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 for one bacterium to divide into two.
• If you know it, you can work out how many bacteria a single cell produces in a given period, in two steps:
  1. Number of divisions = total time ÷ mean division time (both in the same units).
  2. Number of bacteria = 2 raised to the power of the number of divisions - because each cell splits into two every division.

Writing large counts in standard form

• Standard form writes a number as a value between 1 and 10 multiplied by a power of 10.
• Move the decimal point so the first part lies between 1 and 10, then count the places it moved to get the power of 10.
• Very large bacterial counts are usually written this way to keep them manageable, and rounded to 2 significant figures for an exam answer.

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Worked example - Bacteria after 5 hours

One bacterium divides every 20 minutes and has enough nutrients at a suitable temperature for 5 hours. Calculate the number of bacteria produced, giving your answer in standard form to 2 significant figures.