MICR221

• dsRNA virsus carry RdRp in there capsid, e.g Rotavirus
• While inside the capsid, RdRp transcribes the -ve strand of dsRNA genome into +ssRNA. +ve ssRNA is then released from the capsid and translated to viral proteins. the RdRp and +ve ssRNA are then packaged into the capsid. In capsid, RdRp replicates the +ve SSRNA into -ve strand to generate dsRNA genome.
• How can a virus be classed as living?

  - Posseses own genetic material (nucleic acids)

  - Capable of reproduction

  - Capability to catalyse biochemical reactions. (encode enzymes)
• How can a virus be classed as non-living?

  - Acellular structure

  - Do not possess own metabolism, (no energy, no digestion)

  - Dependence on another organism for reproduction
• What does a complete virion contain?

  - Nucleic acid in the form of RNA or DNA as a genome

  - Protein coat called a capsid
• Nucleic acid + capsid =


  Nucleocapsid
• What are the two types of virus?

  - Non-enveloped

  - Enveloped
• The capsid contains protein building blocks to form its structures. What are these building blocks called?

  Protomers
• Building block protomers can create what three symmetries?

  - Helix

  - Icosahedral

  - Complex
• Name an example of a Non- enveloped virus with helical symmetry?

  Tobacco mosaic virus (TMV)

  - Rod shaped
• Name an example of a helical symmetry virus?

  Enveloped virus'

  - Influenza

  - Corona virus
• How does icosahedral symmetry work?

  - 20 face polyhedron

  - each face is an equilateral triangle

  - in each triangle protomers are further arranged into units of 5 or 6, these units are called capsomers.
• Name three non-enveloped virus with icosahedral symmetry

  - poliovirus

  - Adenovirus

  - Papillomavirus
• Name one enveloped virus with icosahedral symmetry

  Herpes simplex virus
• Binal symmetry is a property of what?


  A complex virus that has binal symmetry.
• A virus with a capsid with complex symmetry is

  - Poxvirus
• Linear viral genome examples

  - Poxvirus

  - Coronavirus

  - common in eukaryotes.
• Circular viral genome examples

  Simian virus (SV40), common in bacteria
• Segmented viral genome examples

  Influenza virus
• What is the range of viral genome size?

  4,000 > 1 million nucleotides
• Double stranded DNA (dsDNA)

  Common,

  - Poxvirus

  - Bacteriophage
• Single stranded DNA (ssDNA)

  Rare

  - Parovirus
• Single-stranded RNA


  Common,

  -Influenza virus

  - Corona virus
• Double-Stranded RNA (dsRNA)


  Rare

  - Reovirus
• Virus shape and size

  Varies from 10nm - 400nm

  rod, brick,bullet,filament,crystal,moon-landing shuttle, crown.
• What is a virophage


  A virus that infects another virus.
• Name the two types of hosts

  Clinical- Symptomatic and potentially lethal infection. (e.g humans, plants.)

  Reservoir (intermediate) - asymptomatic/non-lethal infection. (bats, birds mosquitos)
• describe the fives main steps of a virus life cycle

  1. Attachment

  2. entry

  3. synthesis

  4. assembly

  5. release
• what is the exception to the normal five main steps of virus life cycle?

  Plant virus do not attach to host cell surface.
• Where is the receptor binding protein found?

  On the virion.

  - in an enveloped virus on the envelope

  - in a non-enveloped virus on the capsid.
• What could a receptor be?

  - Protein molecule i.e CD4 for HIV ACE2 for SARS-CoV-2

  -Carbohydrate molecule i.e Sialic acid for influenza virus.
• Are receptors unique to one virus?

  Sometimes, not all the time.

  i.e Sialic acid for influenze virus, parainfluenze virus and adenovirus.
• Explain a co-receptor


  - primary receptor always binds first then the co-receptor.

  - i.e CCR5 for HIV
• Receptor facts


  - A virus may not be present on all hosts OR on all tissues.

  - Determines the host and tissue tropism, (attraction) of a virus.

  - for plant viruses a receptor has not been found.
• What are three mechanisms for viral entry into host cell.

  - Fusion

  - Endocytosis

  - Direct injection
• Entry by fusion example

  HIV, mainly common enveloped viruses.

  - virus and host cell bind via receptors.

  - lipid bilayer of viral envelope fuses with host cell membrane.

  - NUClEOCAPSID (no envelope) are released into cytoplasm of the sun.
• Entry of enveloped virion via endocytosis


  Influenza, coronavirus

  - envelope receptors bind to cell.

  - binding to receptor triggers receptor mediated endocytosis.

  - virus taken into cell in an endosome, its increased acidity allows nucleocapsid to escape from the endosome and enter the cytoplasm.
• Entry of non-enveloped virion via endocytosis


  Adenovirus

  - capsid proteins bind to receptors on cell surface triggering receptor-mediated endocytosis.

  - taken into an acidic endosome. the capsid is strong and sturdy. structure of capsid altered which allows nucleic acid to leave endosome and into the cytoplasm.
• entry by direct injection

  Bacteriophage T4

  - cell wall is is porus, a pore is created and through high pressure genetic material is injected into the cell.
• Virion entry to plant

  - no active mechanism known.

  - occurs via a wound so virion can enter cell.