(8) Reproduction in prokaryotes: binary fission to replicate cells of microorganisms like bacteria

(and another look at reproduction maths!)

Bacterial reproduction - bacteria usually reproduce by a simple form of asexual reproduction called binary fission (splitting in two).

This differs from the normal process of cell division in higher plants and animals which starts with mitosis.

Prokaryotes like bacteria can replicate themselves by this simple cell division process of binary fission.

Binary fission involves prokaryotes with a single chromosome (its not the same as mitosis in eukaryotic cells.

Step 1. In the parent cell, the large jumbled rings of DNA and the smaller plasmid rings are replicated to provide enough genetic material for two cells.

Step 2. The parent cell becomes enlarged with a greater volume of cytoplasm and the two bundles of DNA separate and move to opposite ends ('poles') of the enlarged cell.

Step 3. The cytoplasm begins to divide and new separate cell walls begin to form.

Step 4. The cytoplasm divides in two, so each of the two 'daughter' cells has its own cell wall AND its own single copy of the jumbled ring of DNA. The copies of the plasmids can be variable.

The arithmetic of cell division by binary fission

The mean cell division time is the average time it takes for one bacteria cell to divide in two (by binary fission).

From the mean division time you can work out how many times a cell will divide in a given time and therefore how many cells will be produced in that time.

The maths of cell division is illustrated above.

Starting with one cell, the number of cells produced = 2ⁿ, where n = the number of cell divisions.

This produces the arithmetical series 1, 2, 4, 8, 16, 32 etc. for n = 0 to 5 etc.

For example:

Suppose a bacterial cell has a mean division time of 15 minutes.

How many daughter cells will be produced in 1.5 hours by binary fission?

1.5 hours = 1.5 x 60 = 90 minutes

since each cell on dividing, makes two cells, the number of cells increases by a factor of 2 for each cell division

cell divisions per cell in 1.5 hours = 90 / 15 = 6

number of cells produced = 2^{cell divisions} = 2⁶ = 64 bacterial cells

After 20 divisions over a million bacterial cells can be produced from just one original cell.

The maths is simple on your scientific calculator: 2²⁰ = 1 048 576 or 1.05 x 10⁶ (to 3sf but still scary!)

Using the above example, this will take 20 x 15 mins = 300 mins = 5 hours, if this is a bacterial pathogen in your body, then this becomes very scary indeed !!!

Cell division in microorganisms - bacterial growth curves

You can estimate the quantity of bacteria in a colony over time and when you plot the results over a long period of time e.g. many hours, you can derive and bacterial growth curve graph like the one shown above.

The x axis is time, the y axis is the logarithm of the number of bacteria - logarithms are used because the range of numbers is to great to fit on an appropriate scale.

1. The lag phase:

During the initial lag phase there is no cell division i.e. no reproduction of the bacteria.

In this lag phase the bacteria are copying their DNA and synthesising the necessary proteins in order to facilitate the binary fission - this mode of cell division.

2. The exponential growth phase:

In this exponential growth phase, lots of food available, so cell division by binary fission rapidly takes place.

The number of bacteria can double in a relatively short time e.g. doubling in number every 10-20 mins, hence the 'acceleration' in the graph line.

3. The stationary phase

However, growth of the bacterial colony cannot be continuously accelerating because the nutrient resources are becoming depleted.

In time, the rate of bacterial growth is matched by the rate of bacterial death, so the graph line becomes horizontal.

However, if you were to introduce more nutrients (food), the colony can grow in number again, but, otherwise .... read on ... !

1. to 3. are described as a standard growth curve or sigmoid curve graph.

4. The death phase:

Finally, in the growth of the colony, not only are the food resources being diminished, but bacteria produce toxins as a waste product.

So, they become poisoned by the build-up of these toxins and the live bacteria in the colony begin to steadily diminish in number.

For more on prokaryotic cell structure see Introduction to plant and animal cell structure and function

See also Culturing microorganisms like bacteria - testing antibiotics and antiseptics

and development of antibiotic resistant bacteria

Summary of learning objectives and key words or phrases

Be able to describe cell division in the reproduction in prokaryotes by binary fission to replicate a microorganism cells like bacteria.

Be able to sketch the sigmoid growth curve and its various phases - the sequence lag phase, exponential growth phase, stationary phase and death phase.

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