Ch.15 Viruses

8/3/2019 Ch.15 Viruses

http://slidepdf.com/reader/full/ch15-viruses 1/19

Ch. 13: Viruses,p. Dr.Kamel Adwan 1

Chapter 15

Viruses

I. Introduction to VirusesVirus originates from Latin word poison .

First virus discovered was tobacco mosaic disease virus (TMV) in

1890s.

In 1930s: TMV was isolated and purified.

By 1950s science of virology was well established.

II. Characteristics of viruses

They are a cellular, that is, they contain no cytoplasm or cellular

organelles.They are totally dependent on a host cell for replication. (They are

strict intracellular parasites)

Viruses contain a single type of nucleic acid (DNA or RNA, never

both) and a protein coat (capsid), sometimes enclosed by an envelope

composed of lipids, proteins, and carbohydrates.

Do not divide by binary fission or mitosis

Viruses are usually much smaller than bacteria and are

submicroscopic. Most range in size from 5 to 300 nanometers (nm),

although some Paramyxoviruses can be up to 14,000nm long




III. General Morphology

An intact infectious viral particle is called a virion

1. Helical viruses (for example, Tobacco mosaic virus) resemble long

rods and their capsids are hollow cylinders surrounding the nucleicacid.

2. Polyhedral viruses (for example, adenovirus) are many-sided. Usually

the capsid is an icosahedron. , with 20 triangular faces and 12 corners.

3. Enveloped viruses are covered by an envelope. There are enveloped

helical viruses (for example, Influenzavirus) and enveloped polyhedral

viruses (for example, Herpesvirus).

4. Complex viruses have complex structures. For example, many

bacteriophages have a polyhedral capsid with a helical tail attached.




Nucleic Acid

Viruses contain either DNA or RNA, never both, and the nucleic acid

may be single- or double-stranded, linear or circular, or divided into

several separate molecules

Smallest viruses have only 4 genes, while largest have several

hundred

Capsid and envelope

The protein coat surrounding the nucleic acid of a virus is called thecapsid.

The capsid is composed of subunits, capsomeres, which can be a

single type of protein or several types.

The capsid of some viruses is enclosed by an envelope consisting of

lipids, proteins, and carbohydrates.

For most viruses, is derived from host cell membranes by a process

called budding

Some envelopes are covered with carbohydrate-protein complexes

called spikes. These glycoprotein spikes function in attaching the virus to receptors

on host cells




IV. Taxonomy of viruses

Classification of viruses is based on type of nucleic acid, strategy for

replication, and morphology.

Nomenclature of viruses

1. Family groups ending in viridae (virus)

Picornaviridae are the small RNA viruses

Pico =small, rna = RNA, viridae = virus

2. Genus names end in virus

Based on location they infect Enterovirus infect the enteric tract and is a subdivision of

Picornaviridae

3. Species names are not italicized and not capitalized

A group of viruses sharing the same genetic information and

ecological niche (host).

Name of the disease used as species name

Poliovirus is a species of Enterovirus

Subspecies are designated by a number




V. Isolation, cultivation, and identification of viruses

1. Growth of bacteriophages in the laboratory

The plaque method mixes bacteriophages with host bacteria and

nutrient agar. After several viral multiplication cycles, the bacteria in the area

surrounding the original virus are destroyed; the area of lysis is called

a plaque.

Each plaque originates with a single viral particle; the concentration

of viruses is given as plaque-forming units (PFUs).




2. Growth of animal viruses in the laboratory

a. In living animals or in embryonic eggs

a. Cell cultures

Cells growing in culture media in the laboratory.

Primary cell lines and embryonic diploid cell lines grow for a

short time in vitro.

Continuous cell lines can be maintained in vitro indefinitely.




3. Viral Identification

Immunologic assays: Detect specific viral proteins or antibodies to

them. Examples are: Western blotting, ELISA,

Biological assays: Detect cytopathic effects (CPE) caused by viral

infection of cells. o Cytopathic effects include the stopping of mitosis, lysis,

formation of inclusion bodies, chromosomal changes, and

transformation

o Some viruses cause cytocidal effects (cell death), and others

cause noncytocidal effects

Plaque assays for lytic viruses

Hemagglutination assay: Many viruses clump red blood cells.

Molecular assays: Assay for viral nucleic acids.

o PCR (Polymerase chain reaction)

o Southerns (DNA) or Northerns (RNA)

Electron microscope




VI. Viral multiplication

1. Multiplication of bacteriophages

Lytic cycle: Phage causes lysis and death of host cellLysogenic cycle: Prophage DNA incorporated in host DNA

a. The lytic life cycle

T-even bacteriophages that infect E . coli

The virus life cycle can be divided into f ive stages:

1. Attachment phase of the lytic cycle, sites on the phage's tail fibers

attach to com-plementary receptor sites on the bacterial cell.

2. Penetration phase, phage lysozyme opens a portion of the bacterial

cell wall, the tail sheath contracts to force the tail core through the cellwall, and phage DNA enters the bacterial cell. The capsid remains

outside.

3. Biosynthesis, transcription of phage DNA produces mRNA coding for

proteins necessary for phage multiplication. Phage DNA is replicated,

and capsid proteins are produced.

4. Maturation, phage DNA and capsids assemble into complete viruses.

5. Release, phage lysozyme breaks down the bacterial cell wall, and the

multiplied phages are released




The one-step growth curve of virus replication

Following adsorption, the infectivity of the virus particles disappears,

a phenomenon called eclipse. This is due to the uncoating of the virus

particles.

