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A) A micrograph of two rod shaped cells attached at their ends. B) A diagram of binary fission. First a cell replicates its DNA and elongates. Then, as the cell continues to elongate, each loop of DNA travels to one end or the other. The cell then starts to constrict in the center. This results in two cells each containing a loop of DNA.
(a) The electron micrograph depicts two cells of Salmonella typhimurium after a binary fission event. (b) Binary fission in bacteria starts with the replication of DNA as the cell elongates. A division septum forms in the center of the cell. Two daughter cells of similar size form and separate, each receiving a copy of the original chromosome. (credit a: modification of work by Centers for Disease Control and Prevention)
A diagram of a cell dividing. The cell is shaped like a figure 8; each end of the cell contains a loop a DNA. The constriction point of the figure 8 is labeled cleavage furrow. A ring of dots in this region is labeled FtsZ ring. Next these dots line up along the constriction point as the constriction point completely separates the two halves of the cell. This region is now called the septum. Finally the two cells separate completely.
FtsZ proteins assemble to form a Z ring that is anchored to the plasma membrane. The Z ring pinches the cell envelope to separate the cytoplasm of the new cells.
  • What is the name of the protein that assembles into a Z ring to initiate cytokinesis and cell division?

Generation time

In eukaryotic organisms, the generation time is the time between the same points of the life cycle in two successive generations. For example, the typical generation time for the human population is 25 years. This definition is not practical for bacteria, which may reproduce rapidly or remain dormant for thousands of years. In prokaryotes (Bacteria and Archaea), the generation time is also called the doubling time and is defined as the time it takes for the population to double through one round of binary fission. Bacterial doubling times vary enormously. Whereas Escherichia coli can double in as little as 20 minutes under optimal growth conditions in the laboratory, bacteria of the same species may need several days to double in especially harsh environments. Most pathogens grow rapidly, like E. coli , but there are exceptions. For example, Mycobacterium tuberculosis , the causative agent of tuberculosis, has a generation time of between 15 and 20 hours. On the other hand, M. leprae , which causes Hansen’s disease (leprosy), grows much more slowly, with a doubling time of 14 days.

Calculating number of cells

It is possible to predict the number of cells in a population when they divide by binary fission at a constant rate. As an example, consider what happens if a single cell divides every 30 minutes for 24 hours. The diagram in [link] shows the increase in cell numbers for the first three generations.

The number of cells increases exponentially and can be expressed as 2 n , where n is the number of generations. If cells divide every 30 minutes, after 24 hours, 48 divisions would have taken place. If we apply the formula 2 n , where n is equal to 48, the single cell would give rise to 2 48 or 281,474,976,710,656 cells at 48 generations (24 hours). When dealing with such huge numbers, it is more practical to use scientific notation. Therefore, we express the number of cells as 2.8 × 10 14 cells.

In our example, we used one cell as the initial number of cells. For any number of starting cells, the formula is adapted as follows:

N n = N 0 2 n

N n is the number of cells at any generation n , N 0 is the initial number of cells, and n is the number of generations.

In generation 0 there is 1 cell. In generation 1 there are 2 cells. In generation 2 there are 4 cells. In generation 3 there are 8 cells.
The parental cell divides and gives rise to two daughter cells. Each of the daughter cells, in turn, divides, giving a total of four cells in the second generation and eight cells in the third generation. Each division doubles the number of cells.

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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