Exam 3: Virology

Question 1

Virus

Answer 1

An acellular infectious agent

Question 2

Viroid

Answer 2

A subviral particle lacking a capsid and copnsist soley as RNA

Question 3

Virusoid

Answer 3

A subviral particle lacking a capsid, copnsist solely as RNA, and requires a virus for replication

Question 4

Prion

Answer 4

A misfolded, infectious protein

Question 5

Capsid

Answer 5

The proteinaceous coat that protects viral genetic information

Question 6

Capsomere

Answer 6

The subunits of a capsid

Question 7

Prophage

Answer 7

A bacteriophage genome that is integrated into the DNA of a host cell
bacterial cell

Question 8

Provirus

Answer 8

A virus genome that is integrated into the DNA of a host cell

Question 9

Latency

Answer 9

The ability of a pathogenic virus to lie dormant within a cell

Question 10

Oncogene

Answer 10

A gene that has the potential to cause cancer

Question 11

Describe the basic characteristics of viruses

Answer 11

Characteristics:
○ Cause many infections of humans, animals, plants, and bacteria
○ Have extracellular & intracellular state
○ NO cytoplasmic membrane, cytosol, organelles
■ Contain a protein coat
○ Neither grow NOR respond to environment
■ Viral particles assemble into viron
○ CAN’T carry out any metabolic pathway
■ Can’t redirect host cell metabolic pathways
○ CAN’T respond to their environment
■ Attach to host receptors for entry, assemble + exit host cell
○ CAN’T reproduce independently
■ Do NOT replicate in host cell

Question 12

structure of viruses

Answer 12

○ Size
■ Virion size range from 10-400 nm in diameter
■ Size is reviewed w/ electron microscope
■ Nucleic acid: DNA or RNA, single or double stranded, linear or circular
○ Contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein
coat (capsid)
○ Envelopes
■ Virions having envelopes= enveloped viruses
■ Virons lacking envelopes= naked viruses

Question 13

capsids of viruses

Answer 13

○ Protein coats that provide protection for viral nucleic acid and means of attachment to
host cell’s
○ Composed of proteinaceous subunits called capsomeres or portomers
■ May be made of single or multiple types of proteins
○ Helical, icosahedral, or complex
■ Helical
● Shaped hollow tubes w/ protein walls
● Capsomeres self assemble
● Size of capsid is a function of nucleic acid
■ Icosahedral capsids
● Is a regular polyhedron w/ 20 equilateral faces
○ Resembles a sphere
● Capsomeres
○ ring/ knob- shaped units made of 5-7 protomers
○ Pentamers (pentons) 5 subunit
○ Hexamers (hexons) 6 subunits
■ Complex
● Do not fin into the category of having helical or icosahedral capsids
● Poxviruses
○ Largest animal virus
● Large bacteriophages
○ Binal symmetry
○ Head resembles icosahedral, tail is helical

Question 14

components of viruses

Answer 14

○ Viral Envelope
■ Many viruses are bound by an outer, flexible virus. Membranous layer called the
envelope
■ Acquired host cell during viral replication or release
● Envelope is portion of membrane system of host
■ Composed of phospholipids bilayer and proteins
● Some proteins are virally encoded glycoproteins (spikes)
■ May project from the envelope surface as spikes or peplomers
● Involved in viral attachment to host cell
● Used for identification of virus
● May have enzymatic or other activity
● May play a role in nucleic acid replication
○ Extracellular state
■ Aka virion
■ Protein coat (capsid) surrounding nucleic acid
■ Nucleic acid + capsid also called nucleocapsid
■ Outermost layer provides protection and recognition sites for host cells
○ Intracellular State
■ Capsid removed
■ Virus exists as nucleic acid
■ Just DNA or RNA!
○ Genetic material of viruses:
■ Show variety in nature of their genomes than cells
■ Primary way scientists categorize & classify viruses
■ May be DNA or RNA, never both
● Can be dsDNA, ssDNA, dsRNA, ssRNA
● Strandedness
■ May be linear and segmented or single and circular
■ Much smaller than genomes of cells
■ Positive single stranded RNA in virus = looks exactly like mRNA
● Positive strand = same sequence as mRNA
● Negative strand = reverse complement of mRNA sequence
■ Coding strand, template strand
● Make mRNA off template strand
○ We will be given viral mRNA strand
○ Positive single stranded mRNA: would be the same

