B. Diversity of Living Things · Diversity of Living Things

Prokaryotes, Eukaryotes and Viruses Compared

B3.2 B2.1 A1.13About 75 minutes

Learning goals

I can compare prokaryotic cells, eukaryotic cells and viruses by genetic material, metabolism and structures.
I can label the structures of a bacterial cell and of two kinds of virus.
I can describe the lytic and lysogenic cycles of a virus.
I can argue whether viruses are living, using the characteristics of life.

Words to know

nucleoid
The region of a prokaryotic cell where its single, circular chromosome is found. It has no membrane around it.

plasmid
A small, circular piece of DNA in a bacterium, separate from the chromosome; it often carries genes such as antibiotic resistance.

ribosome
A structure that builds proteins. Prokaryotes have smaller (70S) ribosomes; eukaryotes have larger (80S) ones in their cytoplasm.

virus
A non-cellular particle made of genetic material (DNA or RNA) inside a protein coat; it can reproduce only inside a living host cell.

capsid
The protein coat that surrounds a virus's genetic material.

envelope
An outer membrane around some viruses, taken from the host cell's membrane.

bacteriophage
A virus that infects bacteria ("phage" for short).

lytic cycle / lysogenic cycle
Lytic: the virus takes over the cell, makes copies and bursts the cell. Lysogenic: the viral DNA joins the host's DNA and is copied quietly each time the cell divides.

host range (specificity)
The particular species or cell types a virus can infect.

Three very different "blueprints"

A Streptococcus bacterium, a cell from your cheek, and the influenza virus can all end up in the same place: your throat. Yet they are built in completely different ways. Two are cells. One is not a cell at all.

In this lesson you will compare all three in a structured way, using the three features the curriculum names: genetic material, metabolism and structures (organelles).

Remember from Grade 10

In Grade 10 you looked at plant and animal cells under a microscope and labelled organelles such as the nucleus, cell membrane, cytoplasm, vacuole, chloroplast and cell wall. You also researched a viral disease (tobacco mosaic virus). Both plant and animal cells are eukaryotic. Today we add the other cell type, the prokaryote, and look closely at viruses.

Prokaryotic cells

Pro means before and karyon means kernel or nucleus: prokaryotes have no nucleus. Bacteria and archaea are prokaryotes.

Genetic material: one main circular chromosome in a region called the nucleoid, plus small DNA circles called plasmids.
Structures: cell membrane, cytoplasm and small (70S) ribosomes. No membrane-bound organelles (no mitochondria, no chloroplasts, no ER). Most have a cell wall; many have a sticky capsule, hair-like pili for attaching, and one or more flagella for swimming.
Metabolism: very diverse. Reactions such as cellular respiration or photosynthesis happen on the cell membrane or in the cytoplasm, because there are no organelles. Some need oxygen (aerobic), some are killed by it (obligate anaerobes), some can switch.
Size: usually about 1 to 10 µm (micrometres) long.

A typical bacterial cell. Its DNA floats in the nucleoid with no nuclear membrane, and it has no membrane-bound organelles. The capsule, pili and flagellum are present in many bacteria but not all.

Eukaryotic cells

Eu means true: eukaryotes have a true nucleus. Protists, fungi, plants and animals are eukaryotes.

Genetic material: several linear chromosomes, wrapped around proteins called histones, inside a nucleus with a double membrane.
Structures: many membrane-bound organelles: mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, and in plants and algae chloroplasts and a large central vacuole. Larger (80S) ribosomes in the cytoplasm. A cytoskeleton gives shape and moves things around.
Metabolism: compartments let different reactions happen in different places, such as cellular respiration in mitochondria and photosynthesis in chloroplasts.
Size: usually about 10 to 100 µm across.

Viruses

A virus is not a cell. It is a package of genetic material in a protein coat.

