Campbell Biology · Chapter 19

Viruses

pp. 381–395 · 3 sections

Viruses are packets of genes in a protein coat that can copy themselves only by taking over a living cell. This chapter covers how they're built, how they hijack bacteria and animal cells, and how viruses, viroids and prions cause disease. For the AP course, the parts that count most are retroviruses and reverse transcriptase (Topic 6.4), viruses moving and mixing genes (Topic 6.7) and pathogens evolving into new diseases (Topic 7.8); the rest is solid background for a class test.

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Chapter quiz (36 questions)Flashcards (43 terms)

19.1 What a virus is made of

pp. 381–384

On the AP exam? Background

Viruses aren't a topic of their own, but Topic 6.1 expects you to know that some viruses keep their genes in RNA instead of DNA. The discovery story, capsid shapes and virus sizes won't be tested.

In the course: Topic 6.1 DNA and RNA Structure, Topic 2.3 Plasma Membrane, Topic 1.7 Proteins, Topic 6.4 Translation (notes, videos and more questions)

Key points

A virus is a set of genes, written in DNA or RNA, packed inside a protein shell. It has no ribosomes, makes no ATP and runs no metabolism, so outside a cell it just sits there.
Are viruses alive? They can't make energy, build proteins or reproduce by themselves, so most biologists don't count them as living. Still, they carry genes written in the same code cells use, and they evolve, so they're clearly tied to the living world.
Viruses were first noticed in the late 1800s as a disease agent small enough to slip through filters that trap bacteria. It multiplied only inside living hosts, never in a lab dish of nutrients. Electron microscopes later made them visible.
A viral genome may be DNA or RNA, and either kind can be single- or double-stranded. Many viruses keep the whole genome on one molecule, a straight strand or a loop; others, such as flu, split it into separate segments.
Sizes vary a lot. The smallest viruses are about 20 nm across and carry only a few genes. Giant viruses found since 2003 can be about 1 µm long, as big as small bacteria, with well over 1,000 genes, yet they still can't make their own ribosomes.
The protein coat, or capsid, is assembled from repeating subunits called capsomeres, usually coded by just one or a few viral genes. The subunits can spiral into a rod (helical viruses), fit together into a 20-sided ball (icosahedral viruses) or build fancier shapes, such as a phage's head and tail.
Many animal viruses also have an envelope: a membrane taken from the host cell, studded with viral glycoproteins that latch onto host cells. Bacteriophages, or phages, are viruses that infect bacteria, and viruses of some kind infect every group of living things.

Key terms (13)

virus: A tiny infectious particle made of genes (DNA or RNA) in a protein coat. It can only make more of itself inside a living host cell.
viral genome: All the genetic material a virus carries. Depending on the virus, it's DNA or RNA, single- or double-stranded, and sometimes in several pieces.
DNA virus: A virus whose genes are stored in DNA, either single- or double-stranded.
RNA virus: A virus whose genes are stored in RNA. Flu, measles and coronaviruses are examples.
capsid: The protein shell that surrounds and protects a virus's genome.
capsomere: One of the many protein building blocks that fit together to form a capsid.
helical virus: A virus whose capsid proteins spiral around the genome to make a rod or long thread.
icosahedral virus: A virus with a roughly ball-shaped capsid made of 20 triangular faces.
viral envelope: An outer membrane around some viruses' capsids. It's made from the host cell's membrane plus proteins coded by the virus.
glycoprotein: A protein with sugar chains attached. On a virus, these often form the spikes that grab onto host cells.
bacteriophage (phage): A virus that infects bacteria. Many have a head holding DNA and a tail used to attach and inject it.
giant virus: A virus as large as a small bacterium, with hundreds to over a thousand genes. It still depends on a host cell's ribosomes.
host cell: The living cell a virus infects and uses to make new viruses.

Check yourself: 19.1 What a virus is made of

4 questions on 19.1 What a virus is made of. Pick an answer to see if you got it, and why.

