7.2Cell Proliferation and DNA Replication


Cell Proliferation Is the Most Fundamental Function of the Cell

All organisms on earth share a commonality that they are composed of cells. In terms of common structures, all cells are covered by phospholipid bilayer membranes, contain proteins as functional macromolecules, and contain DNA as genes that control cell structures and functions. A common property of the cell as a unit of life is that it undergoes cell division and proliferation to yield progeny (daughter) cells. Although multicellular organisms produce offspring as a function at the level of individual organisms, this function is supported by the proliferation of component cells, and cell proliferation is the basic function of the process by which an embryo develops into an individual organism. Proliferation of constituent cells is important for the maintenance of function in organisms. Having survived the long evolutionary process, cell proliferation is one of the most fundamental and common functions of life.

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Special Characteristics of DNA Replication

Fig. 7-1 Outline of cell division

Cells proliferate by binary fission (Fig. 7-1). All intracellular constituents of the parent cell must be doubled prior to cell division. However, components comprising a cell consist of numerous molecules, and the concept of “doubling” here is applied loosely; the components are not always precisely segregated equally into the two cells (daughter cells).

On the other hand, DNA is completely different. While there is generally only one molecule of DNA per cell in prokaryotes (haploid), it is somewhat complex in eukaryotes. For example, human somatic cells are diploid with 46 molecules of DNA in the nucleus. The base sequences of 23 molecules originate from the mother (ovum) and those of the other 23 from the father (sperm). The 23 DNA molecules vary greatly in length and base sequence. Homologous chromosomes from the parents (e.g., the pair of chromosome 1, pair of chromosome 2…) are very similar to each other in structure, but they are not completely identical in base sequence. This indicates that all the 46 DNA molecules in human somatic cells differ in base sequence. Therefore, regardless of whether an organism is prokaryotic or eukaryotic, it is normal that every DNA molecule in each cell has a unique structure (base sequence).

Because DNA molecules carry genetic information, during cell proliferation, replication of the DNA molecule in the parent cell must produce two totally identical molecules before each molecule is accurately distributed to the two daughter cells formed. This phenomenon centered on an individual molecule is a conspicuous characteristic not observed in any other cellular molecule.

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