Genetic material: DNA or RNA, never both. It may be single- or double-stranded.
Structures: a protein capsid; some viruses also have a lipid envelope taken from the host's membrane, studded with spike proteins that attach to host cells. Viruses have no ribosomes, no cytoplasm, no membrane of their own and no organelles.
Metabolism: none. A virus cannot make energy or proteins. Outside a host it is an inert particle.
Reproduction: only by taking over a host cell's machinery.
Size: most are about 20 to 300 nm (0.02 to 0.3 µm), far too small for a light microscope; an electron microscope is needed.

Each virus has a host range: its spike or tail proteins fit specific receptor molecules on certain cells. That is why tobacco mosaic virus infects plants but not people.

Two virus designs. A bacteriophage injects its DNA through its tail into a bacterium. An enveloped virus such as influenza is wrapped in a membrane stolen from its last host cell, with spike proteins that attach to new cells.

Feature	Prokaryotic cell	Eukaryotic cell	Virus
Genetic material	DNA: one circular chromosome + plasmids, in nucleoid	DNA: several linear chromosomes in a nucleus	DNA or RNA in a capsid
Nucleus	no	yes	no (not a cell)
Membrane-bound organelles	no	yes	no
Ribosomes	yes, smaller (70S)	yes, larger (80S)	no
Metabolism	yes, very diverse	yes, in compartments	none outside a host
Reproduction	binary fission	mitosis / meiosis	only inside a host cell
Typical size	1 to 10 µm	10 to 100 µm	20 to 300 nm
Examples	E. coli, cyanobacteria, methanogens	amoeba, yeast, maple cell, human cell	influenza, tobacco mosaic virus, bacteriophage T4

Comparing the three by genetic material, metabolism and structures.

Worked example

Let's compare sizes with unit conversions

A bacterium is 2 µm long. A virus is 100 nm across. A human cheek cell is 60 µm across. (1 µm = 1000 nm.) (a) How many viruses would fit in a row along the bacterium? (b) How many bacteria would fit in a row across the cheek cell?

Put both lengths in the same unit first.
(a) 2 µm = 2 × 1000 nm = 2000 nm. Number of viruses = 2000 nm ÷ 100 nm = 20.
(b) Both are in µm: 60 µm ÷ 2 µm = 30 bacteria.
So the cheek cell is 30 times longer than the bacterium, which is 20 times longer than the virus: the cheek cell is 600 times the width of the virus.

(a) 20 viruses; (b) 30 bacteria.

How viruses reproduce: lytic and lysogenic cycles

Biologists first worked out viral reproduction in bacteriophages.

Lytic cycle (fast and destructive):

Attachment: tail fibres or spike proteins bind to receptors on the host cell.
Entry: the viral DNA or RNA enters the cell.
Synthesis: the viral genes take over. The host's ribosomes, enzymes and energy make viral nucleic acid and viral proteins.
Assembly: new virus particles are put together.
Lysis and release: the cell bursts (or, for many enveloped viruses, the viruses bud out through the membrane), releasing dozens to hundreds of new viruses.

Lysogenic cycle (quiet and hidden): after entry, the viral DNA inserts itself into the host chromosome, where it is called a prophage. Every time the host cell divides, the prophage is copied too. Later, a trigger such as stress or UV light can switch the virus into the lytic cycle. Some human viruses behave in a similar way: the virus that causes chickenpox can stay inactive in nerve cells for years and later reactivate as shingles.

In the lytic cycle (left) the virus quickly makes copies and bursts the cell. In the lysogenic cycle (right) the viral DNA hides inside the host chromosome and is copied with every cell division, until a trigger sends it into the lytic cycle.

Are viruses alive?

Biologists disagree, and both sides use evidence.

Not alive: viruses are not made of cells, have no metabolism, do not grow, and cannot reproduce on their own. Outside a host they can even be crystallized like a chemical (tobacco mosaic virus was crystallized in the 1930s).
Alive, in a way: they contain genetic material, reproduce (with help), and evolve by natural selection: new flu strains appear every year.

Many biologists describe viruses as being "at the edge of life." Because they are not cells, viruses are not placed in any kingdom or domain. They are classified separately, by their type of nucleic acid, capsid shape, envelope and host.

