Binary Fission and Virus Replication
How bacteria reproduce by binary fission (circular DNA and plasmids) and why viruses cannot divide, instead using a host cell to make new virus particles. Includes the 5-mark exam question, a worked model answer and the examiner trap on never saying bacteria divide by mitosis.
Chapters (8)
Exam question [5 marks]
Describe how a bacterium reproduces. Explain why a virus cannot reproduce in the same way.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Circular DNA and plasmids are replicated | "DNA copied", "plasmids copied" (both needed) | "chromosomes replicate"; "DNA replicates" with no mention of plasmids |
| 2 | Cytoplasm divides to give two daughter cells, each with one copy of the circular DNA | "cell splits in two", "binary fission" (must imply one copy of circular DNA each) | "nucleus divides"; "mitosis" |
| 3 | Number of plasmids in each daughter cell is variable | "different numbers of plasmids", "not always equal" | "each cell gets the same plasmids" |
| 4 | Virus is non-living / acellular, so it does not undergo cell division | "not a cell", "no cytoplasm to divide" | "viruses are too small to divide" |
| 5 | Virus injects its nucleic acid into a host cell, which replicates the virus particles | "host cell makes new viruses", "host machinery copies it" | "virus replicates by itself"; "virus divides inside the host" |
Marks 1 to 3 describe binary fission; marks 4 and 5 explain why a virus differs. Naming the process (binary fission) earns nothing without the description. Mitosis is never correct for a bacterium.
Model answer (5/5)
The circular DNA and the plasmids are replicated. The cytoplasm then divides to produce two daughter cells, each with one copy of the circular DNA. The number of plasmids in each daughter cell is variable. A virus is non-living, so it does not undergo cell division. Instead it injects its nucleic acid into a host cell, and the host cell replicates the virus particles.
Examiner trap
- Saying bacteria divide by mitosis
- Saying viruses divide or 'reproduce by binary fission'
- Saying each daughter gets the same number of plasmids
Key terms
- binary fission
- circular DNA
- plasmid
- daughter cell
- viral replication
- host cell
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:19Bacteria: the starting point
Bacteria are prokaryotic cells. They have no nucleus, and their main DNA is one circular molecule floating in the cytoplasm. Many also carry plasmids, which are small extra rings of DNA. There is no nucleus and no spindle, so mitosis is not an option.
Instead they use binary fission. It is a simpler process, with no stages to step through. Remember the name: binary means two, fission means splitting. One cell splits into two.
0:52Binary fission: DNA copied
Step one. The circular DNA is replicated, so now there are two copies. The plasmids are replicated too. The two loops of DNA move apart, to opposite ends of the cell.
1:05Binary fission: cell divides
Step two. The cell gets longer, and the cytoplasm divides. That gives two daughter cells. Each daughter cell has one copy of the circular DNA. But the plasmids are not shared out equally, so the number of plasmids in each daughter cell is variable.
Look at the drawing. One cell may get three plasmids, and the other just one.
1:28Viruses are acellular
Now viruses. A virus particle is a nucleic acid core inside a protein coat. It is acellular. It has no cytoplasm and no cell structure. It is non-living, so it does not undergo cell division. It cannot do binary fission, and it cannot do mitosis.
1:49Virus replication in a host
So how does a virus make more of itself? It needs a host cell. The virus injects its nucleic acid into the host cell. Then it is the host cell that replicates the virus particles. The virus is not dividing. It is being copied by the host.
Think of it like a photocopier. The virus supplies the instructions, but the host cell does all the copying. That is why viruses cannot reproduce on their own, outside a host.
2:53Examiner trap and recap
The traps. Bacteria use binary fission, never mitosis. Viruses do neither. And do not say that each daughter cell gets the same number of plasmids.
Recap. Bacteria copy their circular DNA and plasmids, then split. Viruses are copied by the host.
Next, we meet mitosis in practice, with stained root tip squashes and the mitotic index.
Watch first
This lesson builds on:
- Prokaryotic vs Eukaryotic Cells · AQA 7402 · 3.2.1.2
- Viruses: Structure and Non-Living Status · AQA 7402 · 3.2.1.2
Keep going
More from 3.2 Cells
- Mitosis and the Cell Cycle: Stages · AQA 7402 · 3.2.2
- Cancer: Uncontrolled Cell Division · AQA 7402 · 3.2.2
- Required Practical 2: Root Tip Squash · AQA 7402 · 3.2.2
- Nucleus, Mitochondria and Chloroplasts · AQA 7402 · 3.2.1.1
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-032.