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Lesson 3 of 7Diversity of Living Things
Unit 1 · Lesson 3 of 7

Prokaryotes, Eukaryotes and Viruses

Comparing bacteria and archaea with eukaryotic cells, why viruses are not considered living cells, and the endosymbiotic theory for mitochondria and chloroplasts.

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Key things to remember

  1. 1Prokaryotes: no nucleus, circular DNA in a nucleoid, 70S ribosomes, binary fission. Eukaryotes: nucleus, linear chromosomes, 80S ribosomes, membrane-bound organelles.
  2. 2Typical sizes: prokaryotes about 1–10 µm, eukaryotic cells about 10–100 µm, viruses about 20–300 nm (1 µm = 1000 nm).
  3. 3A virus = DNA or RNA inside a protein capsid, sometimes with an envelope; it can reproduce only inside a host cell.
  4. 4Lytic cycle: attachment, entry, replication, assembly, release (cell bursts).
  5. 5Endosymbiotic theory: mitochondria came from engulfed aerobic bacteria and chloroplasts from engulfed photosynthetic bacteria (cyanobacteria).
  6. 6Evidence: both organelles have circular DNA, 70S-type ribosomes, double membranes and divide like bacteria.

Every living thing is built from cells, but cells come in two basic designs. Prokaryotic cells are small and simple, with no nucleus. Eukaryotic cells are larger and divided into compartments, with their DNA held in a nucleus. For well over a billion years, the only life on Earth was prokaryotic; eukaryotes, including every plant, animal and fungus, came later. Then there are viruses, which cause diseases from the common cold to influenza yet are not cells at all. Comparing these three helps you see what "alive" really means, and how complex cells may have arisen from simpler ones.

Bacterial cellProkaryote · about 1–10 µmNo nucleus: DNA lies in thecytoplasm (nucleoid region)Animal cellEukaryote · about 10–100 µmNucleus plus membrane-boundorganelles such as mitochondriaVirusNot a cell · about 20–300 nmNo cytoplasm or ribosomes; copiesitself only inside a host cellFlagellumCapsuleCell wallCell membraneNucleoid (circular DNA)PlasmidRibosomesNucleusEndoplasmic reticulumCell membraneGolgi apparatusMitochondrionRibosomesSurface proteinsEnvelope(some viruses)Capsid (protein coat)DNA or RNAPanels are not drawn to the same scale. 1 µm = 1000 nm, so a virus is far smaller than a bacterium.
Fig 1A bacterial cell, an animal cell and a virus with key structures labelled. The drawings are not to the same scale.

Prokaryotes: Bacteria and Archaea

A prokaryotic cell is usually about 1 to 10 micrometres () long. Its DNA is a single circular chromosome lying in a region of the cytoplasm called the nucleoid, with no membrane around it. Many prokaryotes also carry plasmids, small extra rings of DNA that often hold genes such as antibiotic resistance. The cell has a cell membrane, a rigid cell wall, and small 70S ribosomes that build proteins. Some have a slimy capsule for protection, pili for sticking to surfaces, or a whip-like flagellum for swimming. Prokaryotes reproduce asexually by binary fission, splitting in two, which lets a population grow very quickly. They can also swap genes, for example by passing plasmids through a pilus in a process called conjugation.

  • Shapes of bacteria — spheres are cocci (e.g. Streptococcus), rods are bacilli (e.g. Escherichia coli), and spirals are spirilla.
  • Gram stain — bacteria with a thick peptidoglycan wall hold a purple dye and are Gram-positive; those with a thin wall plus an outer membrane stain pink and are Gram-negative. The difference affects which antibiotics work.
  • Roles of bacteria — decomposers that recycle nutrients; nitrogen fixers such as Rhizobium, which live in root nodules of legumes; photosynthetic cyanobacteria; helpful gut bacteria; bacteria used to make yogurt and cheese; and a minority that cause disease, such as strep throat.
  • Archaea — also prokaryotic, but their cell walls lack peptidoglycan and their membranes are built from different lipids. Many live in extreme places: methanogens release methane in swamps and animal guts, extreme halophiles thrive in very salty water, and thermophiles live in hot springs. Archaea are also common in ordinary soil and oceans.

