MCD – Microbiology 1 – Virus Properties

Anil Chopra

  1. Describe the nature of viruses: their small size, mode of replication and diversity.

Viruses

  • Viruses are small in size, 20-450 nm
  • They are obligate (cannot survive outside host) intracellular parasites.
  • Composition: nucleic acid (DNA or RNA, can be single stranded or double stranded, linear or circular) and protein, (sometimes lipid and carbohydrate too)
  • They have a unique mode of replication.
  • Diversity: all species are infected by viruses, may cause great plagues or be asymptomatic
  • Classification according to:
  • Type of disease
  • Mode of transmission
  • Structure
  • Immunological relatedness
  • Nucleic acid sequence
  • Mode of replication.
  • Viruses are measured in various ways including:
  • Observing disease in host
  • Plaque assay (infectivity) – A selection of susceptible host cells is infected with the virus. An area of killed cells large enough to be seen = plaque.
  • Electron microscopy
  • Polymerase chain reaction
  • Immunological evidence of infection
  1. Examples of virus replication HIV, polio, influenza, herpes simplex virus

Virus replication occurs in a series of steps. This includes:

  • Binding to host cell: often very specific interaction between virus surface proteins and cell receptors

HIV gp120 : CD4

Epstein-Barr virus gp340 : CD21

Influenza virus haemagglutinin (HA) : sialic acid

  • Penetration: e.g. fusion of virus and cell membranes

(A) enveloped viruses: fusion between virus and cell membranes

  • HIV and measles virus at cell surface
  • Influenza virus with acidified intracellular vesicles

(B) non-enveloped viruses: disruption of host cell membrane integrity, genome or core crosses into cytosol

  • Polio virus
  • T4 in E. coli
  • The eclipse phase: expression of virus proteins and replication of nucleic acid.

Virus particles have been disassembled so no infectious particles are present

Expression of virus proteins

  • Highly regulated: temporal and quantitative

Replication of virus nucleic acid

  • Assembly: production of new infectious particles
  • Release: cell lysis or budding

Polio

  • The virion is 20nm and is very stable to acid pH and proteases.
  • It is a simple mRNA molecule that is used to synthesise giant polyprotiens.
  • It uses a complementary mRNA strand intermediate to replicate itself.

Influenza

  • Genome transcribed into mRNAs and cRNAs by virus RNA-dependent RNA polymerase.
  • mRNAs translated to virus protein.
  • cRNAs are copied into virus RNA (vRNA)

HIV

  • A retrovirus, (replicates via reverse transcription)
  • Enveloped particle, 110 nm
  • ss RNA genome (+ve), 10 kb, diploid
  • RNA genome converted into dsDNA via reverse transcription
  • DNA integrates into host DNA (= provirus), may remain dormant
  • Enables vertical transmission of virus
  • Expressed by host RNA pol II, to make HIV mRNAs, regulated by splicing

Herpes Simplex Virus

  • Virion: icosahedral capsid surrounded by lipid envelope, 130 nm
  • Genome: Linear ds DNA 152 kb, ~ 80 genes
  • Replication: in nucleus, may replicate via lytic cycle, or become latent
  • HSV-1, cold sores; HSV-2, genital herpes
  • Infection: via skin abrasions
  • Lytic replication: cascade of gene expression
  • IE proteins: regulatory
  • DE proteins: replicative, e.g. DNA polymerase
  • Late proteins: structural
  • Each class requires prior expression of proteins of the previous class
  1. Define the following terms as used in the description and classification of viruses: DNA virus, RNA virus, capsid type, envelope non-enveloped

DNA Virus: uses DNA as its genetic material. They may be double or single stranded.

Baltimore Classification
Group / Contains
I / dsDNA viruses
II / ssDNA viruses
III / dsRNA viruses
IV / (+)ssRNA viruses
V / (-)ssRNA viruses
VI / ssRNA-RT viruses
VII / dsDNA-RT viruses
ss: single-stranded, ds: double stranded
RT: reverse transcribing

RNA Virus: uses RNA as genetic material.

Capsid Type: the type of outer shell the virus has.Helical capsids, icosahedral (isometric) capsids, or enveloped.

Enveloped: Coating for virus particle.