Viruses: structure, classification and replication – Introduction to Classification (Senior 4 Biology) | Ganzaa

Introduction to Classification

Lessons

Senior 4 · Biology

Introduction to Classification

5 lessons

Lesson 4 of 5

Viruses: structure, classification and replication

25 min

An infection that passed through a filter

In 1892 the Russian botanist Dmitri Ivanovsky squeezed sap from tobacco plants with mosaic disease, their leaves mottled light and dark green, and passed it through a porcelain filter whose pores were fine enough to hold back every known bacterium. The filtered sap still infected healthy plants. In 1898 the Dutch microbiologist Martinus Beijerinck repeated the experiment and showed that the agent multiplied inside the living plant, so it could not be a simple poison. He called it a virus. In 1935 Wendell Stanley purified tobacco mosaic virus into crystals that were still infectious, something no living cell can do.

Viruses are not cells, so they fit into neither the domains nor the kingdoms of Lesson 2. This lesson describes how they are built, how they are classified, and how they multiply.

Definition

Virus: a non-cellular infectious particle, typically 20–300 nm across, made of a nucleic acid (DNA or RNA, never both) inside a protein coat; it can multiply only inside a living host cell. A complete virus particle outside a cell is a virion.

Capsid: the protein coat of a virus, built from many identical protein units called capsomeres.

Envelope: in some viruses, an outer lipid membrane taken from the host cell when the virus leaves it, studded with viral glycoproteins.

Bacteriophage (phage): a virus that infects bacteria. Example: phage T4, which infects Escherichia coli.

Obligate intracellular parasite: an agent that can reproduce only inside the cells of a host.

How viruses are built

Every virus has a genome of nucleic acid and a capsid. The genome may be DNA or RNA, single-stranded or double-stranded. The capsomeres pack in one of a few regular ways, which give viruses their shapes (Figure 1). Some viruses also carry enzymes, such as the reverse transcriptase of HIV, and some are wrapped in an envelope.

Classifying viruses

Basis	Groups	Examples
Type of nucleic acid	DNA viruses or RNA viruses; single-stranded (ss) or double-stranded (ds)	dsDNA: adenovirus, phage T4, smallpox virus; ssRNA: tobacco mosaic virus, polio, measles, influenza, HIV
Capsid symmetry	Helical (rods), icosahedral (20 triangular faces), complex	Helical: TMV; icosahedral: adenovirus, polio; complex: phage T4
Envelope	Naked or enveloped	Naked: TMV, polio, adenovirus; enveloped: HIV, influenza, measles
Host	Plant, animal or bacterial (bacteriophage) viruses	Plant: TMV, cassava mosaic viruses; animal: rabies, HIV; bacterial: T4

Virologists also use the Baltimore classification, which sorts viruses into seven groups by their genome (DNA or RNA, single- or double-stranded) and the route by which they make messenger RNA. HIV, for example, is a retrovirus: its RNA is copied into DNA by reverse transcriptase, and that DNA is inserted into the host cell's chromosome.

Worked example 1: why viruses passed through the filter

A tobacco mosaic virus particle is about 300 nm long and 18 nm wide; an E. coli cell is about 2 µm long. (a) How many times longer is the bacterium than the virus? (b) The best light microscopes can separate points about 200 nm apart. Explain why TMV can be seen only with an electron microscope.

(a) Convert to the same unit: $2\,\mu\mathrm{m} = 2 \times 1000 = 2000\,\mathrm{nm}$ . Ratio $= \frac{2000}{300} = 6.7$ : the bacterium is about 7 times longer, and its volume is far larger still.
(b) The virus is only 18 nm wide, about $\frac{200}{18} \approx 11$ times smaller than the smallest distance a light microscope can resolve, so it cannot be seen as a separate object. An electron beam has a much shorter wavelength, so an electron microscope resolves objects of a few nanometres (Unit 3).

Transmission electron micrograph of purified tobacco mosaic virus: many thin, straight, rigid rods of the same width lie criss-crossed on the grid. Each rod is a helical capsid around one strand of RNA. The scale bar is 0.2 µm. Image: T. Moravec, public domain, via Wikimedia Commons

How a virus multiplies: the lytic cycle

A virus has no ribosomes and no enzymes for respiration, so it must use the host cell's machinery. Phage T4 follows the lytic cycle:

Attachment: the tail fibres bind to specific receptor molecules on the surface of an E. coli cell.
Penetration: the tail sheath contracts and the phage injects its DNA through the cell wall; the empty capsid stays outside.
Synthesis: phage genes take over the cell, which now copies phage DNA and makes phage proteins on its own ribosomes, using its own ATP.
Assembly: new heads, tails and DNA come together into complete phages.
Release: a phage enzyme breaks down the cell wall, the cell bursts (lysis), and about 100 or more new phages escape to infect other cells.

Some phages instead follow a lysogenic cycle: their DNA is inserted into the bacterial chromosome as a prophage and copied every time the bacterium divides, until a stress such as ultraviolet light switches it into the lytic cycle. Enveloped animal viruses such as HIV leave a cell by budding, wrapping themselves in a piece of its membrane, so the cell need not burst at once.

Worked example 2: how fast can phages multiply?

One phage infects a culture containing millions of bacteria. Each lytic cycle takes 30 minutes and releases 100 phages, and every new phage infects a fresh bacterium. How many phages are there after 90 minutes?

