Biology › Cells, microscopy and biological organisation › Prokaryotic cells and viruses: less machinery, same problems
Prokaryotic cells and viruses: less machinery, same problems
A bacterium solves the problems of being alive with no nucleus and no membrane-bound organelles at all. A virus does not solve them; it borrows a cell that already has. One of these is a living organism and the other is not, and knowing why is worth several marks a year.
Before this Cell ultrastructure and the organelles · Sizes and the resolving limits of microscopes
Before you start
Bacteria are simple cells and viruses are just very small bacteria. The first half is nearly harmless — a bacterium has less equipment than one of your cells, though calling something that lives in boiling springs simple is a stretch. The second half is a real mistake, and it is not a matter of size. A virus has no cytoplasm, no ribosomes, no membrane it built itself and no metabolism of any kind. It cannot make a protein, cannot respire and cannot divide. Antibiotics work on bacteria and do nothing to viruses for exactly this reason: there is no machinery there to poison.
What you should be able to do
- Describe the structure of a prokaryotic cell and give a function for each part.
- Compare prokaryotic and eukaryotic cells, including DNA, ribosomes and cell wall.
- Explain what a plasmid is and why it is not part of the main chromosome.
- Describe binary fission and calculate numbers of bacteria after a given time.
- Describe the structure of a virus, using HIV, and justify the claim that viruses are not living.
What a prokaryote has, and what it does without
Prokaryotic cells are the bacteria and the archaea, and they are small — typically 0.5 to 5 µm across, against 10 to 100 µm for a eukaryotic cell. A whole bacterium is about the size of one of your mitochondria, which looks like a coincidence only until you have met the endosymbiotic theory.
Start at the outside. The cell wall is made of murein, also called peptidoglycan — a mesh of polysaccharide chains cross-linked by short peptides. It is not cellulose and it is not chitin, and the difference is exactly what penicillin exploits: the drug blocks the cross-linking, so a growing bacterium builds a wall it cannot hold together and bursts. Your own cells have no wall to interfere with, which is why the antibiotic is safe to take.
Outside the wall, many species secrete a capsule of slime. It holds water, helps the cell stick to surfaces and to other bacteria, and makes phagocytosis harder for a white blood cell — the capsule is one reason some strains are dangerous and closely related ones are not.
Inside the wall is the plasma membrane, and inside that the cytoplasm, and that is where the differences bite. There is no nucleus. The DNA is a single circular molecule, lying free in the cytoplasm in a region called the nucleoid, with no envelope and no histone proteins wound into it. There is no endoplasmic reticulum, no Golgi apparatus, no mitochondrion and no lysosome. The electron transport chain and ATP synthase sit in the plasma membrane itself, so the one membrane holding the cell together is also the membrane it respires on. Older textbooks drew the chain on infoldings called mesosomes; the infoldings were made by the fixing chemicals and are not there in a living bacterium. The only structure in there that a eukaryotic cell also has is the ribosome — and even that is smaller, at 70S against the eukaryote's 80S.
Two extras appear in many species. Plasmids are small circles of DNA, separate from the main chromosome, carrying a handful of genes that are useful rather than essential — resistance to an antibiotic, for instance. They replicate independently and can be passed from one bacterium to another, which is how resistance spreads through a population far faster than mutation alone could manage. A flagellum is a long protein filament, anchored in the membrane and rotated by a molecular motor, which drives the cell forward. It is not the same structure as a eukaryotic flagellum and does not move the same way: this one spins, like a propeller.
- Prokaryotic cell
- A cell with no nucleus and no membrane-bound organelles, whose DNA is a single circular molecule free in the cytoplasm.
- Plasmid
- A small circular molecule of DNA, separate from the main chromosome, which replicates independently and often carries genes such as antibiotic resistance.
- Capsule
- A layer of slime outside the cell wall which retains water, aids attachment and gives some protection from phagocytosis.
- Nucleoid
- The region of a prokaryotic cell occupied by its circular DNA, with no surrounding membrane.
Where the two kinds of cell really differ
Comparison questions here are common and predictable, and they are marked on pairs: a statement about the prokaryote next to the matching statement about the eukaryote. "Prokaryotes have no nucleus" on its own is half a point at best.
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Size | 0.5–5 µm | 10–100 µm |
| Nucleus | Absent | Present, with a double envelope |
| DNA | One circular molecule, no histones | Several linear molecules wound round histones |
| Plasmids | Often present | Absent from the cytoplasm |
| Membrane-bound organelles | None | Mitochondria, ER, Golgi, lysosomes and more |
| Ribosomes | 70S, smaller | 80S in the cytoplasm |
| Cell wall | Murein (peptidoglycan) | Cellulose in plants, chitin in fungi, none in animals |
| Division | Binary fission | Mitosis or meiosis |
Two rows on that table need care. The DNA row is not simply 'circular against linear' — the absence of histones is a separate mark, and so is the fact that a prokaryote has one chromosome rather than several. And the organelle row does not mean a prokaryote has nothing inside it: it has ribosomes, enzymes, food stores and often internal membranes. What it does not have is any compartment sealed off by its own membrane.
