The VirusesUnit 3

Donna Howell

Medical Microbiology

Gaffney High School

Coming soon to a body near you -

He He He!

History

• In 1892, Dmitri Iwanowski studied the tobacco mosaic virus. He found out that this unknown agent could pass through a filter, unlike bacteria.

• In 1930’s, Wendell Stanley found out that this unknown agent could crystallize, which means it is a chemical.

• In 1933, electron microscope was invented, and could see viruses by 1941.

What is a Virus?

• Any of various simple submicroscopic parasites of plants, animals, and bacteria that often cause disease and that consist essentially of a core of RNA or DNA surrounded by a protein coat. Unable to replicate without a host cell, viruses are typically not considered living organisms.

How Are Viruses Classified?

• Viruses are classifed according to what type of nucleic acid core they contain: either RNA or DNA.

DNA or RNA –

that is the question!

Further Classification

Category Tissue Affected Example Diseases

Dermotropic Skin and subcutaneous tissue

Chicken pox, shingles, measles, mumps, smallpox, rubella, herpes simplex

Neurotropic Brain and central nervous system tissue

Rabies, West Nile virus, polio

Viscerotropic Internal organs Yellow fever, AIDS, Hepatitis A and B, Mono, Dengue fever

Pneumotropic Lungs and other respiratory structures

Influenza, common cold

Viruses can be further classified based on what part of the body is affected:

Size Of Viruses

• Viruses are among the smallest known infectious particles. They come in many sizes:

Did You Know?

• There is a connection between herpes and psychosis due to inflammation of the brain!

Shapes Of Viruses

• Viruses also come in many different shapes:

Components Of Viruses

• Capsid – the outer protein coat

• Capsomere – protein units that make up the capsid

• Genome – DNA or RNA • Envelope – an enclosing

structure similar to cell membrane

• Spikes – projections on outside of virus that allow it to attach to host cell

Viral Replication

Viruses can replicate (reproduce) in one of two ways:

1. Lytic cycle – this is where the virus replicates in the host cell, and causes host cell to “lyse”, or burst to release new viruses

2. Lysogenic cycle – this is where the virus incorporates its DNA into the host DNA as a prophage, and replicates as host DNA replicates.

Lytic Cycle

There are 6 stages of this cycle:1. Attachment – virus attaches to

host2. Penetration – virus enters host3. Uncoating – capsid removed,

nucleic acid released4. Synthesis – parts for new viruses

synthesized5. Assembly – parts are assembled6. Release – host cell lyses, new

viruses released

Ex: Flu virus

Lysogenic Cycle

This is when the virus enters the host cell, but does not replicate immediately. Instead, it incorporates its genes into the host’s genes, and then one day will complete the lytic cycle. Ex: AIDS

Viral Identification

• In many cases, when a virus infect a cell, it causes cell death or at least cell damage. The specific imprint that the virus leaves is called the cytopathic effect (CPE). By observing the effects under the microscope, scientists can sometimes identify the type of viral infection. Many viruses have a characteristic CPE.

How Are Viruses Cultured?

Viruses can be cultured in one of 3 ways:

1. Inside a live animal

2. Inside a fertilized chicken egg

3. In a viral cell culture, which is a thin layer of cells in a petri dish with special nutrients

Touch my egg you die,

buster!

Did You Know?

• There is a link between in-utero exposure to flu and schizophrenia later in life!

Shake ya tailfeathers!

Defenses Against Viruses

What are some defenses against viruses?

• Body can produce antibodies, which attack and kill viruses.

• Some drugs can be used, such as interferon or AZT

• Can have a vaccine against a virus

Go body go!

Types of Vaccines

There are 3 types of vaccines that we use for viruses:

1. Inactivated vaccine – those made from real inactivated virus. The genome is destroyed, but the capsid remains.

2. Attenuated vaccine – those made from “live” viruses, but in such a low dose that it does not usually cause disease.

3. Genetically-engineered vaccine – viral proteins are produced by yeast cells, so NO actual viruses used. Safest!

I got your vaccine,

baby!

Viroids

• An ultramicroscopic, single-stranded molecule of RNA without a protein coat.

• Mostly infect plants• Cause stunted growth

and abnormal development.

Come on viroids!

Prions

• Proteinaceous infectious particle (single piece of protein with no capsid)

• Can survive heat, radiation, and chemicals that normally inactivate viruses.

• Causes kuru and mad cow disease.

I’m a mad cow – get it?

Viruses and Cancer

It is well known that certain viruses have cancer-causing abilities. Here are a few:

• HPV virus – cervical cancer

• Hepatitis B virus – liver cancer

• HTLV virus – leukemia• Epstein-Barr virus –

Burkitt’s lymphoma

Get it – horoscope,

cancer, crab????

HIV Virus (AIDS)

Human Papilloma Virus (Genital Warts)

Herpes Virus (Chicken Pox)

Chickens don’t have

pox!

Filovirus – Ebola Hemorrhagic Fever

Rotavirus – The “Stomach Bug”

HBV – Hepatitis B

Orthomyxovirus – The “Flu”

Paramyxovirus (Measles)

Picornavirus (Polio)

Rhabdovirus (Rabies)

The End!Science is cute

and cuddly, boys!