PART 1 Acellular Microbes:

Virus, Viroids and Prions

Complete virus particles are called virions.

Size range from 10-300 nm in diameter. Virus could only be seen using an EM. Viruses can infect all organisms! Many human diseases are caused by

viruses. Oncogenic viruses or oncoviruses caused

specific types of cancer.

They posses either DNA or RNA They are unable to replicate on their own They do not divide by binary fission, mitosis or

meiosis They lack genes and enzymes necessary for

energy production They depend on the ribosomes, enzymes, and

metabolites

Genetic Material- DNA or RNA Capsid– composed of capsomeres Envelope- composed of lipids and

polysaccharides Some have tail, sheath and tail fibers

Type of genetic material: DNA or RNA Shape of the capsid Number of capsomeres Size of the capsid Presence or absence of an envelope Type of host that it infects Type of disease it produces Target cells Immunologic or antigenic properties

One theory states that viruses existed before cells.

The other theory states that cells came first and that viruses represent ancient derivatives of degenerate cells or cell fragments

Family names end in -viridae Genus names end in -virus Viral species: A group of viruses sharing

the same genetic information and ecological niche (host).

Common names are used for species Subspecies are designated by a number

Herpesviridae- Herpes virus, icosahedral, enveloped, DNA• HSV 1- orofacial

herpes• HSV 2- geniteal herpes• VZV- chicken pox and

shingles

Retroviridae- Retrovirus, RNA, helical, enveloped• Lentivirus- HIV

HIV

Adenoviridae- Adenovirus, icosahedral, non-enveloped, DNA• Human Adenovirus

A- respiratory infections, conjunctivitis, gastroenteritis.

• Viral encephaliltis and meningitis

Picornaviridae- Picornaviruses, icosahedral, RNA, and non-enveloped.• Poliovirus- causes

poliomyelitis• Coxscakie viruses-

hand, foot and mouth diseases

• Echoviruses- nervous disorder

Coronaviridae- coronavirus, helical, RNA and enveloped• SARS• Common colds• Gastroenteritis• Upper and lower

respiratory tract infections

Three categories: Icosahedron bacteriophages- Almost

spherical shape with 20 triangular facets Filamentous bacteriophages- long tubed

form by a capsid proteins assembles into a gelical structure

Complex bacteriophages- icosahedral heads attached to helicaltails

Complex Bacteriophage

Single stranded DNA phages Double stranded DNA phages Single stranded RNA phages Double stranded RNA phages

Attachment Phage attaches by tail fibers to host cell

Penetration Phage lysozyme opens cell wall, tail sheath contracts to force tail core and DNA into cell

Biosynthesis Production of phage DNA and proteins

Maturation Assembly of phage particles Release Phage lysozyme breaks cell wall

LYTIC CYCLE

Figure 13.10.1

Attachment:Phage attaches to host cell.

Penetration:Phage pnetrates host cell and injects its DNA.

1

2

3

Bacterial cell wall

Bacterial chromosome

Capsid DNA

Capsid

Sheath

Tail fiber

Base platePin

Cell wall

Tail

Plasma membrane

Sheath contracted

Tail core

4 Maturation:Viral components are assembled into virions.

Tail

5 Release:Host cell lyses and new virions are released.

DNA

Capsid

Tail fibers

The Lysogenic CycleThe Lysogenic Cycle

Viruses that infects human and animals. Some are DNA viruses; others RNA. May consist solely of nucleic acid

surrounded by a protein coat or they maybe more complex.

Attachment Viruses attaches to cell membrane

Penetration By endocytosis or fusion Uncoating By viral or host enzymes Biosynthesis Production of nucleic acid and

proteins MaturationNucleic acid and capsid proteins

assemble Release By budding (enveloped viruses)

or rupture

Multiplication of Multiplication of Animal virusesAnimal viruses

Attachment, Penetration, Attachment, Penetration, and Uncoatingand Uncoating

Figure 13.14

Release of an enveloped Release of an enveloped virus by buddingvirus by budding

Multiplication of DNA VirusMultiplication of DNA VirusVirion attaches to host cell

Virion penetrates cell and its DNA is uncoated

Early transcription and translation; enzymes are synthesized

1

2

3

DNA

Late transcription; DNA is replicated

4

Late translation; capsid proteins are synthesized

5

Virions mature6

Capsid

Papovavirus

Host cell

DNA

Cytoplasm

Virions are released7

Capsid proteins

mRNA

Latent Viral Infections• Virus remains in asymptomatic host cell for

long periods• Cold sores, shingles

Persistent Viral Infections• Disease processes occurs over a long

period, generally fatal• Subacute sclerosing panencephalitis

(measles virus)

Viral InfectionsViral Infections

Activated oncogens transform normal cells into cancerous cells.

Transformed cells have increased growth, loss of contact inhibition, tumor specific transplant and T antigens.

The genetic material of oncogenic viruses becomes integrated into the host cell's DNA.

Oncogenic VirusesOncogenic Viruses

Epstein-Barr Virus- cause of infectious mononucleosis (not a type of cancer), but also causes 3 types of cancer• Nasopharyngeal cancer• Burkitt’s Lymphoma• B-cell Lymphoma

HBV and HCV• Hepatocellular

carcinoma HSV 8

• Kaposi’s Sarcoma- type of cancer common in AIDS patients.

HPV• Causes different kinds

of cancer, including cervical cancer.

Usually transmitted via insects; mites; nematodes; infected seeds, cuttings and tubers; and contaminated tools.

Ex. TMV

Some Plant VirusesSome Plant Viruses

Consists of short, naked fragments of single stranded RNA.

Can interfere with the metabolism of plant cells.

Transmitted between plants. Ex: potato spindle tuber, citrus exocortis,

diseases of chrysanthemums So far, no animal diseases have been

discovered that are caused by viroids.

Infectious proteins Inherited and

transmissible by ingestion, transplant, & surgical instruments• PrPC, normal cellular

prion protein, on cell surface

• PrPSc, scrapie protein, accumulate in brain cells forming plaques

Awarded: 1997 Nobel Prize for Medicine

He coined the term “prion”

He studied the role of prions in disease

Fatal spongiform encephalopathies: Sheep scrapie Creutzfeldt-Jakob Disease Gerstmann-Sträussler-Scheinker Syndrome Fatal familial insomnia in humans Kuru