Question 15

assembly of viruses

Answer 15

○ Random interactions
○ Random protein interactions, proteins expressed later on in the stages of assembly are
more important cuz they actually distinguish the full virus, sort of regulatory

Question 16

virion release

Answer 16

○ Nonenveloped viruses lyse the host cell
○ Virus replicates inside host cell, cause host cell to lyse and virion is released and can go
and infect other cells
○ Enveloped viruses use budding
■ Viral proteins first enter host cell membrane
■ Nucleocapsid binds to viral proteins
■ Envelope derived from host cell membrane
■ Virus may use host actin tails to propel through host membrane
■ acquire their external lipid envelopes by budding through host cellular
membranes

Question 17

glycoproteins

Answer 17

Viral encoded, spikes on surface
● Viral attachment to host cell
● Identification of virus
● Enzymatic activity
● May help in nucleic acid replication

Question 18

List the criteria used to classify viruses.

Answer 18

● Classification based on multiple characteristics!
● Determined by ICTV (less popular way)
○ Nucleic acid type: DNA/RNA, ss/ds, linear/circular
○ Presence or absence of envelope
○ Capsid symmetry
○ Dimensions of virion and capsid
● Most used : Baltimore classification
○ Based on viral genome
○ See if viruses carry out similar processes
○ 7 life cycle groups
■ Double stranded ds DNA
■ Single stranded ss DNA
■ dsRNA
■ ssRNA, +
■ ssRNA, -
■ ssRNA retrovirus
■ dsDNA retrovirus
● Virus names: species name is ICTV name, most are referred to as Baltimore classification name

Question 19

Lytic replication

Answer 19

= replication results in death and lysis of host cell
○ Attachment of virus to host cell receptors
○ Infected DNA or RNA enters host
○ Virions formed
○ Release of host cell lyses and new virions = rapid replication and cell destruction

Question 20

Lysogenic replication

Answer 20

virus inserts genetic material, and host cell keeps growing and reproduce normally for
generations before lyse
○ Host chromosome didn’t get fragmented, but viral genetic material is part of host cell DNA
now
○ Host cell replicates DNA which has viral genome in it now
○ Prophage = inactive phage, bc virus now exists simply as genetic material
○ Eventually causes lysis bc virus dissociates from the host genome and enter into a lytic
mode of infection (called induction)
The phage infects a bacterium and inserts its DNA into the bacterial chromosome, allowing the phage DNA
(now called a prophage) to be copied and passed on along with the cell’s own DNA

Question 21

Compare and contrast various mechanisms of viral entry and exit

Answer 21

Entry:
● Membrane fusion: Fusion of viral envelope with host membrane; nucleocapsid enters
○ Virus gains phospholipids from host cell membrane
● Endocytosis in vesicle, endosome aids in engulfing virus, and then endosome helps w viral
uncoating, capsule degrading
● Direct injection: Injection of nucleic acid, viral genome injected into host cell (like the gene gun or
injection)

Exit
● Budding
○ Some enveloped viruses exit host cells through a budding process. During budding, the
virus acquires an envelope derived from the host cell membrane. (like membrane fusion)
○ new virions are released from the cell surface when the viral envelope fuses with the host
cell membrane, allowing the virus to be released without causing immediate cell lysis
● Lysis
○ Many viruses induce the lysis (bursting) of the host cell, which releases new virus particles
into the surrounding environment. This is known as a lytic release. The new virions can then
infect other host cells.
● Exocytosis
○ Some viruses, particularly large DNA viruses like poxviruses, exit host cells by hijacking
cellular exocytosis pathways. This process involves packaging new virus particles into
vesicles within the host cell and then releasing these vesicles by exocytosis.

