4 - MICROBIAL DIVERSITY - VIRUS

Viruses

infectious agent that can only replicate within a host organism

are not alive.

virions

Complete virus particles,

• possess either DNA or

  RNA,

• unable to replicate

• do not divide by binary fission,

  mitosis, or meiosis.

• lack the genes and enzymes necessary for

  energy production.

• depend on the ribosomes, enzymes, and

  metabolites (“building blocks”) of the host cell

Viruses are said to have five specific properties that distinguish them from living cells:

simplest of human viruses

consists of nothing more than nucleic acid surrounded by a protein coat (the capsid).

nucleocapsid

The capsid plus the enclosed nucleic acid are referred to as the

enveloped viruses

have an outer envelope composed of lipids and polysaccharides

Bacterial viruses

may also have a tail, sheath, and tail fibers. There are no ribosomes for protein synthesis or sites of energy production; hence, the virus must invade and take over a functioning cell to produce new virions.

1. type of genetic material

2. shape of the capsid

3. number of capsomeres

4. size of the capsid

5. type of host that it infects

6. type of disease it produces

7. target cell

8. immunologic or anti genic properties

Viruses are classified by the following characteristics:

DNA or RNA

type of genetic material

• helical capsid

• icosahedral capsid

• multiple helical capsid

• complex capsid

shape of the capsid

helical capsid

looks like corn

icosahedral capsid

looks like a hexagon

multiple helical capsid

spherical viral envelope

complex capsid

looks like a robot, with icosahedral head and helical tail

capsomere

many small protein units

capsid

protein coat

1. antibody

2. antigen

3. pathogen

4. a different antibody will be made for this antigen

immunologic or anti genic properties

RNA, DNA

Note that some viruses contain ______, whereas

others contain ________

single or double stranded.

the nucleic acid that they possess may either be

single- stranded positive sense RNA

functions as messenger RNA (mRNA),

single- stranded negative sense RNA

serves as a template for the production of mRNA.

envelope

Some of the viruses possess an ______________, whereas others do not.

bacteriophages (phages).

The viruses that infect bacteria

coliphages

those that infect Escherichia coli

Icosahedron bacteriophages, Filamentous bacteriophages, Complex bacteriophages

three categories of bacteriophages, based on their shape:

Icosahedron bacteriophages

an almost spherical shape, with 20 triangular facets; the smallest icosahedron phages are about 25 nm in diameter.

Filamentous bacteriophages

long tubes formed by capsid proteins assembled into a helical structure; they can be up to about 900 nm long.

Complex bacteriophages

icosahedral heads attached to helical tails; may also possess base plates and tail fibers.

Virulent Bacteriophages

always cause what is known as the lytic cycle, which ends with the destruction (lysis) of the bacterial cell.

1. Attachment

2. Penetration

3. Biosynthesis

4. Maturation

5. Lysis

Multiplication of Bacteriophages (Lytic Cycle)

Attachment

the phage attaches to the surface of the host

Penetration

the viral DNA enters the host cell

Biosynthesis

Phage DNA replicates and phage proteins are made

Maturation

New phage particles are assembled

Lysis

the cell lyses releasing the newly made phages

temperate bacteriophages ( lysogenic phages)

—do not immediately initiate the lytic cycle, but rather, their DNA remains integrated into the bacterial cell chromosome, generation after generation.

Animal Viruses

always cause what is known as the lytic cycle, which ends with the destruction (lysis) of the bacterial cell.

can only attach to and invade cells bearing appropriate surface receptors.

Multiplication of Animal Viruses

1. Attachment

2. Penetration

3. Uncoating

4. Biosynthesis

5. Assembly

6. Release

Latent Virus

When a virus is present in the body but exists in a resting (latent) state without producing more virus.

Epstein-Barr virus

example of latent virus

Latent Viral Infection

usually does not cause any noticeable symptoms and can last a long period of time before becoming active and causing symptoms.

Herpes virus infections,

cold sores, A fever, stress, or excessive sunlight can trigger the viral genes to take over the cells and produce more viruses; in the process, cells are destroyed and a cold sore develops.

Shingles,

a painful nerve disease that is also caused by a herpesvirus, is another example of a latent viral infection.

Then, when the body’s immune defenses become weakened by old age or disease, the latent chickenpox virus resurfaces to cause _____________

chickenpox infection

After a ________________________, the virus can remain latent in the human body for many years.

Antiviral Agent

Drugs used to treat viral infections

antibiotics

may be prescribed in an attempt to prevent secondary bacterial infections that might follow the virus infection.

Oncogenic Viruses

Viruses that cause cancer

viruses were shown to be the cause of various types of cancers in rodents, frogs, and cats

Epstein-Barr virus

(a type of herpesvirus) causes infectious mononucleosis (not a type of cancer)

nasopharyngeal carcinoma, Burkitt lymphoma, and B-cell lymphoma.

three types of human cancers caused by Epstein Barr virus

Human immunodeficiency virus

HIV

Human immunodeficiency virus

the cause of AIDS, is an enveloped, single-stranded RNA virus.

lentiviruses,

HIV is a member of a genus of viruses called ________________

Retroviridae (retroviruses)

HIV is in a family of viruses called

Mimivirus

it “mimics” bacteria, It is so large that it can be observed using a standard compound light microscope.

Mimivirus particle

has a 7 nm thick capsid with a diameter of 750 nm. An array of 80- to 125-nm long closely packed fibers project out ward from the capsid surface

Plant Viruses

are usually transmitted via insects (e.g., aphids, leaf hop pers, whiteflies); mites; nematodes (round worms); infected seeds, cuttings, and tubers; and contaminated tools (e.g., hoes, clippers, and saws).

Viroids

are infectious RNA molecules that cause a variety of plant diseases.

consist of short, naked fragments of single-stranded RNA (about 300–400 nucleotides in length) that can interfere with the metabolism of plant cells and stunt the growth of plants, sometimes killing the plants in the process

Prions

are infectious protein molecules that cause a variety of animal and human diseases.

are small infectious proteins that apparently cause fatal neurological diseases in animals,

Scrapie

diseases cause by prions in sheeps and goats

bovine spongiform encephalopathy

mad cow disease