Viruses called bacteriophages can infect and set in motion a genetic

VIRUS-HOST RANGE.• The host range of a virus is the spectrum of host cells the virus can infect.. • Some viruses have broad host ranges which may include several speciese.g.. VIRAL STRU

Introduction

ADOLF MEYER.

• A German scientist demonstrated that the disease was contagious and proposed that the infectious agent was an unusually small bacterium that could not be seen with a

microscope

• He successfully transmitted the disease by spraying sap from infected plants onto the healthy ones

• Using a microscope, he examined the sap and was unable to identify a microbe.

D IVANOWSKY.

• 1890: A Russian scientist proposed that tobacco mosaic disease was caused by a bacterium that was either too small to be trapped by a filter or that produced a filterable toxin.

• To remove bacteria, he filtered sap from

infected leaves.

• Filtered sap still transmitted disease to healthy plants.

• He ruled out the theory that a filterable

toxin caused the disease by demonstrating that the infectious agent in filtered sap

could reproduce

• Plants were sprayed with filtered sap from disease plants -> sprayed plants

developed tobacco mosaic disease ->sap from newly infected plants was used to

infect others

WENDELL M STANLEY.

• 1935:An American Biologist, from the Rockefeller Institute, crystallized the infectious particle now known as Tobacco Mosaic Virus(TMV).

• The purified virus precipitated in the form of

crystals.

• He was able to show that viruses can be better

regarded as chemical matter, than as living

organisms.

• Crystals retained the ability to infect

healthy tissue

• Subsequent determination of chemical

nature of TMV: Protein in combo with

nucleic, TMV were rods 300 nanometers

long, TMV was RNA surrounded by protein coat

GENERAL CHARACTERISTICS.

• Obligatory Intracellular Parasites

• Protein coated fragments of DNA or RNA that have become detached from the

genomes of cells Because they cannot

replicate on their own, they are not

organisms

• Viruses are generally host-specific

• They reproduce only within a certain host.

• Should be as many viruses as there are

kinds of organisms

VIRUS-HOST RANGE.

• The host range of a virus is the spectrum of

host cells the virus can infect.

• Some viruses have broad host ranges which may include several species(e.g swine flu and rabies).

• Some viruses have host ranges so narrow that they can:

• infect only one species(e.g phages of E.coli

• Infect only a single tissue type of one

species(e.g human cold virus infects only cells of the URT; AIDS virus binds only to specific receptors on certain white blood

cells

VIRAL SIZE.

• In the 1950’s, TMV and other viruses were

finally observed with electron microscopes.

• Smallest = 17 nanometers in diameter.

• Largest = 1000 nanometers(1 micrometer) in greatest dimension.

• Few are barely visible at light microscope level.

• Most are visible only via EM.

VIRAL STRUCTURE.

• The virus or virion, is just nucleic acid

enclosed by a protein coat, it’s a complete, fully developed infectious viral particle that

is a vehicle of transmission from one host to another

• Viruses are classified by differences in the structures of these coats

NUCLEIC ACID- VIRAL

GENOMES.

• Depending on the virus, viral genomes:

• May be double-stranded DNA, single-stranded DNA, double-stranded RNA or single-

CAPSID AND ENVELOPE.

• Capsid: Protein coat that encloses the viral genome

• It’s structure may be rod-shaped, polyhedral

or complex

• Composed of many capsomeres: protein

subunits made from only one or a few types

of protein

• Membrane that cloaks some viral capsid:

• Helps viruses infect their host

• Derived from host cell membrane which is usually virus-modified and contains

proteins and glycoproteins of viral origin

TAXONOMY OF VIRUSES.

• Viruses are not organisms and are not

classified in the kingdoms of life.

• Regarded as self-replicating portions of the genomes of organisms.

• In comparison to living things, vruses are acellular(not cells and do not consist of

cells), do not metabolize energy, no

photosynthesis, cell respiration or ferment.

• Viruses are cell parasites as they infect

organisms at all taxonomic levels.

• Oldest classification based on symptoms.

• ICTV group vruses into families, based on:

• Nucleic acid type, strategy for replication, and morphology.

• E.G.: Family= ends with viridae.

• Genus= virus.

GROWING VRUSES IN THE

LAB.

• Plaque method: Bacteriophages

• Living animals: Animal viruses

• Embryonated eggs: Animal viruses

• Cell cultures: Animal viruses (CPE)

• Primary cell lines, Diploid cell lines, and continuous cell lines

• Coopting host ceel’s resources to:

• Manufacture capsid protein,

• Assemble newly produced viral nucleic acid and capsomeres.

• Several mechanisms used to infect host cells with viral DNA.

• For example, T-even phages use an

elaborate tailpiece to inject DNA into the host cell

• Once the viral genome is inside its host cell,

it commandeers the host’s resources and

reprograms the cell to copy the viral genes and manufacture capsid protein

• Three possible patterns of viral genome

replication:

• 1 DNA >DNA: If viral DNA is stranded, DNA replication resembles that of cellular DNA, and the virus uses DNA

double-polymerase produced by the host

• 2 RNA ->RNA: Since host cells lack the enzyme to copy RNA, most RNA viruses contain a gene that codes for RNA replicase

• RNA replicase is an enzyme that uses viral RNA as a template to produce

complementary RNA

• 3 RNA ->DNA ->RNA: Some RNA viruses encode reverse transcriptase, an

enzyme that transcribes DNA from an RNA template

MULTIPLICATION OF BACTERIOPHAGES.

• Studies on lambda phage of E coli showed that double-stranded DNA viruses

reproduce by two alternative mechanisms: The lytic cycle and the lysogenic cycle

LYTIC AND LYSOGENIC

CYCLES.

• Lytic cycle:

• Results in the death or

lysis of the host cell.

• Bacteriophage takes

over the machinery of

the cell, so viral

replication and release

occur.

• Lysogenic cycle:

• Involves the incorporation of the viral genome into the host cell genome.

• Phage becomes a prophage, integrated into the host genome.

• Virulent bacteriophage

reproduce by a lytic

replication cycle= that

lyse their host cells.

• Later, the phage may reenter the lytic cycle and replicate itself.

• Temperate viruses= integrate and remain latent.

THE LYTIC CYCE.

THE LYTIC CYCLE.

THE LYSOGENIC CYCLE.

• Lysogeny

• Prophage

• Phage conversion

• Specilized transduction

THE LYSOGENIC CYCLE.

MULTIPLICATION OF ANIMAL VIRUSES.

• Attachment

• Penetration

• Uncoating

• Biosynthesis of DNA viruses

• Biosynthesis of RNA viruses

• Maturation and Release

DNA-CONTAINING VIRAL

MULTIPLICATION.

RNA-CONTAINING VIRAL

MULTIPLICATION.

RNA VIRAL MULTIPLICATION CONTD.

VIRUSES AND CANCER.

LATENT VIRAL INFECTIONS.

• Latent Infection:

• Herpes simplex

• Chicken pox (varicella)

• Shingles(zoster)

• Proteinaceous infectious particle that lacks nucleic acid

• Nine animal diseases fall into this category

• All nine are neurological diseases called

spongiform encephalopathies

• These diseases run in families, which

indicates a possible genetic cause

SPOKE TOO SOON.

• However, The diseases cannot be purely

inherited because mad cow disease arose

from feeding scrapie-infected sheep meat to cattle, and the new (bovine) variant was

transmitted to humans who ate undercooked beef from infected cattle