CH. 13 | Viruses, Viroids, and Prions

13 - 1 Differentiate a virus from a bacterium

Virus

• Host specific

  • Attachment happens through receptor sites

• Obligate intracellular parasites (However, some bacteria have this as well)

  • Requires living host cells / Needs to be inside a host in order to multiply

• Acellular organisms (Do not contain cellular membranes, cellular structure, etc)

• Contain a single type of nucleic acid; DNA or RNA (Cannot have both)

  • Can either be single-stranded or double-stranded, but never both

  • Can either be linear or circular, but never both

• Contains a protein coat

• Lack enzymes for protein synthesis and ATP generation
  
  
  

Bacteriophages / Phages

• Viruses that infect bacteria
  
  
  

Host Range

• A variety of cell types/host cells that the virus can infect
  
  
  

• Two factors determine host range:

  1. Attachment compatibility — The virus's outer surface must chemically match specific receptor sites on the target cell's surface. This is essentially a lock-and-key mechanism held together by weak bonds (like hydrogen bonds). The more attachment points, the stronger the association.

  2. Internal cellular machinery — Even if a virus successfully attaches, it also needs the host cell to have the right internal factors to support viral replication.
     
     
     

What sets viruses apart from bacteria?

• Viruses are acellular organisms that contain either DNA or RNA, but never both

13 - 2 Describe the chemical and physical structure of both an enveloped and a nonenveloped virus

Virion

• A complete, infectious viral particle made of nucleic acid surrounded by a protein coat
  
  
  

Nucleic Acid

• Encoded by either DNA or RNA (Never both)

• Can be single or double stranded, linear, or circular
  
  
  

Capsid

• Used for protection, protects the nucleic acid from nuclease enzymes and promotes attachment to host cells

• The protein coat that protects the nucleic acid

• Made of protein subunits called capsomeres

• Determines most of the virus’s mass

---

Enveloped Virus

• Has nucleic acid + capsid + an outer envelope (membrane)

• The envelope is composed of lipids, proteins, and carbohydrates

• Derived from the host cell’s plasma membrane during extrusion

• May be covered by spikes — carbohydrate-protein complexes that:

  • Helps the virus attach to host cells

  • Can cause hemagglutination

  • Are used as identification markers

• Makes the virus more versatile in attaching to host cells, but makes it more vulnerable to destruction

Nonenveloped Virus

• Consists of only nucleic acid + capsid (Acts as the outermost layer)

  • Capsid serves 2 key functions:

    • Protects the nucleic acid from nuclease enzymes in biological fluids

    • Promotes attachment to susceptible host cells

• More resistant to destruction

13 - 3 Define viral species

A group of viruses sharing the same genetic information and host range (ecological niche)

• An example of this is SIV vs. HIV

• SIV affects apes. Whereas HIV affects humans

13 - 4 Give an example of a family, genus, and common name for a virus

Viruses don’t follow scientific nomenclature for naming. Instead, they use common names. An example of this is HIV: Human immunodeficiency Virus

Examples

---

Family: Coronoavindae

Genus: Betacoronavirus

Common name: Severe acute respiratory syndrome coronavirus 2

• SARS-CoV 2
  
  
  

Family: Herpesviridae

Genus: Simplexvirus

Species: Human herpesvirus-2

Viral Taxonomy

Family, Genus, Common Name

Family names end in -viridae
Genus names end in -virus

13 - 5 Describe how bacteriophages are cultured

Bacteriophages can be grown either in liquid or solid media

• A bacteriophage sample is mixed with host bacteria and melted agar

• Poured into a Petri plate

• Virus-bacteria mixture solidifies into a thin top layer of bacteria

• Virus multiplication occurs
  
  
  

Plaque

• Lawn inoculation

• Corresponds to a single virus in the suspension

13 - 6 Describe how animal viruses are cultured

3 Methods

• Using living animals

• Embryonated eggs

• Cell cultures
  
  
  

In Living Animals

• Viruses are grown in living animals (mice, rabbits, guinea pigs). The animal is inoculated with the specimen, then observed for signs of disease or sacrificed so infected tissues can be examined. Used when a virus cannot be grown any other way.
  
  
  

Embryonated Eggs

• A fertilized egg with a developing chick embryo. A hole is drilled in the shell, and the virus is injected near the appropriate membrane. Viral growth is indicated by embryo death, cell damage, or pock/lesion formation on the membranes. Still used today to grow viruses for some vaccines, which is why you may be asked about egg allergies before vaccination.
  
  
  

Cell Cultures

• ells grown in culture media in the lab — more convenient than living animals or eggs. Started by treating animal tissue with enzymes to separate individual cells, which are then suspended in nutrient solution and grow into a monolayer
  
  
  

Viral infection of the monolayer causes cell deterioration called cytopathic effect (CPE).

