10.1: Binary Fission and Generation Time

Learning Objectives

• Describe the process of binary fission and identify the key steps
• Define generation time and explain the significance in microbial growth
• Calculate the number of bacterial cells after a given number of generations using generation time

The bacterial cell cycle involves the formation of new cells through the replication of DNA and partitioning of cellular components into two daughter cells. In prokaryotes, reproduction is always asexual, although extensive genetic recombination in the form of horizontal gene transfer takes place, as will be explored in a different chapter. Most bacteria have a single circular chromosome; however, some exceptions exist. For example, Borrelia burgdorferi, the causative agent of Lyme disease, has a linear chromosome.

Binary Fission

The most common mechanism of cell replication in bacteria is a process called binary fission, which is depicted in Figure \(\PageIndex{1}\). Before dividing, the cell grows and increases its number of cellular components. Next, the replication of DNA starts at a location on the circular chromosome called the origin of replication, where the chromosome is attached to the inner cell membrane. Replication continues in opposite directions along the chromosome until the terminus is reached.

Figure \(\PageIndex{1}\): (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).

The center of the enlarged cell constricts until two daughter cells are formed, each offspring receiving a complete copy of the parental genome and a division of the cytoplasm (cytokinesis). This process of cytokinesis and cell division is directed by a protein called FtsZ. FtsZ assembles into a Z ring on the cytoplasmic membrane (Figure \(\PageIndex{2}\)). The Z ring is anchored by FtsZ-binding proteins and defines the division plane between the two daughter cells. Additional proteins required for cell division are added to the Z ring to form a structure called the divisome. The divisome activates to produce a peptidoglycan cell wall and build a septum that divides the two daughter cells. The daughter cells are separated by the division septum, where all of the cells’ outer layers (the cell wall and outer membranes, if present) must be remodeled to complete division. For example, we know that specific enzymes break bonds between the monomers in peptidoglycans and allow addition of new subunits along the division septum.

Figure \(\PageIndex{2}\): 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.

Query \(\PageIndex{1}\)

 

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 Figure \(\PageIndex{3}\) shows the increase in cell numbers for the first three generations.

The number of cells increases exponentially and can be expressed as 2ⁿ, 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 2ⁿ, where n is equal to 48, the single cell would give rise to 2⁴⁸ 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 × 10¹⁴ 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:

\[N_n = N_02^n\]

N_n is the number of cells at any generation n, N₀ is the initial number of cells, and n is the number of generations.

Figure \(\PageIndex{3}\): 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.

Query \(\PageIndex{1}\)

Query \(\PageIndex{1}\)

 

Interactive Element

Case Study Preview: “The Raw Deal”

Jeni, a healthy 24-year-old in her second trimester, thought her flu-like symptoms were no big deal - until her doctor found a red flag: she drinks raw milk. With a possible Listeria monocytogenes infection on the table, Jeni's pregnancy puts both her and her fetus at serious risk.

In this case, you’ll trace how Listeria is identified through blood agar, catalase testing, and molecular diagnostics, and why pregnant people are especially vulnerable. You'll also learn how Listeria’s ability to thrive in cold, salty environments makes it a stealthy threat in food safety - and a reason to rethink that unpasteurized cheese.

This time, it’s not what you eat - it’s what’s hiding in it.

Chapter 9 Case Study - The Raw Deal

 

Key Concepts and Summary

• Most bacterial cells divide by binary fission. Generation time in bacterial growth is defined as the doubling timeof the population.