Number of cycles: $\frac{90\,\text{min}}{30\,\text{min}} = 3$ .
Each cycle multiplies the number of phages by 100: after 1 cycle 100, after 2 cycles $100^2 = 10\,000$ , after 3 cycles $100^3 = 1\,000\,000$ .
So up to $10^{6}$ phages in 90 minutes. In practice the number is lower, because not every phage meets a bacterium. This explosive multiplication is why a clear patch (a plaque) appears in a lawn of bacteria within hours.

Shadowed transmission electron micrograph of two T4 bacteriophages: each has an angular head, a straight tail and thin, bent tail fibres spreading from the end of the tail, as in Figure 1D. Image: AFADadcADSasd, CC BY 4.0, via Wikimedia Commons

Living or not?

Features of living things that viruses show	Features of living things that viruses lack
They contain genetic material (DNA or RNA)	They are not made of cells
They reproduce, though only inside host cells	They do not respire, feed or excrete; they have no metabolism of their own
Their genes mutate, so they evolve (new strains of influenza and HIV)	They do not grow or respond to stimuli; outside a cell they are inert and some can be crystallised

Most biologists therefore place viruses on the border between living and non-living. Because they use the host's own ribosomes and enzymes, antibiotics, which target bacterial structures such as peptidoglycan and 70S ribosomes, have no effect on them. Viral diseases are prevented by vaccines (measles, polio, hepatitis B) and some are treated with antiviral drugs, such as the antiretroviral drugs used against HIV.

Amoeba Sisters: "Viruses (Updated)". Follow the steps of the lytic and lysogenic cycles, and note why viruses need a host cell to reproduce.

Warning

Common mistakes

Calling viruses "very small bacteria". Viruses are not cells: no cytoplasm, no ribosomes, no cell membrane of their own.
Saying a virus contains both DNA and RNA. Each virus has one or the other.
Treating a viral infection, such as a cold or influenza, with antibiotics. They do not work on viruses and their misuse breeds resistant bacteria.
Writing that the envelope is made by the virus. Its lipid comes from the host cell's membrane; only the glycoproteins in it are viral.

Tip

Memory hook for the lytic cycle: "All Phages Should Avoid Rain": Attachment, Penetration, Synthesis, Assembly, Release.

✦

Practice: viruses

Name the two parts that every virus has, and two parts that only some viruses have.
Explain what Ivanovsky's filtration experiment showed, and why Beijerinck's further result mattered.
Classify HIV by nucleic acid, envelope and host.
Describe the five stages of the lytic cycle of phage T4.
A virus particle is 120 nm across. What is this in micrometres? How many would fit side by side across a lymphocyte 7 µm wide?
Explain why antibiotics cannot cure influenza, but vaccination can prevent it.
Evaluate the statement "Viruses are living organisms."

Show answersHide answers

All: nucleic acid (DNA or RNA) and a protein capsid. Some: a lipid envelope with glycoprotein spikes; enzymes such as reverse transcriptase; tail fibres (phages).
The agent passed through filters that held back bacteria, so it was smaller than any bacterium. Beijerinck showed it multiplied in living plant tissue, so it was an infectious agent that reproduced, not a toxin.
Single-stranded RNA (a retrovirus), enveloped, infects animal (human) cells, especially lymphocytes.
Attachment of tail fibres to receptors; penetration (injection of DNA); synthesis of phage DNA and proteins by the host's machinery; assembly of new phages; release by lysis of the cell.
$120\,\mathrm{nm} = \frac{120}{1000} = 0.12\,\mu\mathrm{m}$ ; $\frac{7}{0.12} = 58$ particles (58.3).
Antibiotics act on bacterial structures (cell walls, 70S ribosomes) that viruses do not have; the virus uses the host's own machinery. A vaccine primes the immune system to make antibodies and memory cells against the virus's antigens, so a later infection is stopped quickly.
For: they have genes, reproduce and evolve. Against: they are not cells, have no metabolism, do not grow or respond, and can only reproduce inside a host cell; outside it they are inert particles that can be crystallised. A reasonable conclusion is that they lie on the border: they show some characteristics of life only when inside a host cell.

Quick check

Put the stages of the lytic cycle of phage T4 in order.

Tap the cards in the right order.

Key points

A virus is a non-cellular particle of DNA or RNA in a protein capsid, sometimes with a lipid envelope; it reproduces only inside host cells.
Viruses are classified by nucleic acid (DNA or RNA, single or double strand), capsid symmetry (helical, icosahedral, complex), envelope and host.
Lytic cycle: attachment, penetration, synthesis, assembly, release; in the lysogenic cycle the viral DNA hides in the host chromosome.
Viruses lie on the border of life; antibiotics do not affect them, but vaccines and antiviral drugs can.

Sources and credits

5.1.2: Viral Evolution, Morphology, and Classification and 5.1.3: Virus Infections and Hosts, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for virus structure, classification and the lytic and lysogenic cycles. Adapted by Ganzaa.
Tobacco mosaic virus, Wikipedia contributors, licensed under CC BY-SA 4.0. Used for the history of its discovery and its dimensions. Adapted by Ganzaa.
Figure 1, the worked examples, practice and quick check: written by Ganzaa.
Images: T. Moravec, public domain; AFADadcADSasd, CC BY 4.0; Centers for Disease Control and Prevention (C. Goldsmith and colleagues), public domain; all via Wikimedia Commons.
Video: "Viruses (Updated)", Amoeba Sisters, embedded from YouTube.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1An infection that passed through a filter

2How viruses are built

3Classifying viruses

4How a virus multiplies: the lytic cycle

5Living or not?

An infection that passed through a filter

How viruses are built

Classifying viruses

How a virus multiplies: the lytic cycle

Living or not?