CHAPTER 21: VIRUSES

First isolated by Dmitri Ivanovsky
● Filtered sap from an infected
tobacco plant could still infect a healthy tobacco plant because it contained tobacco mosaic virus.
● Later identified that infectious agent was viruses (rather than bacteria)

In the 1930s, a new technology made viruses visible by microscopy
• Electron microscopy

• Almost all viruses are too small to be seen with light microscopes.
• A single virus particle (a “virion”) may be only 20 nm across.
The invention of electron microscopes led to the discovery of many types of viruses in many types of organisms.

They are “acellular” (not cells).
• They have no metabolism and cannot grow.
• They depend on host cells to make copies of themselves.
• They are the ultimate parasite; cannot replicate unless they are in host cell using that
cell’s machinery
• Their origin is uncertain.
• They seem to have collected pieces of nucleic acid from various sources.
• They can infect nearly all lifeforms.
• They are not only species specific but tissue specific (as far as what they will infect)

Viruses are very simple:
○ Capsid: outer protein coat
○ Nucleic acid genome: single- or
double-stranded DNA or RNA
○ Envelope: membrane covering
capsid (not all viruses)
● Viruses cannot replicate on their own. Require cellular machinery of host cells to survive and replicate.
● Detection: Polymerase chain reaction (PCR) or immunoassays.

Viruses evolved from more complex free- living organisms that lost genetic material as they adapted to a parasitic lifestyle.

Viruses may stem from pieces of RNA and DNA that escaped from a host cell and gained the ability to move between cells

Viruses existed before all life as self-replicating entities that over time became more organized and more complex.

Filamentous: long, thin, worm-like shape
● Isometric (or icosahedral): spherical-shape
● Enveloped: have membranes that surrounding the capsids
● Complex / Head and tail: infect bacteria and have a head that is similar to icosahedral viruses and a tail shaped like helical viruses

either DNA or RNA (but not both)
• may be single-stranded or double-stranded • may be circular or linear
• may be in one piece or in multiple segments
• The genome is much smaller than a cell’s genome.
• DNA directs the host cell to make new virus copies.
• RNA may be directly transcribed, or replicated first. • RNA viruses use enzymes that make more errors • RNA viruses mutate more frequently

Often double-stranded, but can be single-stranded
● Replication takes place in the nucleus
● A few have DNA polymerases and can
complete replication in the host cell’s
cytoplasm
● Example: smallpox virus
Often double-stranded, but can be single-stranded
● Replication takes place in the nucleus
● A few have DNA polymerases and can
complete replication in the host cell’s
cytoplasm
● Example: smallpox virus

Usually single-stranded, but can be double-stranded
● Replication takes place in the cytoplasm
● Mutation happens at very high rate because RNA polymerase
does not have proof-reading capabilities
● Examples: influenza viruses, coronaviruses

These cell damaging changes can occur when a virus replicates in a host cell:
lysis (the cell ruptures or bursts open)*
apoptosis (programmed self-destruction)
widespread symptoms due to the body’s immune response
*In some cases, replicated viruses can leave the cell by “budding”, which does not immediately kill the cell.

1. Attachment
2. Entry
3. Replication and Assembly
4. Egress (Release)

Receptors on the surface of the host cell bind to virus capsid proteins or virus envelope glycoproteins.
• Viruses can attach only to cells that have the right receptor molecules.
• Therefore, viruses can be very specific about what species or cell type they can infect.

• Bacteriophage DNA enters the host cell “naked”.
• Viruses may enter eukaryotic cells by endocytosis, or if enveloped, by fusion with the cell’s membrane.

The viral genes then direct the assembly of the new proteins and genomes to make new viruses.

transcribe mRNAmake viral proteins duplicate DNA to make new viral genomes

make complementary RNA if necessary transcribe mRNAmake viral proteins copy RNA to make new viral genomes

reverse transcribe RNA to make DNA, using reverse transcriptase
DNA incorporates into host genome
DNA directs synthesis and assembly of new viruses

May involve lysis and death of the host cell.
• May involve budding, which does not directly kill the host cell. (Like exocystosis)

Viruses that infect bacteria
Most bacteriophages are double stranded viruses
• There are an estimated 1031—ten million trillion trillion

1. Phage infects a cell
2. Phage dna becomes incorporated into the host genome
3. The cell divides and phage dna is passed to daughter cells
Under stressful conditions, the phage dna is excised from the bacteria cell and enters the lytic cycle

1. Phage infects cell
2. Phage dna circularizes and is SEPARATE from the host dna
3. Phage dna replicates and phage proteins are made, new phage particles are made
4. The cell lyses and releases phage

During the viral replication process, certain virus's DNA or RNA affects the host cell's genes in ways that may cause it to become cancerous. These viruses are known as oncogenic viruses, meaning viruses that cause or give rise to tumors.

) Varicella-zoster, the virus that causes chickenpox, has an enveloped
icosahedral capsid visible in this transmission electron micrograph. Its double-
stranded DNA genome becomes incorporated in the host DNA and
can reactivate after latency in the form of (b) shingles, often exhibiting a rash.

HPV, or humans papillomavirus, has a naked icosahedral capsid visible
in this transmission electron micrograph and a double-stranded DNA
genome that is incorporated Into the host DNA. The virus, which is
sexually transmitted, is oncogenic and can lead to cervical cancer.

• Causes COVID-19 disease
• is an enveloped, non-segmented, positive sense single stranded RNA virus

Transmission
• Via respiratory droplets of an infected person (symptomatic, presymptomatic or asymptomatic) when he/she coughs or sneezes (major route)
• Droplets propelled a short distance can spread the virus (BIG)

Molecular test – to detect genetic material (PCR—Curative) • Antigen test – to detect protein on the surface of the virus
• (Binaxhometest)

ANTIBODY TEST
• Looks for antibodies
• To determine if someone has been infected in the past • Not recommended if someone is infected in present

Prime immune system to react when body is exposed to virus. ○ A few can work during early stages of viral infection (e.g.
rabies vaccine).
● Contain a “live” (attenuated) virus, killed virus, or sub-particles.
● If the virus is stable and does not mutate frequently, the vaccine
can work for years without update.
● If the virus mutates frequently:
○ Vaccine may need frequent re-design (e.g. annual flu vaccine)
○ Vaccine may be difficult to design at all (e.g. vaccine against HIV)

Time consuming as it requires 6 stages – discovery, testing in cell cultures or animals models, clinical testing in humans, effectiveness and dosage, approval process, manufacturing and

may come from pollen, another plant, or vectors such as insect bites
Can lead to decreased growth over vigor in plants

virus is transmitted from the parent plant
• May cause hypoplasia (decreased growth and vigor)
• May cause necrosis of the plant or plant tissue
The plant turns into a virus making machine!

Very small
• Contain no nucleic acid
• Cause fatal neurodegenerative diseases
• Mad cow disease (BSE, bovine spongiform encephalopathy) • (v)Creutzfeldt-Jakob disease (in humans)
• Kuru (in humans, spread by cannibalism)
• Scrapie (in sheep)
• Chronic wasting disease (in deer, moose, elk)

Prions are proteins that have adopted an abnormal fold, with this misfolded shape, they convince other proteins to adopt the same shape, which leads to the accumulation of prions, that cause neurodegenerative diseases

Only known to infect plants
• Can replicate within cells
• Do not manufacture any proteins
• Can cause crop failures