Binary fission

Binary fission is the way bacteria and other prokaryotes reproduce. The cell copies its single loop of DNA, grows, and splits into two daughter cells that are genetically identical to it and to each other. It is asexual reproduction, and in good conditions some bacteria divide as often as once every 20 minutes.

What is binary fission?

Binary means two and fission means splitting, so binary fission is splitting in two. It is how prokaryotic cells divide: bacteria and archaea. These cells have no nucleus. Their DNA is one circular molecule lying free in the cytoplasm, and many also carry one or more small rings of DNA called plasmids. With no nucleus and only one chromosome, they have no need for mitosis. The cell copies its DNA and divides.

AQA covers it at two levels. At GCSE, you need to know that bacteria multiply by simple cell division (binary fission), and to work out how many bacteria there are after a given time. At A-level, you need the steps: the circular DNA and the plasmids are copied, then the cytoplasm divides, giving two daughter cells that each have one copy of the circular DNA and a variable number of plasmids.

The stages of binary fission

1. The DNA is copied. Replication starts at one point on the loop, the origin of replication, and runs round the loop in both directions until the whole chromosome has been copied. The plasmids are copied too.
2. The cell grows and the copies move apart. The cell gets longer, and the two DNA loops, each attached to the cell membrane, end up at opposite ends of the cell.
3. The cytoplasm divides. A ring of protein (called FtsZ) forms around the middle of the cell. The membrane pinches inwards there, and new cell wall grows across the cell to make a dividing wall called the septum.
4. Two daughter cells separate. Each has one complete copy of the circular DNA and some of the plasmids, so the two cells can end up with different numbers of plasmids.

The plasmids in the figure show why the specification says a variable number. The parent cell had two, they were copied to make four, and the daughter cells got three and one. The main DNA loop is always shared exactly: one copy each.

Binary fission vs mitosis

Both make two genetically identical cells, but they happen in different kinds of cell and work in different ways.

The stages of the eukaryotic version are in the mitosis guide, and the Cell Biology course compares plant, animal and bacterial cells side by side.

Binary fission in mitochondria and chloroplasts

Mitochondria and chloroplasts in eukaryotic cells also divide by a process that resembles binary fission, and they have their own circular DNA. This is part of the evidence that they began as bacteria that were taken in by an ancestral cell, the idea called endosymbiosis.

Do viruses reproduce by binary fission?

No. Viruses are not cells, and AQA's A-level specification states that, being non-living, they do not undergo cell division. A virus injects its nucleic acid into a host cell, and the host cell then makes new virus particles.

How fast do bacteria divide?

The time from one division to the next is the mean division time, also called the generation time or doubling time. AQA's GCSE specification says bacteria can divide as often as once every 20 minutes when they have enough nutrients and a suitable temperature. E. coli can manage this in a laboratory, but not every species is quick: Mycobacterium tuberculosis, which causes TB, takes 15 to 20 hours.

Every division doubles the number of cells, so while conditions stay good the population grows exponentially:

Example 1: from one bacterium

One bacterium has a mean division time of 30 minutes. How many bacteria are there after 5 hours?

1. Use the same units: 5 hours = 5 × 60 = 300 minutes.
2. Number of divisions: n = 300 ÷ 30 = 10.
3. Number of bacteria: 1 × 2¹⁰ = 1024.

Example 2: in standard form (Higher)

A culture starts with 250 bacteria, which divide every 25 minutes. How many are there after 200 minutes? Give your answer in standard form.

1. n = 200 ÷ 25 = 8 divisions.
2. N = 250 × 2⁸ = 250 × 256 = 64,000.
3. In standard form: 6.4 × 10⁴ bacteria.

Example 3: working out the time

A population of 500 bacteria grows to 32,000. The mean division time is 15 minutes. How long did it take?

1. How many times bigger: 32,000 ÷ 500 = 64.
2. 64 = 2⁶, so there were 6 divisions.
3. Time = 6 × 15 = 90 minutes.

When the ratio is not a neat power of 2, use logarithms:n=\log_2\left(\frac{N}{N_0}\right). Growing from 1,000 to 50,000 cells, for example, takes log₂ 50 ≈ 5.64 divisions.

The population growth simulator models this kind of exponential growth, and shows what happens when food and space start to run out.

Why bacteria don't keep doubling

Doubling every 30 minutes for a day would give 2⁴⁸ cells from one, about 2.8 × 10¹⁴, so it can't go on for long. A real culture follows a growth curve with four phases:

• Lag phase: the number stays about the same while the cells grow and make the enzymes they need.
• Log (exponential) phase: the cells divide by binary fission at a steady rate and the number doubles every generation.
• Stationary phase: nutrients run short and waste builds up, so new cells are made only as fast as others die.
• Death phase: more cells die than divide, and the number falls.

Why binary fission matters

Binary fission makes clones. Every daughter cell inherits the parent's DNA, so if a mutation makes one bacterium resistant to an antibiotic, all of its descendants are resistant too. Once the antibiotic kills off the bacteria that aren't resistant, a resistant strain can take over within days, because there are so many generations in a short time. That speed is why antibiotic resistance is such a problem, and why bacteria grown on agar plates in the lab must be handled with aseptic technique.

Common mistakes

• Calling it mitosis. Bacteria have no nucleus and no spindle, so binary fission isn't mitosis.
• Forgetting the plasmids. At A-level, say the plasmids are copied too, and shared out in variable numbers.
• Mixing units. Convert hours to minutes before dividing by the division time: 5 hours is 300 minutes.
• Doubling once instead of n times. After n divisions there are 2ⁿ times as many cells, not 2 × n.
• Giving viruses binary fission. Viruses aren't cells. Host cells make new virus particles.

To practise cell division and microorganism questions alongside other topics, try today's GCSE Biology Higher or A-level Biology: Year 1 short timed paper.

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