Chapter 13: Infectious Agent Particles - Part I: Viruses & Viral Replication

Characteristics of Viruses, Viroids, and Prions

Lake many of the characteristics of life: acellular

• Lack of both nucleic acids → only one type, or none

• no ribosomes

• no metabolism

• no growth → no division

• no independent reproduction → requires a host cell

Host Range

Group of hosts a virus can infect

• determined by viral receptors (and co-receptors) that are complementary to host cell surface proteins

Characteristics of Viruses

1. Host Range

2. Most infect a particular host only

3. some are generalist: infect several different hosts

4. Others infect only specific types of cells within one host type

• affinity of viral receptors to cell surface proteins

5. obligatory intracellular parasites

Virus Classification

Based on:

1. Nucleic acid type and sequence

2. Morphology

3. Replication Strategy

Virion

complete, fully developed, infectious particle that is extracellular and inert

• outside & NOT active, waiting to infect a cell

Virus

Any stage of infection, intracellular

• INSIDE

Viral Structure

1. Nucleic acid: genome

2. Capsid

3. Envelope for enveloped viruses

Virus Nucleic Acid/genome

• DNA or RNA

• linear , circular, or in segments

• ss or ds

Capsid

• Capsomers: individual protein units

• protect nucleic acid

• viral receptors: attachment for non-enveloped (naked) viruses

• enzymes

Capsomeres

individual protein units

Viral Receptors

attachment for non-enveloped naked viruses

Spike Proteins

receptors found on envelope NOT capsid

Enveloped Virus - Characteristics

• phospholipid bilayer

• stolen from infected cell as they exit

• are easier to sterilize/disinfect

  • alcohols, heat, detergents

• limited survival outside host env.

• can avoid host immune response

Why can enveloped viruses avoid host immune response ?

They look similar to host → have a phospholipid bilayer

Envelope is made of

Membrane

• phospholipid bilayer

How do enveloped viruses attach to host cell ?

Spike proteins → glycoproteins

allow virus to bind to cells

How do enveloped viruses enter host cell ?

fusion

How do enveloped viruses leave host cell ?

budding

Why are enveloped viruses an animal virus trait ?

Because plant cells and bacterial cells have cell walls made up of peptidoglycan and glucose. These cell walls are rigid and do not allow viruses to take a piece with them

Name the shape of this virus

Polyhedral Virus

Formed by plane faces

• some are near-spherical

Name the shape of this virus

Cylindrical capsid with helical symmetry

Name the shape of this virus

Complex Virus

• extra structures such as protein tails

• unusual morphologies

Virus Vs. Bacteria: Intracellular Parasite ?

Bacteria: NO

Viruses: YES

Virus Vs. Bacteria: Plasma Membrane ?

Bacteria: YES

Viruses: NO , but can aquire a portion - envelope

Virus Vs. Bacteria: Binary Fission

Bacteria: YES

Virus: NO

Virus Vs. Bacteria: Pass through bacteriological Filters

Bacteria: NO - larger than viruses

Viruses: Yes

Virus Vs. Bacteria: Possess both DNA and RNA

Bacteria: YES

Viruses: NO

Virus Vs. Bacteria: ATP- Generating metabolism

Bacteria: YES

Viruses: NO

Virus Vs. Bacteria: Ribosomes ?

Bacteria: YES

Viruses: NO

Virus Vs. Bacteria: Sensitive to Antibiotics ?

Bacteria: YES

Virus: NO

Growing Viruses in the Lab Requires

living host → do not grow on agar plates

Plaques

represent cell lysis , dead bacteria

CPE: Cytopathic Effect

Morphological changes and structural damage in cells due to infection

• cell rounding

• swelling or shrinking

• detachment from surface

• death by lysis or inability to reproduce

• can lead to induction of apoptosis: cells avoid completion of viral replication to prevent spread of infection

Apoptosis

• cell avoids completion of viral replication to prevent spread of infection

Lytic Cycle: Bacteriophages

1. Attachment

   1. via viral receptors

2. Entry

   1. do not enter cell, only injects DNA into the cell which circualarizes

3. Biosynthesis

   1. hijacks bacterial cell components for replication, transcription, and translation to make viral proteins

4. Maturation

   1. packages viral particles

5. Release

   1. cell bursts - lysis

Lysogenic Cycle: Becteriophage

Lysogenic Phase

1. Attachment

2. Entry

3. DNA Integrates:

   1. dna integrates into host chromosome - prophage

4. Bacteria replicate normally

   1. binary fission making new infected cells

5. Re-Activation

   1. prophage is excised from bacterial genome

Lytic Phase

6. Biosynthesis

7. Maturation /Assembly

8. Exit → lysis

Transduction

• other bacteria benefit

• must have bacteriophages

• when bacteriophages integrate as prophages into bacterial genome, they are eventually excised. As a result along with the viral genome some bacterial genome is excised as well that when that bacteriophage infects another cell, can be integrated into a newly infected bacteria cell

Phage Conversion

• benefits infected bacteria

• phage gives bacterial gene that benefits bacteria ex. toxins

Consequences of Lysogeny

• prophage can give bacteria viral genes that code for toxins

• viral genes continue to be passes through generations through binary fission as long as lysogeny occurs