Question 1 of 4

An enveloped virus particle is examined after it leaves an infected cell. Which part of the particle was built from material that the host cell made using its own genes, not viral genes?

Question 2 of 4

Researchers measure the bases in the genome of a newly discovered virus and find: A 31%, U 22%, G 28%, C 19%, and no T. Which best describes this genome?

Question 3 of 4

Alcohol hand sanitizer quickly inactivates many enveloped viruses, such as flu viruses and coronaviruses, but it works poorly against norovirus, which has no envelope. What best explains the difference?

Question 4 of 4

A student argues that viruses should be called living things because their genes mutate and they evolve by natural selection. Which observation gives the strongest evidence for the opposing view?

0 of 4 answered

19.2 How viruses copy themselves inside cells

pp. 384–390

On the AP exam? Yes

Topic 6.4 tests retroviruses: reverse transcriptase copies RNA into DNA, which joins the host's chromosome. Topic 6.7 covers phages carrying genes between bacteria. The lytic and lysogenic cycles aren't named in the current course, but they help you explain both; virus classes and the origin debate are background.

In the course: Topic 6.4 Translation, Topic 6.7 Mutations, Topic 6.5 Regulation of Gene Expression, Topic 7.2 Natural Selection, Topic 2.3 Plasma Membrane (notes, videos and more questions)

Key points

A virus can multiply only inside a host cell. It brings the instructions and sometimes a few enzymes; the building blocks, the energy and the protein-making machinery all come from the host.
Infection starts when viral proteins fit specific receptor molecules on a cell's surface. That fit sets the host range: most viruses infect only a few species, and often only certain tissues.
The basic cycle: attach, get the genome inside (by injection, membrane fusion or endocytosis), copy the genome, make viral proteins, let the parts assemble on their own, then leave, often killing the cell. Most DNA viruses can use the cell's own DNA-copying enzymes, but cells have no enzyme for making RNA from an RNA template, so RNA viruses must carry or encode one. Some RNA genomes can be read directly as mRNA; others must first be copied into a matching strand.
Phages show two strategies. In a lytic cycle, the phage makes many copies of itself and bursts the cell. In a lysogenic cycle, a temperate phage slips its DNA into the bacterial chromosome as a prophage, which gets copied every time the cell divides. A stress such as UV light can switch it back to the lytic cycle, and some prophage genes change the host, like the toxin genes that make cholera bacteria dangerous.
Bacteria fight back. Mutant cells with changed receptors escape infection, restriction enzymes cut up foreign DNA (the cell's own DNA is protected by methyl tags), and CRISPR-Cas systems store bits of old phage DNA so they can recognize and cut that phage next time. Phages evolve in return, so the two are locked in an arms race.
Enveloped animal viruses make their spike glycoproteins on the rough ER, finish them in the Golgi and ship them to the plasma membrane. New viruses bud out wrapped in that membrane. Retroviruses like HIV carry reverse transcriptase, which copies their RNA genome into DNA. That DNA becomes a permanent provirus in a host chromosome, and the cell's own RNA polymerase reads it to make viral mRNA and new genomes.
Phages sometimes package bacterial DNA by mistake and deliver it to another cell (transduction), one way genes move between bacteria. Because viruses need cells, most biologists think they arose after cells did, maybe on several separate occasions, from mobile genetic elements such as plasmids or transposons that picked up genes for a coat. Leftover retrovirus DNA makes up about 8% of the human genome.