Just for fun

A true surprise: in 2003 scientists announced that an organism they had first taken for a small bacterium was really a giant virus, now called mimivirus. It is larger than some bacteria. The name even comes from "mimicking microbe."

Hands-on activity

Build a scale model of the microscopic world

You need: a measuring tape (at least 3 m), a dried pea or bead about 1 cm across, a ruler, sticky notes, and some floor space or a hallway.

Scale: 1 cm in the model = 100 nm in real life.

The pea is your virus (100 nm → 1 cm).
Work out the model size of a bacterium 2 µm long. (2 µm = 2000 nm; 2000 ÷ 100 = 20, so 20 cm.) Cut or draw a 20 cm strip of paper.
Work out a cheek cell 60 µm across. (60 000 nm ÷ 100 = 600 cm = 6 m.) Measure 6 m along a hallway and mark both ends with sticky notes.
Work out the width of a human hair, about 70 µm. (Answer: 7 m.)
Stand at one end of your 6 m "cell" holding the pea. Write three sentences about what surprised you, and explain why we need an electron microscope to see viruses.

What to look for: correct conversions (20 cm, 6 m, 7 m) and an understanding that a virus is tiny compared with a bacterium, which is tiny compared with a eukaryotic cell. Keep the pea away from small children (choking hazard).

Your turn

List three structures found in all prokaryotic cells.

Give two structural differences between prokaryotic and eukaryotic cells.

Where does cellular respiration take place in a bacterium, which has no mitochondria?

What are the two parts that every virus has?

Why can't antibiotics that stop bacterial ribosomes work against viruses?

In the lysogenic cycle, what is a prophage?

Convert 250 nm to µm.

More practice: warm-up

What does prokaryote mean?

Name the protein coat of a virus.

Which kind of microscope is needed to see most viruses?

What is a plasmid?

In which cycle does the host cell burst?

More practice: on your own

Explain why an enveloped virus has a membrane even though viruses do not make membranes.

Tobacco mosaic virus infects tobacco plants but not humans. Explain using host range.

A bacterium is 3 µm long and a eukaryotic cell is 45 µm across. How many times wider is the eukaryotic cell? Show units.

Why might a lysogenic virus be harder for a host population to get rid of than a lytic virus?

Give one argument that viruses are alive and one that they are not.

Challenge

A biologist finds a new structure 150 nm across with RNA inside a protein coat, no ribosomes and no ability to make ATP. Should she classify it in a kingdom? Explain.

A eukaryotic cell is about 10 times wider than a typical bacterium. About how many times greater is its volume? Explain, assuming both are spheres.

Mitochondria contain their own small circular DNA and 70S-type ribosomes. What does this suggest about their origin? (You will study this more in the lesson on how structures evolved.)

Check your understanding

Quiz

Which structure is found in prokaryotic cells?

A virus contains:

Where is the DNA of a bacterium found?

In the lysogenic cycle, the viral DNA:

Most viruses are about:

Why are viruses not placed in any kingdom?

How many 200 nm viruses fit in a row along a 4 µm bacterium?

For parents and teachers

Big idea (B3.2): prokaryotes, eukaryotes and viruses differ in their genetic material, metabolism and structures. Viruses are not cells and reproduce only inside hosts (lytic and lysogenic cycles). Unit conversions (nm, µm) practise A1.13.

How to help: The scale-model activity makes sizes real. Ask your teen to explain to you why antibiotics do not work against colds or flu (viruses have no bacterial structures to target).

Common difficulties:

Saying bacteria have no DNA because they have no nucleus. They do; it is in the nucleoid.
Saying viruses are tiny bacteria. They are not cells at all.
Unit errors: 1 µm = 1000 nm; convert before dividing.
Mixing up lytic (burst) and lysogenic (hide).

Sources: cell and virus features and sizes from standard biology texts; mimivirus announced by La Scola et al. (2003, Science); TMV crystallized by Wendell Stanley (1935).