Eukaryotic cells are typically about 10 to 100 across, roughly ten times the length of a typical prokaryote. Their extra size is possible because their interior is divided into membrane-bound organelles, each doing a specialized job. DNA is packaged into several linear chromosomes inside a nucleus. Ribosomes are the larger 80S type, and a cytoskeleton of protein fibres gives the cell shape and moves materials around. Eukaryotic cells divide by mitosis, and most eukaryotes can also reproduce sexually using meiosis. Among single-celled eukaryotes, Amoeba moves and engulfs food with pseudopods (extensions of the cell), Paramecium swims with hair-like cilia, and algae photosynthesize. Fungi release digestive enzymes into their food and absorb the nutrients. Most fungi are made of thread-like hyphae forming a network called a mycelium; a mushroom is only the spore-producing part, and yeasts also reproduce by budding.

FeatureProkaryotic cellEukaryotic cell
DomainsBacteria, ArchaeaEukarya
Typical sizeAbout 1–10 µmAbout 10–100 µm
NucleusNone; DNA in a nucleoidYes, surrounded by a double membrane
DNAOne circular chromosome, often plasmidsSeveral linear chromosomes
Ribosomes70S (smaller)80S (larger)
Membrane-bound organellesNoneMitochondria, ER, Golgi, and chloroplasts in plants and algae
Cell wallUsually presentPlants (cellulose), fungi (chitin), some protists; animals have none
Cell divisionBinary fissionMitosis; meiosis for sexual reproduction
Body formUnicellularUnicellular or multicellular
Comparing prokaryotic and eukaryotic cells.

Viruses: on the edge of life

A virus is a tiny particle, usually about 20 to 300 nanometres (nm) across, so most can only be seen with an electron microscope. It has just two essential parts: a core of genetic material, either DNA or RNA (never both), and a protective protein coat called a capsid. Some viruses, such as influenza, also wrap themselves in an envelope made from a piece of the host cell's membrane. Viruses are host-specific: a virus's surface proteins must fit receptor molecules on a particular type of cell, which is why a bacteriophage infects bacteria but not your cells.

AViruses resemble living things because they
  • contain genetic material (DNA or RNA)
  • make copies of themselves, inside a host cell
  • mutate and evolve over time
BViruses are not considered cells or fully alive because they
  • have no cell membrane, cytoplasm or ribosomes
  • carry out no metabolism of their own and make no energy
  • cannot grow or reproduce outside a host cell; they must hijack the host's machinery
  • can exist as inert particles, and some can even be crystallized
  1. Attachment — the virus binds to specific receptors on the host cell surface.
  2. Entry — the viral genetic material enters the cell (a bacteriophage injects its DNA; many animal viruses are taken in whole).
  3. Replication — the host's ribosomes, enzymes and energy are used to copy viral genetic material and make viral proteins.
  4. Assembly — new capsids and genetic material are put together into new virus particles.
  5. Release — in the lytic cycle, the cell bursts (lyses) and releases many new viruses, killing the host.
Lytic and lysogenic cycles
In the lysogenic cycle, the viral DNA joins the host's chromosome and is copied quietly every time the cell divides, without making new viruses. Later, a trigger such as UV light or other stress on the host cell can switch it into the lytic cycle. Some animal viruses have a similar dormant stage, such as the herpes virus that causes cold sores, which is why some viral infections can go quiet and flare up again. Antibiotics target bacterial structures such as cell walls and 70S ribosomes, which viruses do not have, so antibiotics do not work against viruses.

The endosymbiotic theory

Where did the organelles of eukaryotic cells come from? The endosymbiotic theory, championed by the American biologist Lynn Margulis, proposes that mitochondria and chloroplasts were once free-living prokaryotes. A larger host cell engulfed an aerobic (oxygen-using) bacterium but did not digest it. The two came to depend on each other: the host supplied protection and food molecules, and the bacterium supplied a lot of usable energy through cellular respiration. Over many generations the bacterium became the mitochondrion. Later, some of these cells engulfed a photosynthetic bacterium related to today's cyanobacteria, which became the chloroplast, giving rise to algae and eventually plants.

