 2+ hours, metaphase 5 to 15 minutes, anaphase 2 to 10 minutes, and telophase 10 to 30 minutes. A fourth interphase stage— G₀—often is present in plants. It can occur at almost any point and is induced by unfavorable growing conditions, such as the onset of cold winter weather or by drought during the summer. It is a holding stage while the plant is dormant.