Introduction To Viruses AMG- DLA Flashcards
Viral genomes can be:
Viral genomes can be:
Single stranded(ss), double stranded(ds), linear, circular, segmented, haploid or diploid
How do viruses appear in light microscopy?
“Invisible” by light microscopy with sizes ranging from 20-400nm
What is a virion?
A virion is a complete infectious form of virus outside the host cell (extracellular)
What do viruses infect?
Viruses infect any living organism e.g., vertebrates, invertebrates, plants, bacteria and fungi
What do viruses consist of?
Viruses are non-cellular organisms that consist of either DNA or RNA that is surrounded by a protein coat
What kind of parasites are viruses?
Obligate “intracellular parasites” that require a host cell for replication
How are virions formed?
Individual virion components self-assemble into a virion
Describe the structure of viruses
- Most viruses are structurally simple, however, some viruses have more complex structure
- The basic biophysical and biochemical rules of the viral structure and assembly are same for all viruses
What are the viral structure components?
- Genome
- Capsomere
- Nucleocapsid
- Virus specific glycoproteins
- Envelope
- tegument
What is the viral capsomere?
Viral (protein) subunits that assemble into caspid
What is the viral genome composed of?
Either DNA or RNA
What is the nucleocaspid?
Genome assembled into the caspid
Differentiate between the viral tegument and viral envelope
Viral envelope- outer layer that originates from the host membranes and covers the caspid to maintain aqueous.
Viral tegument- a cluster of proteins that line the space between the envelope and nucleocaspid
What is the structure and the main function of the virus caspid?
Structure: consists of single or several different subunits(capsomeres) that have variable size, have identifiable domains and symmetry
Held together by non-covalent, reversible hydrophobic or hydrogen bonds
What are the functions of the viral capsid?
The viral capsid serves to protect the nucleic acid and genome
What is the purpose of the viral capsid proteins?
Capsid proteins have several functions:
- define tissue or species specific transmission by interaction with host receptors to facilitate the host cell entry
- interact with the viral nucleic acid for packaging/assembling the virus.
- Assist in viral and/or host gene regulation
- Evade/block host immune system, and other functions
What are the forms of virus nucleocapsid morphology?
Three generals forms of nucleocapsid morphology:
- icosahedral(roughly spherical)
- helical
- complex (nonsymmetrical)
Describe the axes of symmetry in the Icosahedral Capsid and what causes it
The protomers assemble into pentamer
The icosahedral capsid has a complex 5-3-2 axes of symmetry e.g., poliovirus
Describe the formation of Icosahedral Capsid
- Few proteins assemble in a basic protomers
- The amount of nucleic acid that can be packaged is limited by the size of the particle.
Icosahedral capsid form independently of the genome
How do helical capsid appear?
Like rod-like, filamentous structures(May be rigid or flexible depending on the specific virus)
Where do helical capsule form?
- Capsomers bind to the viral genome in a regular fashion
- Hellical capsid are “open-ended “ and form around the genome. No empty helical capsid can form
What is common about all hellically shapes animal viruses?
All known examples of animal viruses with helical symmetry contain RNA genomes and have flexible nucleocapsid wound into a ball and surrounded by envelope, except: Rhabdoviruses
Describe the structure of the complex capsid
Bacteriophages or phages (bacterial viruses) exhibit complex symmetry
Head- nucleic acid and protein
Tail and contractive sheath
Tail fibers and tail pins
Base/end plate
Describe the capsid structure of a retrovirus
Retroviruses possess a distinctive cone-shaped capsid structure.
Retroviruses posses a 3 layer capsid that contains 11 dsRNA segments
Give an example of a retrovirus
E.g. human immunodeficiency virus (HIV)
What is the viral envelope?
Viral envelope is a lipoprotein membrane derived from host membrane can be:
- cell plasma membrane (HIV; most of the enveloped viruses)
- Nuclear or other internal (endoplasmic reticulum) membranes (herpesvirus) and contains virus specific proteins.
How much host proteins are in viral envelopes ?
