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Binary fission and virus replicationAQA A-Level Biology: Revision notes

Section 1

Binary fission in prokaryotes

Prokaryotic cells such as bacteria divide by binary fission. They have no nucleus and no mitosis. Their genetic material is a single circular DNA molecule, and many also carry small circular plasmids.

Steps:

  1. The circular DNA is replicated.
  2. The plasmids are replicated.
  3. The cytoplasm divides and the cell splits in two, producing two daughter cells.

Each daughter has one copy of the circular DNA and a variable number of copies of plasmids, because plasmids are shared out unequally.

Key termsbinary fissionplasmid
Common mistake

Do not describe binary fission as mitosis. There is no spindle, no nuclear envelope and no chromosome condensation.

Section 2

Population growth by binary fission

Each division doubles the number of cells. With a fixed generation time, the number after n divisions is 2ⁿ (starting from one cell).

Worked example: a generation time of 20 minutes gives 6 divisions in 2 hours, so 2⁶ = 64 cells.

Growth is rapid when warmth, nutrients and moisture are suitable and no cells die.

Key termsgeneration time
Exam tip

Divide the total time by the generation time to find the number of divisions, then use 2ⁿ.

Section 3

Viruses are non-living

Viruses are acellular and non-living. They have no cytoplasm, ribosomes, enzymes or metabolism of their own, so they cannot carry out cell division. A virus particle is nucleic acid (DNA or RNA) inside a protein coat.

Outside a host cell a virus cannot replicate.

Key termsvirushost cell

Section 4

Virus replication

  1. The virus attaches to a host cell.
  2. It injects its nucleic acid into the host cell.
  3. The host cell's enzymes and ribosomes replicate the viral nucleic acid and make the viral proteins.
  4. The host cell assembles new virus particles, which are released and can infect other cells.

The infected host cell does the work of replicating the virus particles; the virus itself does not divide.

Key termsbacteriophage

Section 5

Comparing the two processes

In binary fission a living cell divides itself to give two daughter cells, using its own enzymes. In virus replication the host cell replicates the viral nucleic acid and makes many virus particles.

Both begin with replication of nucleic acid. Neither process involves mitosis.

That's the notes covered.

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Exam questions on Binary fission and virus replication

  1. A microbiologist inoculated a flask of sterile nutrient broth with a single Escherichia coli cell. Under these conditions each cell divides by binary fission once every 20 minutes and none of the cells die.
    Explain why the daughter cells each contain exactly one copy of the circular DNA but may contain different numbers of plasmids.2 marks
  2. A virologist is studying a bacteriophage, a virus that infects bacteria. When a phage attaches to a bacterium it injects its nucleic acid into the host cell and its protein coat stays outside.
    Explain why phages cannot replicate in sterile nutrient broth that contains no bacteria.2 marks
  3. A hospital laboratory is investigating two patients: one has a bacterial infection and the other has a viral infection.
    Describe how a virus replicates after it has attached to a host cell.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).