Viruses, Viroids, and Prions |
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Viruses do not contain enzymes for energy production or protein synthesis.
For a virus to multiply, it must invade a host cell and direct the host’s metabolic machinery to produce viral enzymes and components.
Viral particles disappear upon penetration, none are seen during biosynthesis and assembly, and eventually all cells die so no new virions can be produced.
The eclipse period is the period when all viral particles are present but before they are assembled.
Burst time is the time from phage adsorption to release.
Burst size is the number of newly synthesized phages produced from one infected cell.
The virus may cause lysis or lysogeny.
Events of the lytic cycle:
Attachment or adsorption
Requires a receptor
Penetration
T-evens release lysozyme to break down a portion of the cell wall.
The tail sheath contracts and the tail core is driven through the hole in the wall to the plasma membrane.
The viral genome is then injected into the bacterium.
Biosynthesis
Viral DNA and proteins are synthesized.
Host protein synthesis is stopped by degradation of host DNA, interference with transcription, or repression of translation.
Maturation
During maturation or assembly phage DNA and capsids are assembled into complete viruses.
Release
Release occurs when phage lysozyme breaks down the cell wall and newly synthesized phage particles are released.
Lysogeny is a cycle in which the phage DNA recombines with the bacterial chromosome.
The incorporated viral DNA is now a prophage.
The prophage genes are regulated by a repressor coded for by the prophage, the prophage is replicated each time the host DNA is replicated.
Exposure to mutagens can lead to excision of the prophage and initiation of the lytic cycle.
Outcomes of lysogeny
Immune to reinfection by the same kind of phage
Host cell exhibits new properties due to viral genes carried on the prophage
Specialized transduction - host cell gains new bacterial genes packaged with the phage
Attachment or adsorption
Penetration
a) Endocytosis (pinocytosis) - togavirus
b) Fusion - herpesvirus
Uncoating of viral nucleic acid may be accomplished by host or viral enzymes. Bacteriophages don't require uncoating because their nucleic acid is injected into the host cell.
Biosynthesis of DNA viruses
DNA of most DNA viruses is released into the nucleus of the host cell.
Transcription and translation of early genes produces enzymes to reproduce viral DNA
Transcription and translation of late genes produces capsid proteins in the cytoplasm.
DNA-containing animal viruses
Some examples of DNA viruses:
Adenoviridae - from adenoids, cause respiratory diseases.
Poxviridae - pox refers to the pus-filled lesions that accompanies the diseases caused by these viruses
Herpesviridae - named after spreading (herpetic) appearance of cold sores
Papoviridae - named for papillomas (warts), polyomas (tumors), and vacuolation (cytoplasmic vacuoles)
Hepadnaviridae - name comes from the fact that they cause hepatitis and contain DNA.
Biosynthesis of RNA Viruses
RNA viruses multiply in the cytoplasm. RNA-dependent RNA polymerase synthesizes a double-stranded RNA.
Sense strand (+ strand) can act as mRNA directly and as a template for antisense strand (- strand) synthesis.
ss + strand RNA viral replication: The viral genome, a single stranded sense strand, is transcribed to make antisense (-) strands. The antisense strands serve as the template for making mRNA (sense, or + strands), which code for viral proteins and serve as the viral genome that is packaged inside the capsid during assembly.
ss - strand RNA viral replication: The viral genome, a single stranded antisense strand, is transcribed to make sense (+) strands, which serve as mRNA to code for viral proteins and also as a template to make more copies of the viral genome, single stranded antisense (-) strands, which will be packaged inside the capsid during assembly.
ds +/- RNA viral replication: transcription of - strand makes more copies of the + strand, which serves as mRNA. Transcription of the + strand provides viral protiens (including RNA-directed RNA polymerase) and more copies of - strand, which is packaged along with the complementary + strands in the capsid during assembly.
RNA-containing animal viruses
Picornaviridae - some of the smallest viruses (pico-); contain RNA, name comes from pico + RNA. Single stranded + strand viruses.
Example: poliovirus
Togaviridae - enveloped, name comes from toga (covering). Single stranded + strand viruses - transcription of a - strand serves as a template, the + strands transcribed from the - strand template are produced as a short strand mRNA that codes for envelope proteins and a long strand mRNA that codes for capsid proteins.
Examples: Arthropod-borne arboviruses or alphaviruses which cause viral encephalitis.
Rhabdoviridae - Rhabdo- is from the Greek for rod (they're really more bullet shaped). Single stranded - strand viruses.
Example: Lyssavirus (rabiesvirus)
Reoviridae - named for habitat, respiratory and enteric tract. Before they were associated with disease they were considered orphan viruses, name comes from respiratory, enteric, and orphan. Double stranded RNA viruses.
Example: Rotavirus
Retroviridae - Lentivirus (HIV-1, HIV-2, HTLV-1, HTLV-2)
Multiplication of Retroviruses
Retroviruses use reverse transcriptase (RNA-dependent DNA polymerase) to transcribe DNA from RNA.
Both viral RNA strands are + strands (making the virus diploid, how about that?) which are transcribed by reverse transcriptase to make complementary DNA strands.
The original viral RNA is degraded and the DNA copies integrate into the host cell's genome.
Maturation or Assembly
Release
Rupture – naked viruses
Budding – enveloped viruses
