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VIRUSES

Obligate intracellular parasites that contain either DNA or RNA which depends on the synthetic machinery of the cell for replication of specialized elements that can transfer the viral genome to other elements

10 to 300 nm

Virus particles (virion) are small & vary in size from _

nucleic acid

Contain only a single type of _ as their genome

bacteria

Smaller compared to _, even the largest virus is smaller

replication strategies

Replicate from their own genetic material utilizing different _, not by the process of binary fission

cellular energy production

cellular machinery

Obligate intracellular parasites that are dependent on _ and _ for protein synthesis

energy

substrates

Lack the capacity to make _ or _, cannot make their own proteins

low viruses

They cannot multiply unless they invade a specific host cell and instruct its genetic metabolic machinery to make and release quantities of _

spontaneous macromolecular interactions

Synthesize their parts separately and then rely on _ for their maturation

antibiotics

Viruses are not sensitive to the usual _

Nucleic Acid (Viral) Genome

Contain the information necessary for virus multiplication

Either DNA or RNA, double

stranded or single

criterion for classification

Nucleic Acid (Viral) Genome is used as _

Capsid

Composed of structural units called capsomers which are clusters of polypeptide (surrounding the nucleic acid)

Protection of Viral Genome

Receptor Site

Stimulates antibody production

Site of Antigenic determinants

Functions of capsid:

Protection of Viral Genome

from both physical destruction and enzymatic hydrolysis of host cell nucleases

Receptor Site

necessary for naked viruses to initiate infection

Site of Antigenic determinants

important in some serologic tests

Nucleocapsid (Nucleic Acid + Capsid)

Appear in different geometric symmetry because of the arrangement of the capsomers

Icosahedral Symmetry

3

Helical Symmetry

rod

Complex

cannot be categorized as helical or icosahedral, no regular symmetry

Envelope

Surrounds the nucleocapsid of enveloped viruses

Membrane

like lipid bilayer with viral coded glycoprotein (spikes or peplomers) and cell derived lipids

Contains molecules necessary for enveloped viruses to initiate infection (if virus has no envelope, it is the function of the capsid)

Acts as stimulus for antibody production

Serves as antigen in serologic tests

Importance of the Envelope

Capable of binding to structures on target cells → Viral attachment factors → Erythrocytes = Hemagglutination

Hemagglutination process

Virion or Viral Particle

The entire virus, including nucleic acid, capsid, envelope, and glycoprotein spikes

Enveloped Viruses

With Envelope(icosahedral, helical)

Naked Viruses

Without envelope, complete virus (naked icosahedral, naked helical)

VIRAL CLASSIFICATION

Viral features that play important role in classification

Type of nucleic acid (DNA / RNA)

Strandedness (single

stranded / double

Physical nature of viral nucleic acid (segmented; some contains 2 DNA strands with different lengths)

Size of viral genome (molecular weight)

Location of nucleic acid replication and assembly

nucleic acid viral classification

poxvirus

DNA replicates in the nucleus except _

Orthomyxovirus and Rotavirus

RNA replicates in the cytoplasm except _ and _

Helical

Icosahedral

Complex

symmetry viral classification

viral messenger RNA

Replication Strategy

The molecular biology of genome replication and the method for producing _ differ strikingly for viral families

VIRION DIAMETER AND CAPSOMER NUMBER

It differs in sometimes with the size of the virus, they have also more number of capsomer

240 capsomers

Adenovirus capsomers

32 capsomers

Polio capsomers

SITE OF NUCLEOCAPSID INVOLVEMENT

useful for alphavirus

Togaviridae

bud through plasma membrane

Flaviviridae

bud through intracytoplasmic membrane

neuraminidase or membrane fusion protein

Enzymes
The presence of specific virion

ETHER SENSITIVITY

Usually exhibited by those with envelope

ANTIGENIC CROSS REACTIVITY

Antigenic determinants shared with a genus (some genus have same antigenic determinant as others)

Dermatrophic

Affects the skin

Neurotrophic

Affects the nervous system

Pox virus, Herpes virus, Hepadnavirus

Enveloped, double

Adenovirus, Papovavirus

Non

Parvovirus

Non

Orthomyxovirus, Bunyavirus, Arenavirus

Enveloped, single

Paramyxovirus, Rhabdovirus, Filovirus, Togavirus, Flavivirus, Coronavirus

Enveloped, single

Retrovirus (causes HIV)

Enveloped, single

Picornavirus, Calicivirus

Non

Reovirus

Non

Lytic Interaction

The most frequent and best understood relationship

Invading viruses multiply and eventually kill and lyse the host cells

Transforming Interaction

The viral genetic information becomes associated with the host cells genetic information

ECLIPSE PERIOD

Results in the loss of virion infectivity

Adsorption Or Attachment

Glycoprotein spikes bind to host cell carbohydrate receptors

Viruses recognize and attach to a limited number of host cell types allowing infection of some tissues but not others

viral tropism

Adsorption Or Attachment is referred to as _ (virus does not enter in all cell types, they choose their viral receptors)

Penetration

Entrance of the virion into the host cell

Endocytosis (Phagocytosis

like)

endocytotic vesicle

Endocytosis results in the formation of an _

The virus will then be released into the cytoplasm

Viropexis (Direct penetration of the membrane)

Binding of a virus to a host cell membrane receptor, then fusion of the viral envelope with the host cell membrane, and it enters the cell

Uncoating

Removal of capsid and core proteins, liberating viral nucleic acid

Enveloped viruses

Their envelopes fuse with the plasma membrane to deliver the nucleocapsid directly to the cytoplasm.

MACROMOLECULAR SYTHESIS

includes replication of viral genome and synthesis of protein polymers

Transcription

DNA VIRUSES

Occurs by host DNA

dependent RNA polymerases

Occurs in a specific temporal pattern, such as immediate early, delayed early, and late mRNA transcription

poxvirus

DNA VIRUSES

Transcription occurs in the nucleus except for _

Translation

Occurs on cytoplasmic polysomes

Followed by transport of synthesized proteins to the nucleus

Genome Replication

DNA VIRUSES

performed by a DNA

dependent DNA polymerase, which may be supplied by the host cell or be virus specific

occurs following the synthesis of the early proteins

Transcription

RNA VIRUSES

Involves an RNA

dependent RNA polymerase for all viruses, except retroviruses, which use a host

Involves a virion associated enzyme with a negative sense

Translation

RNA VIRUSES

May result in the synthesis of a large polyprotein that is subsequently cleaved into individual viral polypeptides

cytoplasmic polysomes

RNA VIRUSES

Translation occurs on _