1. EngulfmentA larger host cell engulfs anaerobic (oxygen-using) bacterium2. Living insideIt is not digested: it survivesinside, wrapped in two membranes3. MitochondrionOver many generations it becomesa mitochondrion (cell respiration)4. ChloroplastA later engulfed photosyntheticbacterium becomes a chloroplastAerobic bacteriumTwo membranesPhotosynthetic bacteriumMitochondriaChloroplastsLine leading to animals and fungiLine leading to plants and algaeEvidence: like bacteria, mitochondria and chloroplasts have their own circular DNA and small (70S) ribosomes,divide by a fission-like process, and are surrounded by two membranes.Mitochondria are most closely related to aerobic alphaproteobacteria; chloroplasts are related to cyanobacteria.
Fig 2A host cell engulfs an aerobic bacterium, which becomes a mitochondrion, and later a photosynthetic bacterium, which becomes a chloroplast.
Evidence for endosymbiosis
Mitochondria and chloroplasts (1) are about the size of bacteria; (2) have their own circular DNA, like bacterial chromosomes; (3) have their own 70S-type ribosomes, like bacteria rather than the 80S ribosomes of the surrounding cytoplasm; (4) are surrounded by a double membrane, as expected if a cell engulfed a bacterium inside a pocket of its own membrane; and (5) reproduce independently by splitting in two, like binary fission. The cell cannot make new mitochondria from scratch, only by dividing existing ones. DNA comparisons also show that mitochondrial genes most resemble those of certain aerobic bacteria, and chloroplast genes most resemble those of cyanobacteria.

Worked examples

Reveal one step at a time and try to beat it to the next line.

Example 1
A newly discovered microorganism has no nucleus, a single circular chromosome, 70S ribosomes, and a cell wall that contains no peptidoglycan. It was collected from a hot spring. Classify it to domain and explain your reasoning.
Example 2
A bacterium is 2 µm long, and a bacteriophage that infects it is 100 nm long. How many times longer is the bacterium than the virus?
Example 3
A classmate says, "The flu virus is alive because it reproduces and evolves." Evaluate this claim using the characteristics of living things.
Example 4
Explain how each observation supports the endosymbiotic theory: (a) mitochondria have circular DNA; (b) mitochondria have a double membrane; (c) a cell whose mitochondria are removed cannot make new ones.

Where marks get lost

  • Saying prokaryotes have no DNA or no ribosomes. They have both; the DNA is simply not enclosed in a nucleus, and the ribosomes are the smaller 70S type.
  • Mixing up micrometres and nanometres when comparing sizes. 1 µm = 1000 nm, so a 2 µm bacterium is 2000 nm, not 2 nm.
  • Thinking antibiotics can cure viral infections. Antibiotics attack bacterial structures such as cell walls and 70S ribosomes, which viruses do not have.
  • Stating that a virus contains both DNA and RNA. Each kind of virus uses one or the other as its genetic material.
  • Assuming all archaea live in extreme environments. Many do, but archaea are also common in soil, oceans and animal guts; the defining differences are molecular.

Flashcards 8

Nucleoid
Plasmid
Binary fission
Capsid
+ 4 more

Practice

1
Which structure is found in both prokaryotic and eukaryotic cells?
2
An animal cell is 20 µm across and a bacterium is 2 µm long. How many times larger is the animal cell's diameter than the bacterium's length?
3
Put the steps of the lytic cycle in order.
  1. 1Release
  2. 2Assembly
  3. 3Replication
  4. 4Attachment
  5. 5Entry
4
Match each structure to its function.
Capsid
Flagellum
Plasmid
Pilus
Mitochondrion
5
Which observation most strongly supports the idea that chloroplasts were once free-living bacteria?
6
What is the name of the protein coat that surrounds a virus's DNA or RNA?
7
Why are viruses not classified in any of the six kingdoms?
8
A prokaryotic cell has a thin cell wall surrounded by an outer membrane and stains pink in a Gram stain. It is:
9
Name the asexual process by which a bacterium divides into two identical cells.

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