The viral envelope is poor in host call proteins, but rich in virus specific glycoproteins-viral attachment protein (VAP)
What is the shape of enveloped viruses?
-Enveloped viruses are usually spherical or pleomorphic in shape except rhabdovirus (Rabies, bullet shaped) and poxvirus (smallpox, complex)
How are enveloped viruses transmitted?
Usually transmitted by secretions, large droplets, blood or sexual contact e.g., measles viruses
How many an enveloped virus be inactivated?
They are sensitive to inactivation by organic solvents (alcohol, chloroform, ether, etc.) detergents, drying, acid, heat.
What is a naked virus?
-viral nucleocapsid is naked
Describe the difficulty of inactivation to non-enveloped viruses
Non-enveloped(naked) viruses are tough and relatively resistant to inactivation by organic solvents (alcohol, chloroform, ether etc.,) detergents, drying, acid and heat
Hoe are non-enveloped (naked) viruses usually transmitted?
Usually transmitted by fecal /oral route, vomited or small droplets
How are non-enveloped viruses released from infected cells?
By lysis e.g., poliovirus, adenovirus
What are the functions of the Viral Attachment Proteins or peplomers (VAP)?
Facilitate host cell entry, e.g., hemaglutinin (HA) on influenza virus which binds erythrocytes, VAP on Epstein-Barr virus(EBV) binds C3d receptor(CR2) on B cells
What is the function of viral polymerase?
Present within the genome of viruses and involved in transcription and replication of viral genome
Where are viral matrix proteins?
Matrix proteins is present between the nucleocapsid and envelope
What is the function of viral matrix proteins?
Stabilize the organization of viral glycoprotein, directs the viral genome to intracellular sites of the viral assembly, facilitate virus assembly and budding
How can the viral proteins be divided?
Depending on the virus and it’s expression pattern, viral proteins are divided into :
- immediate early phase proteins
- early phase proteins
- late phase proteins
What are the characteristics that are involved viral nomenclature?
- Morphology
- Physicochemical properties: thermal stability, detergent stability, ,molecular mass, etc.
- Genome: size, type of nucleic acid, strandedness
- Proteins:
- Lipids: content, character
- Carbohydrates: content, character
- Genome organization and replication
- Antigenic properties: seriological relationships
- Biological properties: Host range, mode of transmission, pathogenicity, tissue tropisms, geographic.
What factors of morphology play a role in viral nomenclature ?
Size, shape, presence of envelope
What factors of genome organization and replication play a role in viral nomenclature?
Strategy of replication, number and position of open reading frames, transcriptionally and translational strategies, site of virion assembly and release
What committee controls viral nomenclature?
The international committee on Taxonomy of Viruses(ICTV, 1966) utilizes a universal system for classifying viruses
What is included in the viral nomenclature system used by the ICTV ?
The system makes use of series of ranked taxons (order/family/ sub-family/genus/ species)
Order Family(-idae) Subfamily(-inae) Genus(-virus) Species [isolates, strains]
Give the nomenclature process of the Measles disease
Order: Mononegavirales
Family: Paramyxoviridae
Subfamily: Paramyxovirinae
Genus: Morbillivirus
Species: Measles virus
What is the Baltimore Classification system ?
A nomenclature classification system that places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), sense, and method of replication.
Named after David a Baltimore, a Nobel prize-winning biologist, these groups are designated by Roman numerals
Give examples of dsDNA viruses
E.g., adenoviruses, Herpes viruses, Poxviruses
Give an example of dsRNA viruses
Reoviruses
What are ssDNA viruses, give an example
+ve strand or “sense” DNA (e.g. Paroviruses)
What are (+ve )ssRNA viruses , give an example
(+ve strand or sense) RNA (e.g. Picornaviruses, togaviruses)
What are ssRNA-RT viruses? Give an example
(+ve strand or sense) RNA with DNA intermediate in life cycle (e.g. Retroviruses)
What is the dsDNA-RT viruses? Give an example
DNA with RNA intermediate in life cycle (e.g. Hepadnaviruses)