Two types of cell lines:

• Primary cell lines — derived from tissue slices; die after a few generations

• Continuous cell lines — transformed (cancerous) cells that survive indefinitely; called "immortal cell lines" (e.g., HeLa cells from Henrietta Lacks)

Virus multiplication | One-step growth curve

13 - 8 Describe the lytic cycle of T-even bacteriophages (infects E. coli)

Lytic Cycle

• A mechanism of phage multiplication that ends with lysis and death of the host cell

—

5 Stages

• Attachment

  • The phage collides with the bacterium

  • Attach to receptor sites on the bacterial cell wall

• Penetration

  • Phage releases phage lysozyme to break down part of the cell wall

  • Phage injects its DNA into the host — capsid stays outside

• Biosynthesis

  • Phage DNA takes over the host cell

  • Host DNA is degraded; protein synthesis is stopped

  • Host cellular components are hijacked to make viral components

  • Eclipse period

• Maturation

  • Viral components self-assemble into complete virions spontaneously

• Release

  • Lysozyme (encoded by Phage DNA) breaks down the bacterial cell wall

  • Plasma membrane bursts open → host cell dies

  • New virions are released and go on to infect neighboring cells
    
    
    

Key Details to Remember

• The host cell always dies in the lytic cycle (unlike the lysogenic cycle where it survives)

• The virus contributes DNA only — all machinery (ribosomes, enzymes, energy) comes from the host

• The eclipse period = viral components present, but no detectable complete virions yet

• Lysozyme is used twice — once to enter (penetration) and once to exit (release)

13 - 9 Describe the lysogenic cycle of bacterophage Lambdavirus

Lysogenic Cycle

• Does not immediately kill the host cell. Instead, it is the incorporation of viral DNA into host DNA, which remains dormant — sometimes for many generations

—

5 Stages

• Attachment & Penetration

  • Same as the lytic cycle: Phage attaches to the host cell and injects its DNA

• Integration

  • Instead of taking over and destroying the host, the phage DNA integrates directly into the host’s chromosome, now called a prophage

    • Repressor proteins encoded by the phage silence all other phage genes, keeping them latent and inactive

    • The host cell has no idea that the extra DNA is there

• Replication

  • Every time the host cell divides, it copies the prophage as well → all daughter cells carry the prophage (lysogenic cells)

• Excision (Trigger required)

  • A spontaneous event.

  • UV light, or certain chemicals, can cause the prophage to pop out of the host chromosome → this initiates the lytic cycle

3 Important Consequences of Lysogeny

1. Phage Conversion

• The prophage carries extra genes that give the host cell new properties — most importantly, toxin production

• Examples:

  • Corynebacterium diphtheriae → causes diphtheria only when carrying a lysogenic phage (the prophage encodes the toxin)

  • Streptococci → can cause toxic shock syndrome only when lysogenic

  • Clostridium botulinum → botulism toxin is also prophage-encoded

• This is clinically significant — a normally harmless bacterium can become dangerous through lysogeny
  
  
  

2. Immunity to Reinfection

• Lysogenic cells are immune to reinfection by the same phage

• The repressor proteins that keep the prophage silent also block any new incoming phage of the same type

• Note: the cell is NOT immune to other phage types
  
  
  

3. Specialized Transduction

• When the prophage excises, it sometimes makes an unclean cut and accidentally carries a piece of the host's bacterial DNA with it

• When this phage infects a new host cell, it delivers both its own DNA and that bacterial DNA fragment

• The new host now has genes from a completely different bacterium — this is how traits can transfer between bacterial cells via a virus

Specialized Transduction

13 - 11 Define oncogene and transformed cell

Oncogene

• A mutated proto-oncogene that triggers abnormal, uncontrolled cell growth that can lead to cancer

• Can be activated, and when activated, it can lead to cancer

  • Proto-oncogenes

    • Normal genes in a healthy cell that encode proteins involved in stimulating normal cell growth

    • When mutated → become oncogenes

    • Mutations can be triggered by mutagenic chemicals, high-energy radiation, or viruses (oncoviruses)

• Oncoviruses

  • Viruses that cause cancer by activating oncogenes → activation makes the cell abnormal → these abnormal cells are called transformed cells
    
    
    

Transformed Cell

• A tumor cell that has acquired new properties distinct from normal cells, including uncontrolled growth, lack of apoptosis, irregular shape, and a virus-specific surface marker called a tumor-specific transplantation antigen (TSTA)

• An abnormal cell that results from oncogene activation

13 - 14 Differentiate between latent and persistent viral infections

Latent Viral Infection

• The virus remains in the host for a long period without producing disease — it is inactive/dormant until triggered

  • Examples include:

  • Simplexirus (cold sores)

  • Varicellovirus (chickenpox)
    
    
    

Persistent (Chronic) Viral Infection

• The virus stays in the host but symptoms gradually increase in severity over a long period. Virions slowly build up over time rather than spiking suddenly

• Typically fatal

Latent and Persistent Viral Infections

Red curve (Acute infection) — measured in days. Virions spike fast and high, then drop quickly. This is your typical infection where you get sick, your immune system fights it off, and it's over. Not latent or persistent — just included for comparison.

Green curve (Latent infection) — measured in months/years. Virion levels are flat/undetectable for a long time (the dormant period), then spike suddenly when a trigger reactivates the virus. That sudden spike is the key visual — it mirrors the acute curve but happens much later.

Blue curve (Persistent infection) — measured in months/years. Virion levels never fully drop to zero after the initial infection — they slowly and gradually build up over a long period. No sudden spike, just a slow creeping rise.

13 - 15 Differentiate virus, viroid, and prion

Virus

• Contains either DNA or RNA enclosed in a protein coat (capsid)

• Requires a host cell in order to replicate

• Can infect animals, plants, bacteria
  
  
  

Viroid

• Infectious RNA

• Short pieces of naked circular RNA only, with no protein coat

• Smaller and simpler than a virus

• Causes disease in plants by gene silencing — does not code for any proteins
  
  
  

Virusoid

• A type of viroid that does not have a protein coat, but can only cause disease when the plant cell is already infected by a virus

  • Hepatitis D is a suspected virusoid in humans

13 - 17 Discuss how a protein can be infectious