Question 22

Differentiate between animal viruses and bacteriophages

Answer 22

Animal viruses
○ Attach via glycoprotein spikes, may infect specific tissues (tropism)
○ Commonly enter by endocytosis or fusion
○ Often cause persistent infections leading to latency
○ DNA viruses replicate within host cell nucleus
○ RNA viruses replicate in cytoplasm
○ Release through budding, lysis, or exocytosis
● Bacteriophages
○ Bind to bacterial cell receptors
○ Phages infect specific bacterial species
○ Typically enter by direct injection of nucleic acid
○ Follow lytic or lysogenic cycles
○ Replicate in bacterial cytoplasm
○ Release through lytic and lysogenic cycle

Question 23

Describe the potential effects of viral infection on host cells, including latency, inclusions and
Oncogenesis.

Answer 23

● Oncogenesis: process of normal cells transforming into cancer cells
○ specific genes (called oncogenes) are capable of inducing cell transformation
● Latency: can cause latent infections, like Lysogenic cycle, can be lytic or lysogenic: when exit
latency, they go back
● Lysogeny: inserting genetic material into your genetic material, run risk of oncogenesis

Question 24

Describe the role of viruses in oncogenesis. Discuss the functioning of oncogenes.

Answer 24

Role of viruses in cancer
● Viruses can carry oncogenes or activate them
● Animal’s genes regulate when cells can no longer divide AND prevent cells from unlimited division
hun
● Neoplasia: uncontrolled cell division in multicellular animal
● Viruses cause 20-25% of human cancers
○ Certain viruses have the ability to directly transform normal cells into cancerous cells
○ Viruses insert genetic material into normal cells, disruption cells’ normal pathways, causing
uncontrolled replication
● Certain cancers associated with certain viruses

Question 25

Discuss ways to culture and visualize viruses

Answer 25

● Culturing viruses in mature organisms
○ in bacteria and plants and animals
● Culturing viruses in embryonated chicken eggs
○ cheap, among largest of cells, free of contaminating microbes, have nourishing yoke
● Culturing viruses in cell Tissue culture; has cells isolated from organism and grown on broth/medium
○ 2 types: diploid and continuous cell cultures
● to visualize: Plaque Assay
○ Dilutions of virus preparation made and plated on lawn of host cells
○ If infected cells are killed, a region free of cells (the plaque)
○ Count plaques, plaque forming units PFU

Question 26

Define viroid, virusoid, and prion. Compare these subviral agents to viruses and other living Organisms.

Answer 26

● All acellular
● Virusoids
○ circular, single stranded RNAs dependent on PLANT viruses for replication and
encapsidation
● Prions
○ Misfolded forms of a normal cellular protein
○ Proteinaceous infectious agents
○ cellular PrP
■ Made by all mammals unknown function
■ misfolded version interacts w wild type, causing misfolding and then that protein
causes more proteins to misfold
○ Prion PrP, disease causing
■ Converts cellular PrP into prin PrP by inducing conformational change
● Viruses: Contain DNA or RNA, protein coat, and replicate in a host cell
● Subviral agents: Lack full viral structure (e.g., no capsid in viroids) and have different replication
requirements
● Living organisms: Have cellular structure, metabolism, and growth—all absent in viruses and subviral
agents

Question 27

Are viruses alive? Discuss why the status of the virus as a living organism is
controversial. How does viral infection lead to morbidity and mortality?

Answer 27

● NO, viruses are not alive
● Controversial
● They have genetic material, they can replicate but need a host cell to reproduce
● Metabolism: viruses LACK metabolism
● Viruses do not have cellular structures like membranes or organelles
● They do NOT respond to the environment
● They DO evolve through mutation and natural selection
● Whether they are alive or not depends on how one defines life; it has characteristics of living and
nonliving