Viruses, Prions, & Viroids

General Characteristics

• Small; 20-1000 nm

• Obligatory intracellular parasites — require a host to multiply

• Contain single type of NA (DNA, RNA)

• Contain protein coat (enclosed by an envelope of lipids, proteins, & carbohydrates)

• Multiply inside living cells using synthesizing machinery of the cell

• Cause synthesis of specialized structures that can transfer the viral NA to other cells

• Have few or no enzymes of their own

  • Any antiviral drug with affect the host cell

Virion

Complete, fully developed, infectious viral particle composed of NA & surrounded by a protein coat

Host Range

• Most viruses are able to infect specific types of cells of only one host species

• Host range is spectrum of host cells a virus can infect

• Viruses, in rare occasions, can cross host-species barrier & expand host range

• Host range depends on requirements for the virus

  • Specific attachment to the host cell

  • Availability within host of cellular factors needed for viral multiplication

Nucleic Acid

• For virus can be single or double stranded DNA or RNA

• Linear or circular

• In some viruses (influenza) it is in several separate segments

Capsid

• NA is protected by a protein coat

• Structure is determined by viral NA

• Accounts for most of mass of virus

• Composed of protein subunits (capsomeres)

• Arrangement of capsomeres is characteristic of particular types of virus

Envelope

• Covers the capsid

• Usually consists of combination of lipids, proteins, carbohydrates

• Contains proteins determined by viral NA & materials derived from host cell components

• May or may not be covered in spikes

• Some viruses attach to host using spikes

  • Used to identify virus

• Viruses susceptible to mutations can alter their surface proteins & escape any antibodies produced by the host

  • Ex: getting sick with influenza more than once

Helical Morphology

• Long rods

• Rigid or flexible

• NA found within a hollow, cylindrical capsid that has a _ structure

• Ex: viruses causing rabies & Ebola hemorrhagic fever

Nonenveloped Virsues

• Viruses with capsid NOT covered by an envelope

• Capsid functions to protect NA & promote attachment to host cells

Polyhedral Morphology

• Many-sides

• Most are shaped as an icosahedron (2- triangular faces & 12 corners)

• Capsomeres of each face form an equilateral triangle

• Adenovirus & poliovirus

Enveloped, Complex

• E: Roughly spherical

• C: Different shapes, poxviruses

Taxonomy

• Genus: virus

• Family: viridae

• Order: ales

• Viral species: group of viruses sharing same genetic info & host range

• Specific epithets are not used. Descriptive common names are used

  • Ex: Human immunodeficiency virus (HIV)

  • Subspecies may receive a number

  • Family Herpesviridae, Genus: Simplexvirus, human herpesvirus 2

Growing Phages in Labs

• Phage & host bacteria are mixed with melted agar & poured over hardened layer of agar growth medium

• Virus infects bacterium, multiplies, & released many more viruses

• After multiple cycles, bacteria in surrounding area are destroyed & effect → Clearing of Plaques

• Ex:

  • Living animals

  • Embryonated eggs

  • Cell cultures

Living Animals

• Not all viruses can grow and/or cause disease in lab animals

• After animal is inoculated, animal is observed for signs of disease or killed so that infected tissues can be examined

Embryonated Eggs

• Hole is drilled in shell of an egg, viral suspension is inserted into egg’s fluid

• Embryo dies or is damaged

• Still used for certain vaccine replication

Cell Cultures

• Replaced eggs as the preferred method

• Cells are separated from specific tissues & are provided with a suspension for nutrients & growth factors

• Normal tissue cells grow in a monolayer

• Viruses can cause a deterioration of that monolayer (cytopathic effect)

  • Continuous cell lines, used from transformed (cancerous) cells that multiply indefinitely

Identifying Viruses

• Serological methods like western blotting are the most common methods used

• PCR can be used to amplify viral DNA

Attachment

Chance collision between phage particles & bacteria, attachment (adsorption) occurs

Penetration

• Phage’s tail release enzyme called phage lysozyme that breaks the CW

• Virus injects viral DNA into bacteria through core of tail

Biosynthesis

• Host protein synthesis is stopped, & bacterial DNA is degraded

• Viral DNA is copied instead, & all translation is redirected to make phage proteins instead

• For several minutes complete phages are not found but only the separate components (eclipse period)

Maturation

Phage DNA & capsids are assembled into complete virions

Release

• Plasma membrane breaks open using the lysozyme protein that breaks CW

• Phages are released to infect new hosts

Lysogenic Cycle

• LC are immune to reinfection by same phage (but not by other phage types)

• Phage conversion — bacteria gains new properties, often more pathogenic

• Specialized transduction — bacterial genes are picked up in a phage coat & are transferred into another bacterium

Attachment (Animal)

• Receptor sites on plasma membrane

• May be spikes on surface of envelope of virus

Entry (Animal Viruses)

• Viruses enter by receptor-mediated endocytosis

  • Cell membrane folds inwards & forms a vesicles

• Viruses can also enter by fusion

  • Viral envelope fuses with cellular membrane & capsid is released into cell

Uncoating (Animal Viruses)

Separation of viral NA from its protein coat

Biosynthesis (Animal Viruses)

• DNA viruses: herpeviruses, papovaviruses, adenoviruses, & hepadnaviruses

• RNA viruses: enteroviruses, poliovirus, toga viruses, rhabdoviruses, reoviruses

• RNA viruses that use DNA: retroviruses

Mutations & Release (Animal Viruses)

• Capsid is assembled spontaneously

• Budding:

  • Envelope proteins are embedded in plasma membrane then a portion wraps around the capsid & adheres to the virus. Does not kill host cell

  • Nonenveloped viruses: released through ruptures in host cell membrane. Causes cell death

Oncogenic Viruses

• Viruses that can cause transformation of normal cells & result with cancer

  • Human papillomavirus (HPV) can cause cervical & anal cancers

  • T-cell leukemia viruses can cause leukemia & lymphoma

Latent Viral Infections

• Virus can remain in EQ with the host & not produce any disease for years until reactivation by a stimulus such as fever, sunburn, immunocompromised state

  • Ex:

    • Herpesviruses

    • Chickenpox & shingles

Persistent Viral Infections

• AKA Chronic Viral Infections

• Occur gradually over time & can be fatal

• Different from latent as in most chronic infections, detectable virus gradually build up over a long period, rather than appearing suddenly

Prions

• Proteinaceous infectious particles

• 1st discovered with a neurological condition that would only respond to treatment with proteases & not radiation suggesting that the infectious agent is only protein

• Ex: mad cow disease

• All 9 conditions are neurological diseases that cause large vacuoles in the brain to develop

Viroids

• Some plant diseases are caused by __, short pieces of RNA 300-400 nucleotides with no protein coat

  • Usually a closed folded, 3D structure that protects itself

  • Only known to cause disease in plants

Animal Virus

• Attachment

• Entry

• Uncoating

• Biosynthesis

• Mutations & Release

Lytic Cycle

• Attachment

• Penetration

• Biosynthesis

• Maturation

• Release