Bacteria are simple, and people take that to mean primitive or feeble. They are neither. A cell a couple of micrometres long, with no internal rooms at all, can divide every twenty minutes and survive in places that would kill anything else. Simple here means stripped down, not badly built.
📚 What you need to know
Prokaryotic cells have no nucleus and no membrane-bound organelles.
The DNA is a single circular chromosome lying in a region called the nucleoid. It is naked, meaning it is not wrapped around histone proteins.
Many also carry plasmids — small extra circles of DNA that can be copied and passed on separately.
They have 70S ribosomes, smaller than the 80S ones in eukaryotic cells.
The cell wall is made of peptidoglycan and stops the cell bursting.
Extra structures include a capsule, flagella for movement and pili for attaching to surfaces and swapping DNA.
They divide by binary fission, a simple split into two identical cells.
What a prokaryotic cell looks like
Work from the outside in. That order also happens to be the order that makes sense in an exam answer.
The dotted outline marks the nucleoid. It is only a region of the cytoplasm where the DNA sits — draw a membrane round it and you have drawn a eukaryotic cell by mistake.
Each part and what it does
Structure
What it is
What it does
Cell wall
A rigid layer of peptidoglycan outside the membrane
Gives shape and stops the cell bursting when water moves in
Capsule
A slimy layer outside the wall
Protects the cell, helps it stick to surfaces, and makes it harder for the host to attack
Plasma membrane
A partially permeable phospholipid layer
Controls what enters and leaves; also where respiration happens, since there are no mitochondria
Cytoplasm
Watery jelly with enzymes dissolved in it
The site of the cell’s chemical reactions
Nucleoid
A region, not an organelle, holding one circular chromosome
Carries the genes the cell needs to survive
Plasmids
Small circles of DNA separate from the main chromosome
Carry extra genes, such as antibiotic resistance, and can be passed to other bacteria
70S ribosomes
Small ribosomes free in the cytoplasm
Build proteins by joining amino acids
Flagellum
A long tail that rotates like a propeller
Moves the cell towards food or away from harm
Pili
Short hair-like projections
Attach the cell to surfaces and to other bacteria for transferring DNA
Notice the plasma membrane doing double duty. In a eukaryotic cell, respiration happens inside mitochondria. A prokaryote has none, so the membrane itself carries the proteins for respiration. Whenever a prokaryote lacks an organelle, ask yourself where that job went instead — the answer is nearly always the membrane or the cytoplasm.
The DNA: circular, naked, and often doubled up
Three details about prokaryotic DNA come up over and over.
Plasmids can be copied on their own and handed to another bacterium through a pilus. That is how antibiotic resistance spreads so quickly through a population.
Circular. The main chromosome is one continuous loop, with no free ends. Eukaryotic chromosomes are linear.
Naked. It is not wound around histone proteins the way eukaryotic DNA is. There is far less of it, so it does not need that packing.
Plus plasmids. These small rings sit separately in the cytoplasm. They are not needed for basic survival, but the genes on them can be extremely useful — resistance to an antibiotic, for example.
Why plasmids matter beyond the exam. Because a plasmid can be passed from one bacterium to another, a useful gene does not have to wait for the slow route of inheritance through generations. Resistance can spread sideways through a population within days. This is also why plasmids are the standard tool for putting genes into bacteria in genetic engineering.
How they divide
Prokaryotes reproduce by binary fission. The circular chromosome is copied, the two copies move to opposite ends of the cell, the cell grows longer, and then it pinches in the middle to make two cells.
There is no mitosis here, because mitosis is the process for sorting several linear chromosomes inside a nucleus, and a prokaryote has neither. Binary fission is much faster, which is how a single bacterium can become millions overnight.
🧠
Outside in
Capsule, Wall, Membrane, then the contents: cytoplasm, nucleoid, plasmids, ribosomes. Answering in that order means you never leave one out.
Worked examples
WE 1
Describe the DNA of a prokaryotic cell
Describe how the genetic material of a prokaryotic cell differs from that of a eukaryotic cell. (3 marks)
Difference 1: shape
Prokaryotic DNA is a single circular chromosome, while eukaryotic DNA is in several linear chromosomes.
Difference 2: packaging
Prokaryotic DNA is naked, meaning it is not associated with histone proteins.
Difference 3: location and extras
It lies free in the nucleoid rather than inside a nucleus, and prokaryotes may also carry plasmids.
Circular, naked, in the nucleoid, plus plasmidsa “differs” question needs both sides of each comparison, not just the prokaryote
WE 2
Explain the role of the cell wall
Explain why a bacterium placed in pure water does not burst. (3 marks)
Point 1: what happens first
Water moves into the cell by osmosis, because the cytoplasm is more concentrated than pure water.
Point 2: the pressure
The cell swells and pushes outwards on the peptidoglycan cell wall.
Point 3: why it holds
The wall is rigid and inelastic, so it resists that pressure and stops the cell expanding any further.
The rigid peptidoglycan wall resists the pressure from osmosisname the material — “cell wall” alone often misses a mark here
WE 3
Suggest how resistance spreads
A gene for antibiotic resistance is carried on a plasmid. Suggest why this allows resistance to spread rapidly through a bacterial population. (3 marks)
Point 1: plasmids can be copied alone
A plasmid is separate from the main chromosome, so it can be replicated on its own.
Point 2: it can be transferred
Bacteria can pass a plasmid to another cell through a pilus, including to a different species.
Point 3: why that is fast
Resistance therefore spreads sideways between living cells, rather than only being inherited by offspring, and bacteria also divide very quickly.
Plasmids move between cells, so resistance does not have to wait for reproduction“suggest” means apply what you know — the answer will not be a memorised fact
💡 Exam tips
Use the three key words for the DNA every time: circular, naked, in the nucleoid.
Name the wall material as peptidoglycan. It is a single word worth a mark.
If asked where respiration happens in a bacterium, the answer is the plasma membrane and cytoplasm, not mitochondria.
Flagella move the cell; pili grip surfaces and transfer DNA. Do not swap them.
Say binary fission, never mitosis, for prokaryotic division.
In a drawing, never put a membrane around the nucleoid.
⚠ Common mistakes
Drawing a nucleus. The DNA is loose in a region of cytoplasm.
Saying prokaryotes have no organelles at all. They have ribosomes, which are organelles without membranes. The accurate phrase is no membrane-bound organelles.
Calling plasmids essential. They carry extra genes; the cell survives without them.
Writing that the cell wall is made of cellulose. That is plants. Bacteria use peptidoglycan.
Mixing up the capsule and the cell wall. The capsule is the slimy outer layer, outside the wall.
Giving 80S ribosomes. Prokaryotes have 70S.
Up next: Eukaryotic Cell Structure — the organelles, what each one does, and how they work together to make and export a protein.
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