2.1Cell Multiplication Through Cell Division
Cell Division and Cell Cycle
Cell growth or proliferation basically occurs by division. For a cell to proliferate, all cell components must first be doubled. In particular, the genetic material must be doubled accurately. The genetic material carries genetic information (see Chapter 3), and all this genetic information in the cell (the genome) must be duplicated because partial replication will not allow normal cell function. In prokaryotes, genomic DNA is accurately replicated and segregated to both poles of the cell, and the cell (cytoplasm and cell membrane) then undergoes division. In eukaryotes, because the genetic material is present in units called chromosomes, each chromosome is replicated to precisely double the number of chromosomes, and cytokinesis (cytoplasmic division) occurs after each set is segregated to the two poles of the cell. Thus, cell proliferation occurs through the following processes; replication of the genetic material, segregation, and cytokinesis. This cycle is called the cell cycle. The cell cycle is irreversible and therefore should accurately proceed step by step according to the cell state. If the genetic material or intracellular structures are not accurately replicated, segregation and division will only proceed after the replication processes are completed. Thus, accurate control of the cell cycle is the most essential aspect of cell proliferation. see Chapter 13 describes cell proliferation and cell cycle regulation in greater detail.
Fig. 2-1 Cell Division in Escherichia coli (Prokaryotic cell)
(A) In cells with mutations affecting cell division (cell division is no longer possible), invagination does not occur and the cell becomes elongated (electron microscope image). The white parts are the nucleoid region containing DNA. DNA is replicated and segregated, but cell division does not occur. (B) Proteins related to cell division are detected using black particles by a special method. Black dots are visible in the region of cell membrane invagination (electron microscope image). (C) The rings that are formed first in division are observed using a fluorescent microscope. When these rings appear, substances accumulate around them to cause invagination (Goehring, N. W. & Beckwith, J. Curr. Biol., 15: R514-R526, 2005, reprinted from Fig. 1).
Let us take a closer look at the aspects of cell division. In prokaryotic organisms (Fig. 2-1), the cell receives nourishment to grow, and it replicates its DNA that is the genetic material when sufficient nutrition is available. Prokaryotic organisms include rod-shaped bacilli and spherical cocci. Bacilli are cells that are elongated by membrane synthesis at the center of the long axis. Escherichia coli, which are bacilliform cells, require 40 min for complete replication of the genetic material, but cell division can occur in a shorter period of time (e.g., 20 min) if the nutritional conditions are good. In such cases, DNA replication occurs in succession without waiting for cell division, resulting in a very complex situation. However, according to recent research, E. coli cells proliferating at a sufficiently slow rate show simultaneous polar genomic DNA segregation and replication. In such cells, the cell wall begins to constrict only after replication and segregation are complete. In general, even in prokaryotic cells, replication, segregation, and division occur in a regular and ordered sequence. Prior to division, ring-shaped structures are formed along a predetermined division plane. These structures are assembled as footholds by a substance involved in cell membrane invagination (Fig. 2-1B, C). Cell membrane invagination results in the formation of two new cells after their cell walls are synthesized.
Fig. 2-2 Cytokinesis (mitosis)
(A) Division in onion root tip cells. The chromosomes are separated and segregated to the two poles.
(B) Schematic diagram of plant and animal cytokinesis. Once the chromosomes are segregated, cytokinesis occurs. Animal cytokinesis proceeds from cell membrane invagination in the central plane, while in plant cytokinesis, vesicles containing the cell membrane and cell wall gather on the division plane (formation of the cell plate) and fuse to complete cell division.
Eukaryotes divide by the process called mitosis. Let us have a look at the typical examples of plant and animal cells, although the mitotic process may vary in different species (Fig. 2-2). Chromatin, which comprises DNA, is replicated during the S phase (S = synthesis), but this process causes no changes in the cell appearance. After entering the division period (M phase), condensation of chromatin occurs, and then chromatin clearly separates into individual units. Previously, to observe chromatin, cells had to be fixed with certain chemicals and stained using particular dyes. Recently, the use of special optics enables in vitro observation of chromatin through differences in refractive indices. In addition, the use of various fluorescent reagents has made observation of detailed behaviors of an individual chromosome possible in living cells. After the nuclear membrane breaks down into vesicles (disintegration of the nuclear membrane), the condensed chromosomes line up along the center of the division plane (the equatorial plane) and are segregated to both poles by several filamentous structures (the spindle apparatus). After segregation is complete, the nuclear membrane is reconstituted.
In animal cells, the cytoplasm is divided in two by cell membrane invagination during continuous cytokinesis. In plant cells, phragmoplasts form on the division plane. They act as a scaffold for cell plate assembly and result in a new cell wall separating two complete cells. However, cytokinesis varies depending on the type of organism. For example, the cell wall sometimes does not disintegrate, as observed in yeast. Furthermore, cell division in eukaryotes also includes meiosis, which accompanies sexual reproduction, and will be discussed later.