Binary Fission and Viruses Revision Notes | A Level AQA Biology

1. Why Cell Division Matters — and Where It Gets Complicated

Every bacterium you have ever encountered, every infection your immune system has fought, traces back to one fundamental question: how do cells make more of themselves? For prokaryotes, the answer is binary fission — a precise, elegant process. For viruses, the answer is stranger still, because viruses do not divide at all.

This lesson covers two contrasting stories of biological reproduction at the cellular level. Understanding both — and the sharp differences between them — is exactly what AQA examiners test. By the end, you will be able to explain each process with the precision the mark scheme demands.

2. Binary Fission in Prokaryotic Cells

Picture a bacterial cell. Unlike a eukaryotic cell, it has no nucleus — its genetic material is a single, circular DNA molecule floating in the cytoplasm, often called the bacterial chromosome. It may also carry one or more plasmids: smaller, circular DNA molecules that replicate independently of the main chromosome.

Binary fission unfolds in a clear sequence. First, the circular DNA molecule is replicated — both strands of the double helix are used as templates, producing two identical circular DNA molecules. This replication begins at a specific point on the chromosome called the origin of replication and proceeds around the circle.

At the same time, any plasmids present in the cell also replicate. This is a separate event from chromosomal replication — plasmids have their own origins of replication and can replicate multiple times, which is why the number of plasmid copies distributed to each daughter cell is variable rather than fixed.

Worked example: Stages of Binary Fission — Described in Sequence

Describe, in order, the stages of binary fission in a prokaryotic cell.

1. The single circular DNA molecule unwinds and both strands act as templates. Semi-conservative replication produces two identical circular DNA molecules. — This is the critical first stage. Examiners expect you to state that the DNA is circular and that replication produces two identical copies — not just 'DNA copies itself'.
2. Plasmids replicate independently of the chromosomal DNA. Multiple copies may be produced, so the number of plasmid copies per daughter cell is variable. — Many students forget plasmids entirely. AQA specifically requires you to mention plasmid replication as a distinct step.
3. The two circular DNA molecules attach to different regions of the plasma membrane and are pulled apart as the cell elongates. — There is no mitotic spindle in prokaryotes — membrane growth drives the separation of chromosomes.
4. The plasma membrane grows inward at the centre of the cell, forming a new cell wall and membrane. The cytoplasm is divided, producing two daughter cells. — Each daughter cell receives one copy of the circular chromosomal DNA. Plasmid copies are distributed variably — this is a key AQA distinction.

Answer: Binary fission produces two daughter cells, each with a single copy of the circular chromosomal DNA and a variable number of plasmid copies.

Notice the contrast with mitosis in eukaryotes: there is no spindle apparatus, no condensation of chromosomes into visible structures, and no nuclear envelope breaking down — because there is no nucleus to begin with. Binary fission is faster and structurally simpler, which is why bacteria can divide every 20 minutes under ideal conditions.

3. Not All Cells Retain the Ability to Divide

Binary fission applies to prokaryotes, but the broader principle — that all cells arise from pre-existing cells — extends to eukaryotes too. Within multicellular organisms, however, not every cell retains the capacity to divide. This is a crucial AQA specification point that students frequently overlook.

During development, cells differentiate — they switch on specific genes and develop specialised structures for particular functions. This specialisation often comes at the cost of the cell's ability to divide. Mature red blood cells in humans, for example, have no nucleus and therefore no DNA to replicate; they cannot divide. Mature neurones in the central nervous system are another well-known example of cells that have largely lost the capacity for division.

This is not a failure of the organism — it is a controlled biological strategy. Highly specialised cells can perform their functions more efficiently when they commit fully to that role. The trade-off is that damage to these cell populations is difficult to repair, which is why spinal cord injuries and heart muscle damage are so clinically challenging.

This spec point connects directly back to the broader theme of cell specialisation: differentiation produces cells with specific functions, and that process can permanently restrict the cell's ability to replicate its DNA and divide. Keeping this link clear will help you answer 'explain' questions that ask why certain tissues cannot regenerate.

4. Viruses — Replication Without Cell Division

Here is where the story takes a sharp turn. Everything covered so far — binary fission, the replication of circular DNA, the division of cytoplasm — applies to living cells. Viruses are fundamentally different, and the reason matters enormously for your exam answers.

Because viruses are non-living, they do not undergo cell division of any kind. They cannot replicate on their own. Instead, a virus introduces its nucleic acid into a host cell—some viruses (such as bacteriophages) inject their nucleic acid, while others (like many animal viruses) enter via endocytosis or membrane fusion. The host cell's own ribosomes, enzymes, and energy systems are then used to transcribe and translate the viral genetic information, producing the components needed to assemble new virus particles.

The host cell effectively becomes a virus factory. New viral nucleic acid molecules are replicated using the host's nucleotides and polymerases. Viral coat proteins are synthesised using the host's ribosomes. These components self-assemble into new virus particles, which are eventually released — often destroying the host cell in the process.

A useful way to anchor this: think of a virus as a set of instructions with no printer. The instructions (nucleic acid) are useless until they reach a printer (the host cell). The printer does all the work — the instructions redirect it. This is why antiviral drugs are so difficult to develop: targeting the virus risks damaging the host cell's own machinery.

5. Comparing Binary Fission and Viral Replication

AQA examiners frequently ask you to compare these two processes. A strong comparison answer identifies both similarities and differences, using precise biological language. The table below gives you the framework — learn it, then practise writing it in prose.

The key conceptual distinction is independence. A bacterium undergoing binary fission is an autonomous agent — it controls its own replication. A virus is entirely parasitic at the molecular level, contributing only its genetic instructions while the host cell supplies everything else.

6. Exam Technique and Mark-Winning Strategies

Now that the biology is clear, the final step is translating your understanding into marks. These topics appear in a range of question styles — from two-mark 'describe' questions to six-mark 'compare and explain' extended responses. The guidance below is drawn directly from how AQA mark schemes are structured.

Across this lesson, the central thread has been the principle that all cells arise from pre-existing cells — but the mechanism differs sharply depending on whether you are dealing with a living prokaryote or a non-living virus. Binary fission is an autonomous, precisely regulated cellular process; viral replication is a molecular hijack. Keeping that distinction sharp, and supporting it with the correct terminology, is what separates a good answer from a full-mark answer.