34-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...

34-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 34: Viruses


Discovery of viruses

• Unusual nature of viruses recognised in late 19th century

• Studies of tobacco mosaic disease showed that– the pathogen could be transmitted in sap– even when precipitated in ethanol the pathogen was still

capable of causing an infection– it was capable of passing through filters with pores so

small they filtered out bacteria

• Studies on other diseases of plants and animals showed that these pathogens behaved similarly

(cont.)


Discovery of viruses (cont.)

• Dutch pathologist, W.M. Beijerinck, described the tobacco mosaic pathogen as a ‘contagium vivum fluidum’, a contagious living fluid

• Later known as – virus

Latin for slimy liquid or poison

– bacteriophage bacterium eater

(cont.)


Discovery of viruses (cont.)

• 1930s: serological tests detected novel antigens in plants infected with tobacco mosaic

• After treatment, sap from infected plants showed streaming birefringence

– changed the plane of polarisation of light– indicated that sap contained disc- or rod-shaped particles

• Electron microscopy revealed rod-shape particles• Structure determined with X-ray diffractometry


Virions

• Virions are metabolically inert transmission phase of virus

– virus genome in protective coat

• When virion enters a host cell, the virion’s genome hijacks the cell’s metabolic systems

• Metabolically active reproductive stage uses – host’s metabolic systems to replicate – host’s ribosomes to produce viral proteins

(cont.)


Virions (cont.)

• Virions have characteristic shapes imposed by the covering of virus-coded virion or coat proteins

– enclose viral genome

• Rod-shaped virions with helically coiled genome• Isometric virions with folded genome


Fig. 34.4: Virion structure

(a) Rod-shaped virion

(b) Isometric virion

(cont.)


Fig. 34.4: Virion structure (cont.)

(c) Virion enclosed in a lipoprotein envelope


Bacteriophages

• Complex bacteria-infecting virions– rounded ‘head’ enclosing genome– ‘tail’ of outer sheath and inner core

• Terminal fibres of tail attach to host cell• Tail core forced through host wall• Viral genome enters cell


Fig. 34.5 a + b: Virions of phages


Viral genome

• Virus genome consists of genes required for infection

• Usually contained in a single virion but some viruses with divided genomes

• Depending on species, genome may be– RNA or DNA– single stranded (ss) or double-stranded (ds)


Replication• Viruses reproduce asexually

– may recombine with other viruses in mixed infections– those with divided genome may reassort parts of genome

• Double-stranded genomes– replicate using pathways similar to those of host

• Single-stranded genomes – transcribed into complementary strand by replicase

complex– enzyme includes host and viral proteins– complementary strand transcribed repeatedly to produce

multiple progeny

(cont.)


Replication (cont.)

• Plus-stranded genomes– are translated directly or– are translated from subgenomic mRNAs with same

sense as genome

• Negative-stranded genomes – must be transcribed into complementary plus strand

before translation– virions carry viral replicase


Fig. 34.6: Life cycle of simple virus


Retroviruses• ssRNA genomes transcribed into dsDNA on

infection of host• dsDNA incorporated into host chromosomes• DNA transcribed to produce ssRNA genomes• Replication strategy of retroviruses

RNA DNA RNA• Replication strategy of viruses with dsDNA

DNA RNA DNA


Fig. 34.7: Life cycle of retrovirus


Protein production

• Viral proteins may be – translated from mRNAs transcribed from genome– produced as polyproteins and hydrolysed by virus-

encoded proteases

• Many viral genomes – have overlapping genes that are read with different

reading frames– are ambisense, so that different parts of genome are

translated in opposite directions


Viral ecology

• Contagious viruses spread from host to host– body fluids– pollen – seed

• Viruses may be spread by vectors, which are often organisms that feed on viral host

– mosquitoes and ticks are vectors for viruses that infect animal hosts

– aphids, nematodes and fungi are vectors for viruses that infect plant hosts

– some viruses are spread by plants to sap-feeding insects


Viral infections

• Symptoms of viral infections are specific to that virus

• Often cause enhanced chance of transmission– coughing and sneezing expel fluids containing virions

– increased body temperature and CO2 production resulting from fevers attract mosquitoes