There is one genuine overlap that examiners like. Mitochondria and chloroplasts contain circular DNA without histones and 70S ribosomes — prokaryotic features, inside a eukaryotic cell. So if a question asks where 70S ribosomes are found in a leaf, the honest answer includes the chloroplasts and mitochondria as well as any bacteria on the surface.
Binary fission, and why bacterial numbers get out of hand
A prokaryote does not use mitosis. It has no nucleus to break down, no spindle and no chromosomes to line up. Instead it divides by binary fission, and the sequence is short: the circular DNA replicates, any plasmids replicate separately and in variable numbers, the cell grows and the two DNA circles move apart, then the cytoplasm divides and new wall material is laid down between the daughter cells.
The two daughters get one copy of the chromosome each, which is tidy. They do not necessarily get the same number of plasmids, which is not — plasmid copies are shared out roughly, so two daughters of the same cell can differ in how many resistance genes they carry.
Fission is fast. Escherichia coli in warm, well-fed conditions divides about every 20 minutes, and doubling every 20 minutes is a habit that gets away from you quickly.
Doubling every twenty minutes
A single bacterium is placed in nutrient broth at 37 °C, where it divides every 20 minutes. Calculate how many bacteria are present after 4 hours, assuming none die.
First count the divisions. Four hours is 240 minutes, and 240 ÷ 20 = 12 divisions.
Each division doubles the population, so the number is 2 raised to the number of divisions: 2¹² = 4096.
The general form is N = N₀ × 2ⁿ, where N₀ is the number you started with and n the number of divisions. Starting with 50 cells instead of one would give 50 × 4096 = 204 800.
The mistake to avoid is multiplying by 12 instead of raising 2 to the power of 12. Real cultures also stop behaving like this once nutrients run low and waste builds up, which is why an unlimited exponential answer is only ever a model of the early growth phase.
Viruses: not small cells, not cells at all
A virus particle is between about 20 and 300 nm across — smaller than the resolving limit of a light microscope, which is why nobody saw one until the electron microscope arrived. But size is not what disqualifies it from being alive. Structure is.
Strip a virus down and you find genetic material, either DNA or RNA, wrapped in a protein coat called a capsid, with attachment proteins on the outside. Some viruses add a lipid envelope, and that envelope is stolen: it is a piece of the membrane of the last cell the virus left. There is no cytoplasm, no ribosome, no mitochondrion and no enzyme system for respiration. A virus cannot make a protein, cannot release energy from food and cannot reproduce by itself. It is acellular — not made of cells — and it is an obligate intracellular parasite, meaning it can only be copied by hijacking a host cell's ribosomes and enzymes.
The attachment proteins are worth dwelling on, because they explain specificity. Each type binds to a particular receptor molecule on a particular host cell, in the same complementary-shape way a substrate binds an enzyme. That is why influenza infects your airway epithelium and not your liver, and why a virus of tobacco plants does nothing to you.
HIV is built in layers. On the outside are glycoprotein attachment proteins, which bind to the CD4 receptor found on helper T cells. Beneath them is the lipid envelope taken from the host membrane, and inside that a layer of matrix protein. At the centre a cone-shaped capsid holds two single strands of RNA and several molecules of the enzyme reverse transcriptase. That enzyme is the point of the whole design: it copies the viral RNA into DNA — the reverse of the usual direction — so the DNA copy can be inserted into the host's own chromosome. A virus that does this is a retrovirus.
- Virus
- An acellular infectious particle consisting of nucleic acid within a protein capsid, sometimes with a lipid envelope, which can only replicate inside a host cell.
- Capsid
- The protein coat surrounding a virus's genetic material.
- Attachment protein
- A protein on a virus's surface with a shape complementary to a receptor on its host cell, which determines which cells it can infect.
- Reverse transcriptase
- An enzyme carried by retroviruses such as HIV which catalyses the synthesis of DNA from an RNA template.
TRY IT — Arguing that something is not alive
A student writes that viruses are living because they reproduce and they evolve. Evaluate that claim.
Check your answer
Both observations are correct and the conclusion still does not follow, because of how the reproduction happens.
A virus does not reproduce; it is reproduced. It has no ribosomes, so it cannot make the proteins of its own capsid, and no enzymes of respiration, so it cannot release the energy that would be needed. Both jobs are done by a host cell's machinery, working from viral instructions. Outside a host, a virus particle does nothing at all — it is chemically inert and can be crystallised like a salt.
Evolution is a weaker argument still. Viral genomes mutate and are selected, which is why influenza vaccines are reformulated every year, but selection acts on any replicating information and does not require the replicator to be alive.
The reasonable position is the one most biologists hold: viruses sit at the boundary, showing some characteristics of life only while inside a cell. For an exam, say acellular, and give the missing structures — no cytoplasm, no ribosomes, no metabolism.
In the exam
- Answer comparison questions in pairs. 'A prokaryote's DNA is circular and has no histones, whereas a eukaryote's is linear and wound round histones' is one complete point; naming only one side of it is not.
