Chap. 13- Viruses, Viroids and Prions

viruses

• acellular particles capable of infecting host cells and causing disease

• not free-living, require host cell to multiply

  • use host metabolic systems

  • usually disrupt normal host cell function

features of viruses -3

• acellular- no plasma membrane

• contains either DNA or RNA

  • protein coast

  • may or may not- additional envelope of lipids

• very dew of their own enzymes

  • take over enzymes of their host

possible host of viruses

• animals

• fungi

• plants

• protozoa

• bacteria

size

range from 20-1000nm in length

which microscope views viruses

electron microscope

all viruses have at least two parts of the three

1.nuleic acid

2. capsid

3. envelope

nucleic acid

• either DNA or RNA as genetic material

• single stranded or double stranded- linear or circular

• segmented

• total amount→ a few thousand to 250 000bp

capsid

• protein coat surrounding the nucleic acid

• made up of individual proteins called capsomers

envolope

• not in all viruses

• lipid bilayer (membrane) acquired from the host cell

• external coating around the nucleocapsid

• additional viral proteins inserted into envelope called spikes

polyhedral

usually icosahedral- shape w 20 triangular faces

helical

long rods- can be rigid or flexible

enveloped

roughly spherical0 dictated by lipid bilater

complex

• polyhedral head w helical tail

• only found in bacteriophages

classification of viruses can be based on:

1. nucleic acid type

2. capsid structure

3. presence of envelope

nucleic acid type

• DNA or RNA

• single stranded or double stranded

• segmented or single molecule

capsid structure

• polyhedral

• helical

naming system of viruses

• family -viridae

  • Herpesviridae

• genus -virus

  • Simplexvirus

• species- specific epithets can also include uppercase and lowercase letter and numbers

  • humanalpha2 (original- Human herpasvirus 2)

adsoption

• viruses have attachment sites

  • recognize protein or glycoprotein of host membrane

penetration

• (most enveloped viruses) enter by fusion- lipids of envelope fuse w host cytoplasmic membrane

• naked virus enters the cell 

uncoating

viral nucleic acid is freed from the capsid

biosynthesis

• viral nucleic acids are replicated

  • DNA- nucleus

  • RNA- cytoplasm

• viral proteins are synthesized in the cytoplasm

• biosynthesis relies on host metabolic machinery

maturation and assembly

• new virions are assembled

  • capsomeres form the capsid

  • nucleic acid enters capsid → form nucleocapsid

release

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

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

  • steady release of mature viruses

  • host cell- alive for long time

interaction between viruses and animal hosts

• host defense plays major role in outcome of viral infection

  • protects against otherwise lethal infection

• most healthy humans carry a number of virus, antibodies to viruses

• if virus is transferred from the immune host to another individual

  • result in infection

acute infection

• usually short duration

• disease symptoms result from tissue damage

  • lysis of host cells- release and spread of virus particles

• host defense systems gradually eliminate virus

  • may take days or weeks

• host may develop long lasting immunity

  • ex. mumps, influenza, polio

acute infection with late complicaitions

• after acute period, some non-infectious particles remain

  • cause serious disease years later

• ex. Measles → subacute sclerosing panencephalitis

• fatal brain disorder- occurs up to ten years after recovery from measles

persistent viral infections

• virus is continuously present in body, but may or may not cause disease

  • no symptoms

• infected host can still serve as a reservoir

  • can transmit virus to others

chronic viral infection

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

  • may or may not cause noticeable symptoms

example of chronic viral infection

• Hepatitis B (serum hepatitis virus)

• transmitted by blood or sexually

• may have acute period- fever, nausea, jaundice

• after acute period, virus numbers stay high for the rest of the patient’s life

what happens when virus numbers stay high forever?

may cause cirrhosis or liver cancer

latent viral infections

• acute infection followed by symptomless period

provirus

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

effects of provirus

• disease can be reactivated years later

• symptoms may be different

Varicella-Zoster virus (Herpes family)

• cause: chicken pox (Varicella) in children

  • remains latent for years to cause shingles (Herpes-Zoster)

tumor

abnormal growth of tissue

benign tumor

does not spread

malignant tumor

• metastasize and invade nearby tissues

  • cancer

two types of genes cell growth is controlled by

1. proto-oncogenes

2. tumor suppressor genes

proto-oncogenes

genes that stimulate cell growth

tumor suppressor genes

genes that inhibit cell growth

tumor formation

• mutations in the genes (proto-oncogenes and tumor suppressor genes) can lead to uncontrolled cell growth

  • tumor formation and cancer

cancer causing viruses

• oncogenic viruses

• carry oncogenes

• most are DNA viruses

  • integrate viral DNA into the host chromosome as a provirus

  • oncogenes continue to be expressed 

oncogenes

genes that interfere with the cell’s control mechanisms

what are believed to cause liver cancer

Hepatitis B and Hepatitis C- believe to cause almost all cases of liver cancer

Epstein-Barr Virus

• causes infectious mononucleosis

• may cause lymphoma (cancer of white blood cells) and some cancers of the nose and throat

Human Papillomavirus (HPV)

• sexually transmitted- genital warts

  • believed to cause almost all cases of cervical cancer

Viroids

• naked RNA

• no protein coat

• results in some diseases in plants

  • not yet found in animals

prions

• infectious protein particles

• no genetic material (RNA or DNA)

• linked to several human and animal diseases

  • transmissible spongiform encephalopathies

  • sponge like holes in the brain

scrapie

sheep infected with prions

mad cow disease

eaten by cows

Varient Creutzfeldt-Jakob disease

MCD → eaten by humans

mode of infection

seem to be transmitted through food

can the infection be destroyed with high temp?

• can be destroyed by heat (480C)

  • or a combination of autoclaving in a solution of sodium hydroxide (strong base)

symptoms of disease in humans and treatment

• onset of disease occurs several years after infection

  • not clear why or how it accumulates in the brain

• always fatal- no treatment or cure

nucleic acid and capsid

• nucleocapsid

  • minimum required structure for a virus