Viruses III

Question 1

describe the classification tree of DNA viruses

Answer 1

Question 2

describe the classification tree of RNA viruses

Answer 2

Question 3

describe the 3 ways of producing viral proteins with a single function

Answer 3

• 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

Question 4

when viruses replicate in the cytoplasm, they can either make their own _____ or be fitted with a 3D RNA structure known as _____

Answer 4

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

Question 5

describe the class I viral genome replication

Answer 5

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

Question 6

describe class II viral genome replication

Answer 6

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

Question 7

describe viral classes III, IV and V

Answer 7

• 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

Question 8

describe class III viral genome replication (e.g. Rotavirus)

Answer 8

Question 9

describe class IV viral genome replication

Answer 9

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

Question 10

describe class V viral genome replication

Answer 10

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

Question 11

describe class VI viral genome replication

Answer 11

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

Question 12

describe class VII viral genome replication

Answer 12

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)

Question 13

describe the replication of hepatitis D virus (class V: (-ve) ssRNA)

Answer 13

• 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)

Question 14

describe recombination (interaction of viruses inside cells)

Answer 14

• recombination between 2 viruses infecting the same host is possibly but usually happens with low frequency, e.g. HSV 1 and HSV 2

Question 15

describe complementation (interaction of viruses inside cells)

Answer 15

• 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

Question 16

describe antigenic drift vs. antigenic shift

Answer 16

• 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

Question 17

describe phenotypic mixing

Answer 17

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