Key terms (15)

obligate intracellular parasite: Something that can reproduce only inside a host cell. All viruses are this way.
host range: The set of species, and often the tissues, a virus can infect. It depends on which cells have receptors the virus can bind.
lytic cycle: A phage life cycle that ends with the host cell bursting open and releasing new phages.
virulent phage: A phage that can only follow the lytic cycle, so it always kills the cell it infects.
lysogenic cycle: A phage life cycle in which the phage's DNA joins the bacterial chromosome and gets copied along with it, without killing the cell.
temperate phage: A phage that can follow either the lytic or the lysogenic cycle.
prophage: Phage DNA that has been inserted into a bacterium's chromosome. It can later pop out and start a lytic cycle.
restriction enzyme: A bacterial enzyme that cuts DNA at a specific short sequence, chopping up invading phage DNA. Scientists use these enzymes as DNA scissors.
CRISPR-Cas system: A bacterial defense that saves pieces of past invaders' DNA and uses them to guide Cas proteins to cut matching DNA. It's also the basis of CRISPR gene editing.
viral RNA polymerase: A virus-made enzyme that builds RNA using an RNA template. Uninfected cells don't have one, so RNA viruses must bring or code for it.
retrovirus: An RNA virus that copies its genome into DNA and inserts that DNA into the host's chromosomes. HIV is one.
reverse transcriptase: The retrovirus enzyme that builds DNA from an RNA template, running information backward from the usual DNA → RNA.
provirus: Retrovirus DNA that has been inserted into a host chromosome. It stays there for the life of the cell and is passed to daughter cells.
transduction: Moving bacterial genes from one cell to another inside a phage that picked them up by mistake.
budding: How many enveloped viruses leave a cell: they push out through the plasma membrane and take a piece of it with them.

Check yourself: 19.2 How viruses copy themselves inside cells

4 questions on 19.2 How viruses copy themselves inside cells. Pick an answer to see if you got it, and why.

Question 1 of 4

Cultured cat cells are infected with feline leukemia virus, a retrovirus. A drug that blocks reverse transcriptase is added either at the moment of infection or 48 hours later, after viral DNA has already joined the host chromosomes. Treatment | Virus particles released on day 5 (% of no-drug control) No drug | 100 Drug added at 0 hours | 2 Drug added at 48 hours | 85 Which best explains why the drug added at 48 hours had little effect?

Question 2 of 4

Every cell in a bacterial culture carries the DNA of phage φ in its chromosome, and the culture has grown normally for many generations. After a brief dose of UV light, the culture turns from cloudy to nearly clear within 90 minutes, and the liquid becomes full of phage particles. What best explains this?

Question 3 of 4

A virus infects human liver cells but not human skin cells, and it can't infect mouse liver cells. What best explains this narrow host range?

Question 4 of 4

Zika virus and respiratory syncytial virus (RSV) both have single-stranded RNA genomes. If you inject purified Zika RNA alone into a cell, it starts an infection, but purified RSV RNA alone does not. What best explains the difference?

0 of 4 answered

19.3 Viral disease, new outbreaks, viroids and prions

pp. 390–394

On the AP exam? Yes

Topic 7.8 tests the idea that pathogens keep evolving and cause emerging diseases, and Topic 6.7 covers related viruses swapping genes in one host cell. Vaccines, antiviral drugs, plant viruses, viroids and prions won't be tested, though prions are a vivid example of a protein's shape deciding what it does (Topic 1.7).

In the course: Topic 7.8 Continuing Evolution, Topic 6.7 Mutations, Topic 1.7 Proteins, Topic 4.1 Cell Communication, Topic 8.5 Community Ecology (notes, videos and more questions)