During the latent period, replication of viral nucleic acid and proteinoccurs. The maturation period and release follow

The time from phage adsorption to release is called burst time (20 to

40 minutes).

Burst size, the number of newly synthesized phages produced from a

single infected cell, ranges from 50 to 200.

b. The Lysogenic Life Cycle

Bacteriophages capable of a lysogenic life cycle are termed temperate

phages.

Phage lambda, infects E. coli

1. Attachment and Penetration: Virus tail binds to specific receptors on

the cell surface and injects genetic material (DNA) into cell.

2. Circularization: Phage DNA circularizes and enters either lytic or

lysogenic cycle. Lysogenic cycle

3. Integration: Phage DNA integrates with bacterial chromosome and

becomes a prophage . Prophage remains latent.

4. Excision: Prophage DNA is removed due to a stimulus (e.g.:

chemicals, UV radiation) and initiates a lytic cycle.




Results of lysogeny

Lysogenic cells are immune to reinfection by the same phage

Host cell may exhibit new properties

C. diphtheria - Toxin production requires phage virus.

2. Multiplication of animal viruses

1. Animal viruses attach to the plasma membrane of the host cell.

2. Penetration occurs by endocytosis or fusion




3. Animal viruses are uncoated by viral or host cell enzymes.

4. Replication of NA and protein synthesis

Animal virusesgrouped by type of nucleic acid

There are three possible patterns of viral genome replication:a. DNA DNA. If viral DNA is double-stranded, DNA replication

resembles that of cellular DNA, and the virus uses DNA polymerase

produced by the host.

b. RNA RNA. Since host cells lack the enzyme to copy RNA, most RNA

viruses contain a gene that codes for RNA replicase, an enzyme that uses

viral RNA as a template to produce complementary RNA.

Picornaviridae + strand RNA acts as mRNA and directs the synthesis




of RNA-dependent RNA polymerase.

Rhabdoviridae - strand RNA is a template for viral RNA-dependent

RNA polymerase, which transcribes mRNA.

Reoviridae (double capsid) are digested in host-cell cytoplasm to

release double-stranded RNA for viral biosynthesis.

c. RNA DNA RNA. Some RNA viruses encode reverse transcriptase,

an enzyme that transcribes DNA from an RNA template

Retrovirus = (Retro = backward) RNA virus that uses reverse

transcriptase to transcribe DNA from the viral RNA genome.

Reverse transcriptase is a type of DNA polymerase that transcribes

DNA from an RNA template.

HIV (human immunodeficiency virus), the virus that causes AIDS

(acquired immunodeficiency syndrome) is a retrovirus.




5. After maturation, viruses are released. One method of release (and

envelope formation) is budding. Nonenveloped viruses are released through

ruptures in the host cell membrane.

VII. Viruses and Cancer

The earliest relationship between cancer and viruses was

demonstrated in the early 1900s, when chicken leukemia and

chicken sarcoma (cancer of connective tissue) were transferred to

healthy animals by cell-free filtrates.

Several viral genes have been identified as oncogenes.

Oncogenes = Genes found in viruses or as part of the normal

eukaryotic genome, that trigger transformation of a cell to a cancerous

state. Code for cellular growth factors or for proteins involved in the

function of growth factors.

Some tumor viruses transform cells by activating cellular oncogenes.

Viruses capable of producing tumors are called oncogenic viruses.

The genetic material of oncogenic viruses becomes integrated into the

host cell's DNA.

Transformed cells lose contact inhibition, exhibit chromosomal

abnormalities, and can produce tumors when injected into susceptible

animals. Certain viruses are implicated in human cancers:

The retroviruses ability to produce tumors is related to the

production of reverse transcriptase. The DNA synthesized from the

viral RNA becomes incorporated as a provirus into the host cell's




DNA.

A provirus can remain latent, can produce viruses, or can

transform the host cell.

Prions

Prions are infectious proteins first discovered in the 1980s.

Prion diseases, such as Creutzfeldt-Jakob disease (CJD) and mad cow

disease, all involve degeneration of brain tissue.

Prion diseases are due to an altered protein; the cause can be a

mutation in the normal gene for PrP or contact with an altered protein

(PrpSc

).

Viroids Infectious pieces of RNA that cause some plant diseases, such as

potatoes, and tomatoes

X. Antiviral therapeutic agents

Amantadine Influenza Inhibits uncoating

Acyclovir Herpes simplex Inhibits DNA polymerase

Azidothymidime HIV Inhibits reverse transcriptase

Interferon Cytomegalovirus Inhibits protein synthesis




Viral vaccines

First vaccine was used by Jenner (1798) against smallpox and

contained live vaccinia (cowpox) virus.

1. Live attenuated vaccines: Mutant viral strains produce an

asymptomatic infection in host.

Examples: Polio (oral, Sabin vaccine), measles, yellow fever, mumps,

rubella, and chickenpox.

Advantages: Better immune response

Disadvantages: May cause disease due to contamination, genetic

instability

2. Killed or inactivated vaccines formalin.




Examples: Rabies, and influenza A & B.

Advantages: Immunization with little or no risk of infection.

Disadvantages: Less effective immune response

3. Recombinant vaccines: Viral subunits are produced by genetically

engineered cells. Example: Hepatitis B

Advantages: Little or no risk of infection.

Disadvantages: Less effective immune response.



This document was created with Win2PDF available at http://www.daneprairie.com.The unregistered version of Win2PDF is for evaluation or non-commercial use only.

http://www.daneprairie.com/




Ch.15 Viruses

Documents

Transcript of Ch.15 Viruses