Viruses III Flashcards
describe the classification tree of DNA viruses
describe the classification tree of RNA viruses
describe the 3 ways of producing viral proteins with a single function
- transcription of individual monocistronic mRNA molecules from the genome
- segmented genome where each molecule gives single monocistronic mRNA
- production of a single long polyprotein that is later cleaved into individual functional peptides
when viruses replicate in the cytoplasm, they can either make their own _____ or be fitted with a 3D RNA structure known as _____
when viruses replicate in the cytoplasm, they can either make their own 5’ cap or be fitted with a 3D RNA structure known as IRES
describe the class I viral genome replication
e.g. Herpesvirus
- all DNA viruses (except poxviruses) replicates in the nucleus
- immediate early proteins take over the cell command
- early proteins target genome replication: like viral DNA polymerase
- late proteins are structural: translocate back into the nucleus to form a capsid
describe class II viral genome replication
e.g. Parvovirus
- class II is a ssDNA genome
- most viruses are either (+ve) or (-ve) ssDNA
- ssDNA is converted to dsDNA by host factors and DNA polymerases
- ssDNA viruses will produce both (+ve) or (-ve) virions
describe viral classes III, IV and V
- they all code RNA-dependent RNA-polymerase because they do not go through DNA intermediates during growth
- class III and V carry viral RNA replicase within their virion
- class IV is (+ve) ssRNA which is used directly as mRNA:
- can be translated by the host machinery
- eventually required to produce viral RNA-replicase for coping to (-ve) ssRNA and then generating (+ve) ssRNA genomes
describe class III viral genome replication (e.g. Rotavirus)
describe class IV viral genome replication
e.g. poliovirus
- these are some of the smallest viruses
- e.g. Hepatitis A virus, poliovirus
- simple replication cycle
- picornavirus mRNA does not have a cap and instead has a IRES element
- viral RNA is translated as a polyprotein which is proteolytically cleaved to give rise to several polypeptides
describe class V viral genome replication
e.g. Rabies virus
- rhabdoviruses, like rabies virus, and paromyxoviruses are representative of (-ve) ssRNA
- rhabdoviruses must carry their own RNA-dependent RNA polymerase for initial mRNA production
- rabies virus give rise to five mRNA molecules and a single long (+ve) ssRNA which serves as a template for genome replication (-ve) ssRNA
describe class VI viral genome replication
retroviruses (e.g. HIV)
- enveloped viruses with complex replication that involves reverse transcriptase (RNA-dependent DNA polymerase)
- contain 2 identical (+ve) ssRNA
- the viral genome becomes permanently integrated anywhere in the host genome
- known as a provirus and similar to bacteriophages without the excision part
- 8% of the human genome is from retroviral origin
- can be tumorigenic
describe class VII viral genome replication
e.g. hepatitis B virus
- this class of dsDNA genomes has a partial single stranded DNA overhang
- the DNA replication requires processing through RNA intermediate e.g. hepatitis B virus (HBV)
- once inside the host cell, the partial dsDNA is completed and further transcribed into mRNA and pre-genomic (+ve) ssRNA
- new partial dsDNA genomes are generated from pre-genomic ssRNA intermediates by reverse transcriptase (RNA-dependent DNA-polymerase)
describe the replication of hepatitis D virus (class V: (-ve) ssRNA)
- HDV RNA genome is translocated from the cytoplasm to the nucleolus (steps 1-3)
- the (-ve) ssRNA is transcribed by cellular RNA polymerase II and mRNA is subsequently transferred to the cytoplasm (steps 4-7)
- hepatitis D antigen produced from HDV mRNA is translocated from the cytoplasm to the nucleolus
- hepatitis B surface antigen is produced from mRNA of the HBV helper virus (steps 7-9)
describe recombination (interaction of viruses inside cells)
- recombination between 2 viruses infecting the same host is possibly but usually happens with low frequency, e.g. HSV 1 and HSV 2
describe complementation (interaction of viruses inside cells)
- occurs when one or both viruses which are simultaneously infecting the host cell have a defective function but at different gene locations
- they can successfully propagate because of complementing and rescuing each others defective function(s) e.g. the relationship between helper HBV and defective HDV
describe antigenic drift vs. antigenic shift
- antigenic drift: evolution due to small changes in the genome sequence within a define strain which usually occurs at a slower rate compared to antigenic shift
- e.g. influenza virus, rhinovirus
- antigenic shift: usually occurs in segmented viruses, notably influenza virus, resulting in exchange (reassortment) of RNA segments between 2 different influenza viruses that subsequently gives rise to a highly pathogenic (reassorted) strain virus
describe phenotypic mixing
association of a genotype with a heterologous phenotype
- phenotypic mixing or transcapsidation: genome of one virus randomly incorporated in capsid composed of proteins from a different, or both, viruses
- pseudotypes: formed when the nucleocapsid of one virus acquires envelope from another type of virus
