Taxonomy of Viruses

Question 1

General characteristics of viruses

Ecological interactions? Hosts? Consists of?

Answer 1

• Obligate intracellular parasites
• Host cells include all groups of organisms
• Piece of nucleic acid enclosed within a protein coat
• 1 type of nucleic acid found in the virion of a given virus

Question 2

Virions

Answer 2

• Outside of the host
• inert infectious particles

Question 3

Capsid

Answer 3

Protein coat around the nucleic acid

Question 4

Nucleocapsid

Answer 4

Nucleic acid + protein coat

Question 5

Capsomeres

Answer 5

Protein subunit that makes up the capsid

Question 6

Envelope

Answer 6

Lipid-containing layer with embedded proteins
* Proteins in the envelope are virus specific

Question 7

Shapes of Virions

Answer 7

• Helical virus: Typical of many plant viruses
• Polyhedral virus: most common is icosahedron
• Complex virus: Composed of several parts

Question 8

Viroids

Answer 8

• Closed circles of single-stranded RNA containing 240-380 nucleotides
• Replication is dependent on host machinery

Question 9

Prions

Answer 9

• Consists solely of 1 protein
• Commonly found in neurons
• Induces misfolding of normal prion proteins who’s accumulation kills cells
• Cause of many neurological degenerative disorders

Question 10

Taxonomy

Virus

Answer 10

• Grouped into families (end in “-viridae”)
• Given a genus name (end in “-virus”)
• Given a species name (usually English)

Question 11

Classificationof viruses

Answer 11

Based on characteristics:
* Nature of the host
* Type of disease caused
* Life cycle
* Naked or enveloped
* Type of nucleic acids and strandedness

Question 12

Baltimore classification scheme

Answer 12

Classification is based on the type of genome

Question 13

Pathway of viruses

Answer 13

• Absorption
• Penetration
• Uncoating
• Replication
• Maturation
• Release

Question 14

Absorption

Life cycle

Answer 14

Attachment of the virus to
specific receptors on the surface of the cell

Question 15

Penetration

Life cycle

Answer 15

Virus genome enters the cell
* The complete virion may enter the cell
* In enveloped viruses: Envelope may be left at the cell surface
* In naked viruses: Capsid may be left at the surface

Question 16

Uncoating

Life cycle

Answer 16

Removal of the envelope and\or
the capsid by host enzymes

Question 17

Replication

Answer 17

Replication of the nucleic acid,
transcription and protein synthesis

Question 18

Maturation

Answer 18

Assembly of virus components, nucleic acid, nucleocapsid and accessory proteins to form new virions

Question 19

Release

Answer 19

Mature virions exit the host cell by means of budding or by causing lysis of the cell.

Question 20

Latent period

Answer 20

Eclipse (replication) + maturaiton
* Eclipse (replication): time necessary for the host cells to replicate the viral genome and to synthesize the viral components
* Maturation: time needed for the different components to be assembled

Question 21

Rise period (release)

Answer 21

Virions are detected outside the cell
* Lysis: virus-encoded proteins damage the CM
* Budding

Question 22

Burst size

Answer 22

Number of virions released

Question 23

Bacteriophages

Answer 23

• Viruses of bacteria
• Most phages contain linear dsDNA genomes
• Most are naked, but some possess lipid envelopes

Question 24

Bacteriophage Types

Answer 24

• Virulent phages
• Temperate phages

Question 25

Lytic pathway

Answer 25

Infection of host cells always leads to replication resulting in host cell lysis * Virulent and temperate phages

Question 26

Lysogenic pathway

Answer 26

Genome becomes incorporated into the bacterial host genome * Temperate phages

Question 27

Virulent Phage: T4

Answer 27

* T4 attaches to the core region of LPS by the tail fibers * Tail lysozymes digest the peptidoglycan layer, forming a small pore * Phage DNA is injected into the cytoplasm of the host cell * Replication of T4 dsDNA * T4 lysosome production leads to cell lysis

