MIDTERM 1/4
• Virion – complete infective virus particle; may be identical to the nucleocapsid
• more complex virions include the nucleocapsid + envelope
• Viroids– smallest infectious agents causing disease in plants
• Capsid – protein shell that encloses the nucleic acid core
• Nucleocapsid – the capsid together with the enclosed nucleic acid
• Capsomeres – morphologic units seen on the surfaces of icosahedral virus particles; represent clusters of structure units
• Envelope – outer coat some viruses acquire as they penetrate or are budded from the nuclear or cytoplasmic membrane; always contain altered host-cell membrane components
• Peplomers – morphologic units composed of structural units embedded in the envelope
• Complementation – general term to describe situations where mixed infections result in enhanced yields of one or both viruses in the mixture
• Bacteriophages – viruses that infect bacterial cells
• Translation – the mechanism by which a particular base sequence in messenger RNA produces a specific amino acid sequence in a protein
• Transcription – the means by which specific information encoded in a nucleic acid chain is transferred to messenger RNA
• Helper virus – required by certain defective closely related viruses to replicate
• Defective virus– functionally deficient particles in some aspect of replication and may interfere with the replication of normal viruses
• Satellite viruses – viruses that cannot reproduce from their own nucleic acid and virions are non-infectious; require a helper virus for transmission and multiplication
Characteristics of Viruses
• Filterable agents that could not be cultivated on artificial media
• All viruses are obligate intracellular parasites
• However, all obligate intracellular parasites are not viruses
• Members of certain bacterial genera also are unable to replicate outside a host cell (eg, Ehrlichia, Anaplasma, Legionella, and Rickettsia)
• Outside living cells → viruses are inert particles
• Inside the cells → viruses utilize host cell processes to produce its proteins and nucleic acid to replicate itself
• Contain only one type of nucleic acid that carries the information for replicating the virus
• Some viruses contain nucleic acid molecules other than their genomic DNA or RNA. (e.g. Retroviruses)
• Retroviruses- cellular transfer (t)RNAs are essential for the reverse transcriptase reaction
• Reverse Transcriptase – enzyme that converts RNA into DNA
Chemical composition of the virion
• Various markedly between those of individual virus families
examples:
• Parvoviruses (family Parvoviridae), simples of viruses; virion is composed of viral structural proteins and DNA
• Picornaviruses (family Picornaviridae) it comprises viral proteins and RNA
• Herpesviridae and Paramyxoviridae – more complex with envelope; mature by budding through different host-cell membranes that are modified by the insertion of viral proteins
•Glycoproteins → the major type of protein present on the exterior of the membrane of enveloped viruses
•Presence of lipid envelope → separates viruses into
2 distinct classes:
1. Inactivated by organic solvents (enveloped)
2. Resistant to organic solvents (non-enveloped)
CLASSIFICATION OF VIRUS - The process of naming viruses and placing them into a taxonomic system
•Two main classification systems:
•ICTV (International Committee on
Taxonomy of Viruses) established in 1966
•Baltimore classification system
Hierarchy of recognized viral tax
(e.g. human respiratory syncytial virus A2)
ORDER = virales - Mononegavirales
FAMILY = viridae - Paramyxoviridae
SUB FAMILY = virinae - Pneumovirinae
GENUS = virus - Pneumovirus
SPECIES - Human respiratory syncytial virus
Viral Nucleic Acids in the Virion
• The type and structural characteristics of the viral genomic nucleic acids are used to classify viruses
• Viruses can simply be divided into RNA viruses and DNA viruses
• Baltimore classification system (7 classes)
• Developed by David Baltimore
• Classes based on genome type and mode of replication and transcription
RNA viruses
• Size of animal RNA viral genomes ranges <2 kb (Deltavirus) to >30kb (Coronaviridae)
Major distinction is whether the virion RNA is:
• Positive sense or polarity
it is of the same sense as mRNA
directly capable of translation to protein
• Negative sense or polarity →
the genomic nucleotide sequence is complementary to the of mRNA
requires transcription of the genome to generate mRNA equivalents
Within negative-strand group:
• Single-strand whole-genome viruses (e.g. Paramyxoviridae)
• Segmented genome viruses (e.g. Orthomyxoviridae – 6,7or 8 segements; Bunyaviridae – 3 segments; Arenaviridae – 2 segments)
• Retroviridae
• Considered diploid (2 copies of their genome); contains two whole-genomic positive-sense RNAs
• Use reverse transcriptase to create a DNA copy of their RNA genome
DNA viruses
•Overall structure of genome is less complex
•Either single molecule of single-stranded (ss)DNA or single molecule of double-stranded (ds)DNA
REPLICATION
Replication of viruses
•Process of reproduction resembles an assembly line
•Various parts of the virus come together from different parts of the host cell to form new virus particles
Replication of viruses
1. The virus attaches to a host cell
2. Enters the cell
3. Intact virus particle ceases to exist (uncoating)
4. Viral genome directs the production of new viral
macromolecules
5. Assembly and appearance of new progeny virus particles
Eclipse period
• Period of time between the penetration of the virus particle into the host cell and the production of the first new virus particle
• Disrupting cells during this period will interrupt the release of significant numbers of infectious virus particles
• If uninterrupted, single infectious particle can replicate to produce thousands of progeny virus particles
Total virus: input virus - (attached and unattached) and progeny virus(intracellular and extracellular)
Total cell free virus: input virus- (unattached) and progency virus (extracellular)
Total cell-associated virus: input virus (attached but not penetrated) and progency virus (intracellular)