1.8 - Binary Fission

How bacteria divide by binary fission

• Bacteria are prokaryotic cells, and they reproduce by a simple type of cell division called binary fission.
• Instead of a nucleus, a bacterium has a single loop of circular DNA floating in its cytoplasm.
• It may also carry one or more small rings of DNA called plasmids.

The stages of binary fission

1. The bacterium copies its circular DNA, and copies any plasmids it has.
2. The cell grows larger, and the copies of the circular DNA move to opposite ends (the two "poles") of the cell.
3. The cytoplasm starts to divide and new cell walls begin to form across the middle.
4. The cytoplasm divides completely and the cell splits in two, producing two daughter cells.

Each daughter cell receives one copy of the circular DNA, and a variable number of copies of any plasmids.

How quickly bacteria divide

• Given the right conditions, bacteria can divide remarkably quickly.
• With enough nutrients and a suitable, warm temperature, some bacteria such as E. coli can divide as often as once every 20 minutes.
• If conditions turn unfavourable - too little food, or the wrong temperature - the bacteria stop dividing and eventually begin to die.

Calculating bacterial numbers after a time

• The mean division time is the average time it takes for one bacterium to divide into two.
• If you know the mean division time, you can work out how many times a cell has divided in a given period, and from that the number of bacteria produced.

The two steps

1. Number of divisions - Divide the total time by the mean division time (both must be in the same units).
2. Number of bacteria - Because each cell divides into two every time, this is 2 raised to the power of the number of divisions (2ⁿ, where n is the number of divisions).

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Worked example - Bacteria after a time period

One bacterium divides every 20 minutes and has enough nutrients at a suitable temperature for 5 hours. How many bacteria are produced?