Lecture 1: Virus properties 1) Describe the nature of viruses: their small size, unique mode of replication and diversity. • Viruses are small in size varying from 20 – 450 nm.(the largest virus is still smaller than the smallest bacteria) • They are obligate intracellular parasites, composed of nucleic acid, protein and sometimes lipid and carbohydrates • Virus replication is unique: binding to the host cell- can be a very specific interaction between virus surface proteins and cell receptors penetration: o enveloped: fusion of virus and cell membranes (HIV,, Measles, Influenza) o non-enveloped: disruption of host cell membrane integrity, genome or core crosses into cytosol the eclipse phase: expression of virus proteins and replication of nucleic acid (highly regulated phages) assembly: production of new infectious particles – may be spontaneous or complex – internally or surface (budding of HIV) release: cell lysis or budding
Plaque forming units
Latent period
Mean burst size Eclipse phase
Time
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The latent period is the time from infection until the 1st new infectious cells are made from the components Lysogency- where latent form of the virus (phage) exists within the bacteria (not destroyed) – enabling transmission of virus to new daughter cells (vertical transmission) • Diversity: all species are infected by viruses. They may cause great plagues or be asymptomatic. 2) Define the following terms as used in the description and classification of viruses: DNA virus, RNA virus, capsid, enveloped, non-enveloped, symmetry. • Viruses can be broadly divided into RNA and DNA viruses according to the type of nucleic acid core, but there are many further subdivisions. • The viral nucleic acid can be either single- or double-stranded. RNA viruses have high mutation rates because their RNA polymerase is incapable of detecting and repairing transcription errors. These mutations lead to antigenicity, enabling RNA viruses to evade host immunity. • The nucleic acid, be it DNA or RNA, can be encapsulated by a nucleocapsid which can be helical or icosahedral (20 faced sphere). • In some cases, there is a lipid envelope surrounding the virus. The envelope is derived from the cell and is fragile; it is destroyed by organic solvents.
Lecture 2: Viral disease 1) Give examples of different viruses associated with infectious disease in humans and describe the way in which they cause disease. • Polio virus Virion 20 nm, icosahedron, very stable to acid pH and proteases Genome = mRNA, translation into a giant polyprotein, proteolytic cleavage into mature proteins Genome replicated via a –ve stranded RNA intermediate Entry via alimentary canal, replication in gut epithelial cells, shed in faeces. Infection of CNS is a rare event, not relevant to polio virus replication or transmission, but devastating consequences Polio likely to be the second disease to be eradicated • Influenza virus Virion enveloped, 100nm, contains helical nucleocapsid ss –ve polarity genome, 8 segments, 13.6 kb Genome transcribed into mRNAs by virus-encoded RNA-dependent RNA polymerase and replicated via +ve sense cRNA intermediate Influenza causes new epidemics because: Virus undergoes antigenic variation (antigenic shift/drift) Existing immunity is of limited value against the new strains Antigenic shift – when a human and an avian virus co infect a cell and undergo reassortment resulting in a new strain of virus. Antigenic drift – this is as a result of the accumulation of point mutations but produces less dramatic epidemics that antigenic shift. • HIV AIDS caused by a retrovirus isolated in 1983 (human immunodeficiency virus) No vaccine, no cure once infected, without drugs death is the outcome