Chapter 13 - Viruses, Viroids, Prions

What are viruses? (1.1)

• Acellular particles capable of infecting host cells - cause disease

  • not free living - need a host cell to multiply

What are obligate intracellular parasites? (1.1)

• viruses

  • use host metabolic systems + disrupt normal host cell function

What are features of viruses? (2.2 + 1.1)

• acellular - no plasma membrane

• contain only one type of nucleic acid - either DNA or RNA

  • surrounded by a protein coat

  • may or may not have an additional lipid envelope

• have few of their own enzymes 

  • tale over enzymes of the host

What is a single host species? (1.1)

• a pathogen/parasite/virus that can only infect a specific host

  • the virus must recognize features on the host cells surface to infect it

What is the range of viral size? (1.1)

• 20-1000 nm in length

  • needs an electron microscope

What are the parts of a virus? (3)

• nucleic acid

• capsid 

• envelope

What is a nucleocapsid?

• nucleic acid + capsid = minimum required structure for a virus

What is the nucleic acid of a virus? (3.1)

• DNA or RNA - not both

• single stranded or double stranded 

  • linear or circular 

• can be segmented - in several pieces 

What is a capsid? (1.1)

• a protein coat surrounding the nucleic acid

  • made of capsomeres

What are capsomeres?

• individual proteins that make up a capsid

What is an envelope? (3.1)

• a lipid bilayer acquired from the host cell

• an external coating around the nucleocapsid

• can have spikes

  • not present in all viruses

What are spikes?

• additional viral proteins inserted → the envelope

What are the shapes of viruses? (4)

• polyhedral

• helical

• enveloped

• complex

What is a polyhedral shape?

• icosahedral - a shape w/ 20 triangular faces

What is a helical shape?

• long rods - ridgid or flexible

What is an enveloped shape?

• roughly spherical - dictated by a lipid bilayer

What is a complex shape? (1.1)

• polyhedral head with a helical tail

  • only found in bacteriophages

How are viruses classified? (1.3 + 1.2 + 1)

• nucleic acid type

  • DNA or RNA

  • single stranded or double stranded

  • segmented or single molecule

• capsid structure

  • polyhedral

  • helical 

• presence of envelope 

How are animal viruses multiplied? (6)

• adsorption 

• penetration

• uncoating

• biosynthesis

• maturation + assembly 

• release

What is adsorption? (1.1)

• attachment to the host cell

  • viruses have attachment sites - recognize protein or glycoprotein of host membrane

What is penetration? (1.2)

• entry into the host cell

  • most enveloped viruses enter by fusion

  • naked virus enters via endocytosis 

What is fusion? (1.1)

• the penetration of enveloped viruses → host cell

  • lipids of envelope fuse with the host cytoplasmic membrane

What is uncoating?

• viral nucleic acid is freed from the capsid

What is biosynthesis? (1.2 + 2)

• process where viral nucleic acids are replicated

  • DNA replication - in nucleus

  • RNA replication - in cytoplasm

• viral proteins (capsomeres) synthesize in cytoplasm

• biosynthesis relies on host metabolic machinery 

What is maturation + assembly? (1.2)

• new virions are assembled

  • capsomeres form the capsid

  • nucleic acid enters the capsid - forms the nucleocapsid

What is release? (1 + 1.2)

• naked viruses - burst out, rupture the cell; host cell dies

• enveloped viruses - bud out, virus pushes through cytoplasmic membrane

  • steady release of mature viruses

  • host cell stays alive for a long time 

What is host defense? (2)

• plays a major role in the outcome of a viral infection

• protects against otherwise lethal injections

What do most healthy humans carry a number of? (2)

• viruses

• antibodies to viruses

What are acute infections? (4.1)

• infections of short duration

• disease symptoms result from tissue damage 

  • lysis of host cells - release + spread virus particles\

• host defense systems gradually eliminate virus 

• the host could develop long lasting immunity

What is the result of acute infection with late complications? (1.1.1)

• after acute period - some non-infectious particles remain

  • can cause serious disease years later

    • ex. measles → subacute sclerosing panencephalitis 

What are persistent viral infections? (2)

• virus is continuously present in the body; may or may not cause disease

• infected host can still serve as a reservoir + transmit virus to others 

What is a chronic viral infection? (1.1)

• the infectious virus remains present at all times after the acute period

  • may or may not cause noticeable symptoms

What are latent viral infections?

• acute infection followed by a symptomless period

• the virus integrates a copy of its DNA → a host cell chromosome and remains dormant

What is provirus? (1.2)

• the copy of the virus’ DNA that’s integrated into the host cell chromosome and remains dormant 

  • disease can be reactivated years later

  • symptoms may be different 

What is the varicella-zoster virus (herpes family)? (1.2)

• a latent virus

  • causes chickenpox in children

  • can reactivate to shingles

What is a tumor?

• abnormal tissue growth

What is a malignant tumor?

• tumor that metastasizes (spreads) and invades nearby tissues 

What is cell growth controlled by? (1.2)

• two types of genes

  • proto-oncogenes

  • tumor suppressor genes 

What are proto-oncogenes?

• genes that stimulate cell growth

What are tumor suppressant genes?

• genes that inhibit cell growth

What make cancer causing viruses, cancer causing? (1.1.2)

• they carry oncogenes - genes that interfere with the cell’s control mechanisms

  • most are DNA viruses 

    • integrate viral DNA → host chromosome as a provirus 

    • oncogenes continue to be expressed 

What are viruses associated with cancer? (3)

• Hep B and Hep C

• Epstein-Barr Virus

• HPV

How is Hep B and Hep C associated with cancer?

• Believed to cause almost all cases of liver cancer

How is Epstein-Barr virus associated with cancer? (1.1)

• causes infectious mononucleosis

  • may cause lymphoma and some cancers of throat and nose

How is HPV associated with cancer? (1.1)

• Sexually transmitted

  • believed to cause almost all cases of cervical cancer

What are viroids? (1.2)

• a virus-like infectious particle 

  • naked RNA - no protein coat

  • diseases in plants, not animals 

What are prions? (1.2.1)

• infectious protein particles

  • has no genetic DNA

  • linked to several human + animal diseases 

    • transmissible spongiform encephalopathies - sponge-like holes in the brain

What are the prions mode of infection?

• through food

What is scrapie?

• sheeps infected with prions

What is mad cow disease?

• cows infected by prions

What is variant creutzfeldt-jakob disease?

• mad cow disease eaten by human

What are characteristics of prions? (1.1 + 1.1)

• not destroyed by high temps

  • can be destroyed by 480^oC heat or a combination of autoclaving in sodium hydroxide (strong base)

• onset of disease occurs several years after infection in humans

  • always fatal