– yellowing of infected plant leaves attracts sap-feeding insects


Epidemics

• Epidemics (outbreaks of disease) occur when a virus

– appears in a new locality (arrival of smallpox in North America in 16th century)

– switches host (HIV moved from monkeys to humans)– mutates (influenza moved from waterbirds to humans)


Classification

• Viruses are polyphyletic– no common genes (unlike cellular organisms)

• Despite free movement of genetic material, viruses have well-defined species

– they have similar genetic sequences– they have similar methods of transmission– infection causes similar symptoms in host


Selection acts on viruses• Studies of Qβ bacteriophages suggest that species

are maintained by selection• Qβ phage genome of ssRNA

– high frequency of replication error– although new variants arise, population remains stable– one sequence favoured in competition—‘master copy’– uncertain whether master copy is real or average of all

variants

• Stabilising selection maintains well-defined species


Viral groups• Viral genera/groups are composed of species that

possess the same– structure and composition of virions– replication strategy– biochemistry– mode of transmission– ecology

• Related species may have differences in host preference

– different species– different parts of the same species


Relationships• Viral genera have complex interrelationships• Many genes form gene families that produce same

proteins – involved in genome replication– proteases– virion proteins

• Some gene families are– shared by viruses of plants, animals and bacteria– related to host genes– unique to virus groups or individual viruses


Satellite viruses

• Satellite viruses replicate in cells in the presence of a helper virus

– prevent replication of the helper virus– genomes of 1000–1400 nucleotides

• Satellite virus (STNV) of tobacco necrosis virus (TNV) encodes one protein (virion protein)

– other functions from helper virus and plant host


Satellite nucleic acids

• Satellite nucleic acids less complex than satellite viruses

– transmitted within virions of helper virus– helper viruses provides all proteins

• Genome is ssRNA circle 300–400 nucleotides in length

– transcribe complementary strand into long ssRNA strand made up of multiple copies of genome

– cut into unit genomes by ribozymes


Transposons

• Repetitive sequences on genomes of cellular organisms

– ‘junk’ or ‘selfish’ DNA– may be up to 10 000 bp long

• Repetitive DNA is complex• Transposons are regions of DNA that exist in

multiple copies– can move between host chromosomes– may compose up to 40 per cent of host genome


Retrotransposons• Retrotransposons (RTs) are related to retroviruses• Long interspersed elements (LINEs) are the most

common RTs in human genome– DNA transcribed to RNA– reverse transcribed to DNA by enzyme encoded by

LINES– resulting DNA integrates into new site on host genome– no virion protein gene– do not produce virions

• Short interspersed elements (SINEs) are also common

– satellites of LINEs using LINE enzymes for replication


Viroids

• Viroids resemble viruses but do not encode proteins

• Possess circular ssRNA genome c. 350 bp long• Resemble satellite RNAs but are able to be

replicated in host without helper viruses


Prions

• Prions are proteinaceous infection particles– encoded in host genome– lack nucleic acids– spread between species

• Diseased prion proteins enter cells and cause healthy cellular proteins to refold into diseased form

– structural alteration causes proteins to accumulate as amyloid plaques

– cells of nervous system


Virus control• Viruses are controlled most effectively by

preventing spread between hosts• Biochemical methods of control are difficult

because viruses use host biochemistry• Replication of some viruses can be retarded with

purine or pyrimidine analogues• Immunisation with

– non-infectious (dead) virions– live non-virulent virions– isolated virion proteins


Influenza

• Influenza orthomyxovirus infects respiratory tract– spreads by droplets produced by coughing

• Virions with outer membrane of lipid with surface proteins

– haemagglutinin (HA)– neuraminidase (NA)

• Influenza A genome in eight parts– reassorts in mixed populations– produce strains with novel combinations of HAs and NAs


Other viruses

• HIV-AIDS– human immunodeficiency lentivirus (HIV) is a retrovirus– causes acquired immunodeficiency syndrome (AIDS)– Lentivirus infects monkeys and other mammals– destroys immune system of host

• Arboviruses– arthropod-borne viruses transmitted between hosts by

ticks, mosquitoes and other arthropods– Murray Valley encephalitis flavivirus (MVEV)– West Nile flavivirus (WNV)

34-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...

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