- 70S is the prokaryotic ribosome and 80S the eukaryotic cytoplasmic one. If you can only remember one, remember that the prokaryote gets the smaller number, since it is the smaller cell.
- Murein is the bacterial wall. Cellulose is plant, chitin is fungal. Losing a mark by writing cellulose for a bacterium is an avoidable way to lose a mark.
- 'Viruses are very small' does not explain why they are not living. The marks are for acellular structure: no cytoplasm, no ribosomes, no organelles, no metabolism of their own.
- For HIV, learn all five parts — attachment proteins, lipid envelope, matrix, capsid, and RNA with reverse transcriptase — and be able to say what each is for. Diagram-labelling questions on HIV appear regularly.
- Binary fission questions are usually 2ⁿ calculations. Work out the number of divisions first, then use it as the power.
Check yourself
A patient is treated with an antibiotic for a bacterial chest infection and recovers. Two months later she has influenza, and asks for the same antibiotic. Explain, in terms of cell structure, why it will not help her, and why some bacterial infections no longer respond to it either.
Answer
An antibiotic works by attacking a structure or process that bacteria have and human cells do not. Many, penicillin among them, prevent the cross-linking of murein in the bacterial cell wall, so a dividing cell builds a wall too weak to resist the water entering by osmosis and bursts. Others bind to the 70S ribosome and block translation, leaving the 80S ribosomes in her cytoplasm alone. That second selectivity is real but not perfect, because her mitochondria carry 70S ribosomes of their own — which is exactly why drugs of this class have side effects the wall-builders do not.
Influenza is a virus. It has no cell wall to weaken, no ribosomes to block and no metabolism to interfere with — it is acellular, and its proteins are made on the ribosomes of her own infected cells. There is nothing there for the drug to act on, so it can do nothing but disturb the useful bacteria she already has.
Resistance in bacteria is a separate matter and comes largely from plasmids. A mutation producing an enzyme that breaks the antibiotic down gives its carrier a selective advantage whenever the drug is present, and because such genes often sit on plasmids they can be passed directly between bacteria rather than only to daughter cells.
That is why a resistant strain can appear in a population within months, and why prescribing an antibiotic for a viral infection makes the problem worse without treating anything.
Questions
Question 14 marks
Compare the genetic material and the ribosomes of a prokaryotic cell with those of a eukaryotic cell.
Mark scheme
- B1 a prokaryote has one circular DNA molecule, whereas a eukaryote has several linear ones
- B1 prokaryotic DNA is not wound round histone proteins, whereas eukaryotic DNA is
- B1 prokaryotic DNA lies free in the cytoplasm in the nucleoid, whereas eukaryotic DNA is enclosed by a nuclear envelope
- B1 prokaryotic ribosomes are 70S and smaller, whereas the ribosomes in eukaryotic cytoplasm are 80S
Question 24 marks
A student writes that a virus is simply a very small cell. Explain why this statement is wrong, referring to the structure of a virus in your answer.
Mark scheme
- B1 a virus is acellular: it has no cytoplasm and no cell-surface membrane of its own
- B1 it has no ribosomes, so it cannot make any protein for itself
- B1 it has no enzyme system for respiration, so it has no metabolism and cannot release energy from food
- B1 it consists only of DNA or RNA inside a protein capsid with attachment proteins outside, sometimes within a lipid envelope taken from a host cell
Question 33 marks
A culture is started with 200 bacteria which divide by binary fission every 30 minutes. Calculate how many bacteria are present after 5 hours, assuming that none of them die.
Mark scheme
- M1 number of divisions = 300 minutes divided by 30, giving 10 divisions
- M1 uses N = N₀ × 2ⁿ, so the calculation is 200 × 2¹⁰
- A1 an answer of 204 800 bacteria
Question 43 marks
A drug is designed to bind to the glycoprotein attachment proteins on the surface of HIV. Suggest why this drug would prevent HIV entering helper T cells, and suggest why it would have no effect on the influenza virus.
Mark scheme
- B1 the attachment proteins of HIV normally bind to the CD4 receptor found on helper T cells
- B1 with the drug bound to them the attachment proteins can no longer bind that receptor, so the virus cannot get into the cell
- B1 influenza carries different attachment proteins, which the drug is not complementary to, so it does not bind them
Question 52 marks
Name the substance from which a bacterial cell wall is built, and name the structure that carries genes such as antibiotic resistance and can be passed between bacteria.
Mark scheme
- A1 murein, also accepted as peptidoglycan
- A1 a plasmid, which is a small circle of DNA
Worth remembering
- Prokaryote: no nucleus, no membrane-bound organelles, one circular DNA molecule with no histones, 70S ribosomes, murein wall.
- Plasmids are small extra DNA circles, replicated separately and shared between cells.
- Mitochondria and chloroplasts also hold circular DNA and 70S ribosomes.
- Binary fission doubles the population each time: N = N₀ × 2ⁿ.
- A virus is acellular — capsid, genetic material, attachment proteins, sometimes an envelope, and nothing else.