Fundamentals of Biochemical Engineering, Part 1 - Bailey, J., Ollis, D. 1989

Molecular Genetics and Regulatory Systems
Growth and Self-Reproduction of an Isolated Cell

The growth of an Organism can be defined as an orderly increase in the mass of all substances that make up the organism. A growing unicellular organism eventually undergoes division. Consequently, the growth of a microorganism population is accompanied by both an increase in Cell number and an increase in the total mass of cellular matter; either of these parameters can be used for the quantitative mathematical analysis of cell growth. In practice, several Units of Measurement for cell mass are employed. For instance, the dry mass of macromolecular substances accounts only for the high-molecular-weight Components of the cell. Total dry mass refers to both high- and low-molecular-weight cellular substances (excluding Water). Finally, cell volume or total cell mass is defined as the mass (or volume) of all cellular substances, including water.

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FIG. 6.29. Events occurring from the birth of a new cell to its division can be represented as a cyclic (a) or linear (b) map of the Cell Cycle. The letters D and A denote Cell Division and the initiation of DNA Synthesis, respectively.

In the next chapter, we will attempt to understand the regularities of growth for a population of Cells in a nutrient medium. In this section, however, we face a simpler task. Here we examine the fundamental events of The Cell cycle, which is defined as the time period from The formation of a given cell As a result of mother cell division to the subsequent division of the daughter cell. The Study of the cell cycle must precede the topic of Chapter 7 for two reasons: first, it will allow us to appreciate the difficulty of analyzing events during the simultaneous growth and division of a multitude of cells, and second, an understanding of the basics of the cell cycle will assist in developing the necessary models required in Chapter 7.

The life cycle of an isolated cell can be schematically represented as a cyclic or linear map (Fig. 6.29). In either case, the point of origin is the moment of cell division. This point is usually placed at the right-hand end of the linear cell cycle diagram, whereas the left-hand end typically corresponds to the youngest daughter cell. To describe the relative chronology of other important cell cycle events, such as the initiation and cessation of DNA synthesis, a special parameter a, called the cell age or cell cycle age, is useful. In the cell cycle, the value of this parameter typically ranges from zero (the moment of cell birth) to t_D, the division time of the daughter organism (for dividing cells); the period from zero to t_D is referred to as the doubling period. In this introductory Structure/133.html">Discussion, we assume that cell division results in the formation of two identical daughter cells. This assumption holds true for simple binary fission, but in budding, as can be demonstrated using baker's Yeast S. cerevisiae, other situations may arise.