Viruses

Viral Structure

•Virion – infectious viral particle

–Assembled with a protein coat around nucleic acid

•Nucleic acid

–RNA or DNA

–Single or double

–Linear or circular

–If RNA, it can be sense (+) or nonsense (-)

•Sense – has codons

•Nonsense – need to make sense strand to translate

–2,000-250,000 nucleotides

•E. coli – 4 million

•Humans – 3 billion

Capsid – the protein coat

Subunits are capsomeres

Some capsids have pentons – protein-carb pointed projections for attachment

Envelop – around the capsid on some viruses (called enveloped viruses)

some envelopes have carb-protein complexes called spikes (also used for attachment)

Coronavirus (cold) and influenza (flu) virus have high mutation rates in spike genes

Mechanism for evading immune system

Slightly different spikes appear new to immune system

Viral Morphology part 1

Capsid structure can be distinct and sometimes help identify a virus

Helical

Cylindrical capsid

Nucleic acid wound up inside

Viral Morphology part 2

Capsid structure can be distinct and sometimes help identify a virus

Helical

Polyhedral

Most are icosahedrons (20 equilateral triangle faces and 12 corners)

Enveloped virus

Appear spherical due to the envelope

Complex viruses – unique shape

Bacteriophage – capsid and accessory structures

Pox virus – no clear capsid, just several protein layers around the nucleic acid

Viral Taxonomy pt 1

Viral “species” is a group of viruses sharing

Same genetic information….viral genomics

Same ecological niche (host range – animal virus, plant virus or bacteriophage)

Species names not used

Usually just given a Genus name that ends in ‘virus’ and a common name

Grouped into families

Names end in ‘-viridae’

Families based on

1. nucleic acid type

2. strategy for replication

3. morphology – capsid, envelope presence, physical dimensions

Viral Taxonomy pt 2

Most commonly identified by their common name

Learning the family groups has little relevance to disease ID

For example

Family: Herpesviridae

Genus: Simplexvirus

Herpes Simplex Virus 2 (HSV2)

aka HHV-2, human herpesvirus 2

Viral Identification

Require electron microscopy for viewing

Some are distinct enough to be recognized on sight

Others are identified by

Disease symptoms

Cytopathic effects

Serological methods (ELISA, Western blot)

Nucleic acid sequencing (RFLP/DNA fingerprinting, PCR)

Cultivating viruses for study

Obligate intracellular parasites Must be grown in living cells (usually their host) Can not be grown in culture media alone Three ways to grow animal viruses in lab: 1. Animal models 2. Embryonated eggs 3. Cell culture

Bacteriophages

Viruses that infect specific bacteria

Well studied example of a virus life cycle because they are easy to grow

Bacteria is easy to grow

Bacteriophages are easy to grow

Bacteriophages – 2 possible types of infections cycle

Lytic cycle: Ends with cell lysis

Host cell dies

Lysogenic cycle: Virus is carried in the host genome

Host cell remains alive

Multiplication of Animal Viruses pt 1

Attachment

Penetration/entry

Uncoating

Biosynthesis

Maturation and release

Viral capsid assembles spontaneously around the viral nucleic acid

Multiplication of Animal Viruses pt 2

Attachment

Penetration/entry

Uncoating is the separation of the viral nucleic acid from its protein coat

Non-enveloped – capsid digested by virl or host enzymes and viral enzymes allow the genetic material to escape the vessicle

Enveloped – performed by host enzymes (proteases) in the host cytoplasm

Biosynthesis of DNA viruses

Viral DNA replicated in host nucleus

Viral proteins made in the cytoplasm

Viral proteins migrate to the nucleus to join the DNA and assemble into virions

Virions transported through host endoplasmic reticulum for release

Both viral RNA AND proteins synthesized in the cytoplasm

Virions assembled in the cytoplasm

Biosynthesis of Retroviruses

Viruses have a ds RNA genome and make reverse transcriptase

Make a ds DNA copy of their RNA using the reverse transcriptase and incorporate the DNA into host cell genome as a provirus

Provirus can remain latent in the genome or be expressed to create virions

E.g. HIV can remain latent for years before it starts replicating

Viruses and cancer

Infectious cancer first observed in mice and chickens in early 1900s

A viral cause of cancer in humans was hard to recognize

Virus discovery part 1

• 1892: Ivanovsky - found the aagnt of tobacco mosaic diseases passes thru filter that retain bacteria

• 1898: Beijeernick made some finding but suggested that pathogen was distinct agnt

Virus discovery part 2

1898: Loeffler & Frosch: agent of hand foot mouth disease is filterable

agents small and only replicate in host. 0.2 micron filters

Virus discovery part 3

1901: first human virus, yellow fever

1903: rabies
1906: variola
1908: chicken leukemia virus, poliovirus

1911: rous sarcoma

1915: bacteriophages

1923: influenza

Darwinian machine

model for survival of fittest concept: too successful viruses may kill host and eliminate themselves. passive viruses may be eliminated