Key points

Viruses cause symptoms by damaging or killing cells, by making toxic products, and by setting off the body's own defenses, which bring fever and aches. Damage lasts longer in tissues that can't replace their cells: the gut lining renews itself within days, but heart muscle cells and the sound-sensing hair cells of the inner ear are rarely or never replaced.
Vaccines train the immune system with a harmless stand-in: a weakened or killed virus, one viral protein, or mRNA that has your cells make one viral protein for a short time. A worldwide vaccination campaign wiped out smallpox, declared gone in 1980.
Antibiotics don't work on viruses because they target bacterial parts, such as cell walls and bacterial ribosomes. Antiviral drugs instead block virus-specific enzymes, like viral polymerases, reverse transcriptase or viral proteases. Combining drugs that hit different steps keeps HIV in check for decades.
Emerging viruses are rarely brand new. They come from mutation of existing viruses, from a virus spreading out of a small, isolated group of people, or from a jump out of an animal host. Most new human diseases come from animals, often from a natural reservoir species that carries the virus without getting sick.
RNA viruses mutate fast because most of their copying enzymes don't proofread (coronaviruses are an exception, with a partial proofreading enzyme). Flu changes in two ways: small mutations in its H and N surface proteins each year, which is why the vaccine is updated, and swapping whole genome pieces when two strains infect one cell, which can create a strain few people are immune to.
Plant viruses, mostly RNA viruses, get in through wounds made by insects, tools or weather (horizontal transmission) or are passed from a parent through seeds or cuttings (vertical transmission). Viral movement proteins widen plasmodesmata so the virus spreads from cell to cell. Infected plants can't be cured, so growers rely on controlling insect carriers, using clean tools and planting resistant varieties.
Viroids are naked circles of RNA, roughly 250 to 400 nucleotides in all, that code for no protein but are copied by plant enzymes and stunt plant growth. Prions are wrongly folded forms of a normal brain protein that bend normal copies into the bad shape. They clump, cause slow, fatal brain diseases such as chronic wasting disease in deer and Creutzfeldt-Jakob disease in people, and survive cooking.

Key terms (15)

pathogen: Anything that causes disease, such as a virus, bacterium, viroid or prion.
vaccine: A harmless version or piece of a pathogen, or instructions for making one piece, that trains your immune system to fight the real thing.
antiviral drug: A medicine that slows a virus by blocking one of its own enzymes or steps, such as copying its genome or cutting its proteins to size.
emerging virus: A virus that suddenly shows up as a disease in a population, usually an existing virus that changed or reached new hosts.
epidemic: A sudden rise in cases of a disease in one region.
pandemic: An epidemic that spreads across many countries or continents.
zoonotic spillover: When a pathogen that normally lives in animals jumps into humans.
natural reservoir: A species that carries a pathogen, often without getting sick, and can pass it to other species.
reassortment: Mixing of whole genome segments when two strains of a segmented virus, like flu, infect the same cell.
hemagglutinin and neuraminidase (H and N): Two flu surface proteins. H helps the virus attach to cells and N helps new viruses get free; flu strains are named by their types, like H3N2.
horizontal transmission: Spread of a plant virus from an outside source, such as an insect, a tool or another plant.
vertical transmission: Passing a plant virus from a parent to its offspring, through seeds or cuttings.
movement protein: A plant-virus protein that widens plasmodesmata so viral genomes can pass from one cell to the next.
viroid: A small circle of RNA with no protein coat and no protein-coding genes that infects plants.
prion: A misfolded protein that spreads disease by forcing normal copies of the same protein into its misfolded shape.

Check yourself: 19.3 Viral disease, new outbreaks, viroids and prions

4 questions on 19.3 Viral disease, new outbreaks, viroids and prions. Pick an answer to see if you got it, and why.

Question 1 of 4

An antibiotic kills bacteria by blocking their ribosomes, which differ in shape from the ribosomes of human cells. Why would this drug do nothing to stop a viral infection of human cells?

Question 2 of 4

Researchers infect a culture of dog kidney cells, a standard lab host for flu, with two flu strains at once: a duck strain (H4N6) and a human strain (H3N2). Among the new viruses released, some are H4N2 and some are H3N6. These new combinations stay the same after ten more rounds of infection in fresh cells. What best explains the result?

Question 3 of 4Calculator allowed

Question 4 of 4

Brain tissue from a deer with chronic wasting disease is tested. It stays just as infectious after treatment with enzymes that destroy all DNA and RNA, and after ultraviolet light strong enough to wreck nucleic acids. It loses most of its infectiousness after treatment with chemicals that unfold proteins. Which conclusion do these results best support?

0 of 4 answered