Question 28

Temperate Phage: Lambda

Answer 28

* Genome = linear dsDNA with cohesive ends that join to form circular molecule * Genome integrated in the bacterial chromosome at the attƛ site (homologous to the att site in the phage) * Infection by temperate phages results in a prolonged, latent state of infection * Sometimes, the prophage can exit the chromosome and continue along the lytic pathway

Question 29

Prophage

Answer 29

phage genome within the host cell chromosome

Question 30

Lysogen

Answer 30

bacterium that contains a prophage

Question 31

Location of replication in animals viruses

Answer 31

* DNA genome: in the nucleus * RNA genome: in the cytoplasm

Question 32

Animals viruses types

Answer 32

DNA genome RNA genome: * plus-strand RNA * minus strand RNA * dsRNA * Retrovirus RNA

Question 33

DNA genome

Answer 33

* Fusion of cell CM with virus envelope * Nucleocapsid transported to nucleus where viral DNA is uncoated * Host apparatus synthetizes immediate early proteins, delayed early proteins, & late proteins * Assembly occurs in the nucleus * envelope is added via a budding process through the inner membrane of the nucleus * Ex: Herpes virus

Question 34

RNA genome: + strand RNA

Answer 34

The genome can be translated directly * Ex: Poliovirus, Hepatitis A virus

Question 35

RNA genome: - strand RNA

Answer 35

The minus-strand is first transcribed into a plus- strand RNA by an RNA-dependent RNA polymerase carried inside the virions * Ex: Measles, Rabies, Influenza

Question 36

RNA genome: dsRNA

Answer 36

* A plus-strand RNA must be synthetized by a viral-encoded RNA-dependent RNA polymerase using the minus strand as the template. * The plus-strand is then translated into proteins and is used as a template to synthesize a minus strand to yield dsRNA genomes * Ex: Rotavirus

Question 37

RNA genome: Retrovirus

Answer 37

* The virion carries two identical copies of the genome (plus-strand ssRNA), reverse transcriptase, integrase and proteases. * The reverse transcriptase reverse transcribes the RNA genome into DNA * The DNA genome travels to the nucleus and is integrated into the host DNA * Ex: HIV

Question 38

Long terminal repeats (LTR)

Answer 38

Contain promoters for transcription and participate in the integration process of viral DNA

Question 39

Provirus

Answer 39

* Integrated viral DNA * Cannot excise from the host genome

Question 40

Consequences of viral infections in animal cells

Answer 40

* Cell fusion * Oncogenic viruses

Question 41

Cell fusion

Answer 41

Enveloped viruses that fuse with the host cell cytoplasmic membrane * results in hybrid cells that have chromosomal aberrations

Question 42

Transformation

Answer 42

Conversion of a normal cell into a tumor cell

Question 43

Oncogenic Viruses

Answer 43

Viral infection is implicated in transformation * Both DNA and RNA viruses are know to cause tumours

Question 44

Mechanisms of oncogenic viruses

Answer 44

* Transduction * Insertion of a strong promoter * Inactivation of a tumor suppressor gene. * Expression of viral protein that induces transformation

Question 45

Expression of viral protein that induces transformation

Answer 45

* DNA virus * The viral protein does not have a cell counterpart * Integration of the viral genome into the host genome may occur or the viral genome may persist in the cell as an extrachromosomal episome * Ex: HPV

Question 46

Nonpermissive host

Answer 46

A host in which a virus cannot complete its infection cycle * The infected undergoes uncontrolled replication (tranformation) * Because the virus cannot complete its replication, the cells will never die

Question 47

Nucleoside analogs

Answer 47

Block reverse transcriptase & production of viral DNA

Question 48

Protease inhibitors

Answer 48

Inhibit processing of large viral proteins into individual components

Question 49

Fusion inhibitors

Answer 49

Prevent viruses from successfully fusing with the host cell