Chapter 19:Viruses

Yes we are finally going to talk about Ebola…

A Slideshow by Miss Valerie

Virus

“A borrowed life”

Particle consisting of genes in a protein coat

Non-living because they need to infect others to replicate DNA

DiscoveryFound by looking at Tobacco Mosaic disease

1883 - Adolf Mayer rubbed infected sap on healthy leaves and spread disease

Speculated that it was a tiny bacteria

1893 – Dimitri Ivanowsky passed infected leaves through bacteria removing filter and sap still produced disease

Martinus Beijerinck – ruled out bacteria and saw agent could replicate

1935 – American scientist (go America!) Wendell Stanley found virus with electron microscope

StructureGenomes may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA

Grouped by type of nucleic acid

May have anywhere from 4 to a thousand genes (still little compared to bacteria)

The protein shell is called a capsidMade up of different protein subunits called capsomeres

Viruses have different shapes depending on protein arrangement

Rod-shaped (helical), Polyhedral (icosadral), other

Accessory Structures

Help viruses infect hosts

Viral Envelope – derived from membranes of the host cell (contain host cell phospholipids and membrane proteins)

How might this help a virus like the influenza virus?

Bacteriophages/phages: infect bacteria, complex capsids

Elongated icosahedral heads with a protein tail and anchoring fibers

Viruses NEED a Host

Viruses are obligate intracellular parasites -like something out of a sci-fi movie, viruses can only “survive” and replicate by taking over host cells

Without a host cell they are just packaged genes

Can only infect certain host species (host range)

For example you can’t give your dog a cold or measlesSome diseases are cross-species – H1N1, Bird flu, etc.

General Viral Replication

Virus binds to host cell

Virus enters cell and is uncoated, releasing its genetic material and capsid proteins

Host enzymes transcribe viral genome into viral mRNA, which in turn make more capsid proteins

Viral genomes and capsid proteins self-assemble into new virus particle, which then exit the cell to infect more cells

Replicative Cycle of Phages

Phages are best understood virus

2 methods that double-stranded DNA viruses can replicate

Lytic Cycle – host cell dies

Lysogenic Cycle – host lives

Some can use both methods – temperate phages

The Lytic Cycle

Virulent phage – replicates only using lytic cycle

A few cycles can destroy an entire bacteria population

Steps: Attachment – tail fibers bind to receptor sites

Entry of phage DNA and degradation of host DNA – phage injects host cell with DNA

Synthesis of viral genomes and proteins

Assembly – parts for new phages get put together

Release – enzyme damages cell wall, cell bursts and phages explode out

Why hasn’t every bacteria been destroyed in a phage explosion?

Natural selection favors bacterial mutants with phage resistant receptors

Sometimes the cell realizes the DNA is foreign before it can start replicating

Restriction enzymes cut up the foreign DNA

There is another way phages reproduce…..

The Lysogenic Cycle

Turns host cell into a phage DNA carrying and reproducing monster

Steps:Phage attaches to host cell and injects DNA

Phage decided to enter lysogenic cycle

Phage DNA integrates into bacterial chromosome, becoming a prophage

Bacteria reproduces normally creating daughter cells with prophage

When it gets a certain environmental signal the phage switches over to lytic cycle

Animal Viruses

Lots of variation among viruses

DNA vs. RNARNA has 3 classes based on their functions in a host cell.

Animal viruses can be more complex than bacterial phages

Can have both RNA and envelopes

How Viral Envelopes Affect Replication

Envelopes used to enter host cellViral glycoproteins bind to receptor sites on surface of host cell

STEPS:1. Binding of glycoproteins to receptors

2. Capsid and viral genome enter the cell; Capsid is digested by enzymes releasing genome

3. Viral genome acts as template for synthesis of complentary strand

4. Viral Genome is copied

5. RNA also works like mRNA making capsid proteins and glycoproteins

6. Vesicles transport glycoproteins to cell membrane

7. Capsid assembles around viral genome

8. New virus buds from host cell

RNA as the Genetic Material

Broadest variety of RNA genomes are animal viruses

Each class has different function in host cell… use enzymes to accomplish tasks:

Viral class IV can directly serve as mRNA – translated into viral protein right after infection

Class V – RNA acts as template for mRNA synthesis

Class VI - most complicated replication… retroviruses

RetrovirusesWork backwards -Use their RNA as a template for DNA using Reverse Transcriptase

Enveloped viruses with two identical molecules of single-stranded RNA and two molecules of reverse transcriptase

Replication:1. Virus enters host cell

2. Reverse transcriptase makes DNA strand complementary to viral RNA

3. Reverse transcriptase makes second strand of DNA complementary to first

4. New Double stranded DNA incorporated into cells DNA (provirus)

5. Cell starts making viral proteins and enzymes

6. Get packaged in vesicles and put themselves together then bud off from host

HIVMore than 35 million people are now living with HIV of those 3.2 million are under the age of 15

Since the beginning of the pandemic, nearly 78 million people have contracted HIV and close to 39 million have died of AIDS-related causes

Human immunodeficiency virus – attacks cells in the immune system (specifically T4 cells)

Destroys these cells when switches to lytic cycle

Remains in lysogenic cycle for many years building up virus in body

In advanced stages of infection AIDS can be diagnosed… AIDS is a syndrome caused by the HIV retrovirus (when you have less than 200 T4 cells per microliter of blood)

Medications that treat virus work at different parts of the replication process – “cocktails”

HIV Treatments

Virus Evolution

Developed after cells – because they infect every type of cell

Mobile pieces of genetic material like plasmids and transposons

Viruses change in response to host cell

Viruses often become drug resistant and can turn into “super viruses” (HIV)

Viruses in Animals

Viral infections cause symptoms through a variety of ways

May destroy cells

May produce toxins that lead to disease symptoms

Some of the molecular components are toxic

How much damage happens depends on the infected cells ability to regenerate itself

Ex: cold doesn’t permanently damage cells because lung cells can be repaired, but polio kills nerve cells which do not regenerate easily if at all

Other symptoms are a result of bodies defenses like fever, and body aches (immune response)

Vaccinations

Major medical technology for preventing viral infections

Vaccine – harmless derivative of pathogen that builds immune systems defenses against specific virus

Successful inoculation can eradicate a virus completely (ex: smallpox, polio, etc.)

The anti-vaccination movement has brought back many viruses once thought under control (whooping cough)

Viruses are nearly impossible to treat once infection has occurred – antibiotics do not work on viruses!!!

Emerging Viruses

Viruses that suddenly become apparent (come out of nowhere)

HIV, West Nile Virus, H1N1, Bird flu, etc.

EBOLA – first recognized in Central Africa in 1976One of several emerging viruses causing hemorrhagic fever

2014 epidemic (general outbreak) is now largest in history

Pandemic is a global epidemic or outbreakEx: H1N1 – by November of 09, 4 months after initial outbreak, 207 countries had cases of the disease

How Viruses Emerge

1. Mutation of existing viruses

2. Dissemination of viral disease from a small, isolated human population

3. Spread of existing viruses from other animals1. About ¾ of new diseases start this way

Example influenza

Plant Viruses

More than 2,000 types of plant viruses known

Kill tons of crops = loss of money

Part of what genetic engineering of plants aims to create is virus resistant plants

2 ways viruses spread in plantsHorizontal transmission – from outside source

Vertical transmission – inherits viral infection from parents

Viroids and PrionsViroids – circular RNA molecules, only a few hundred nucleotides long, that infect plants

Do not encode proteins

A single molecule can be an infectious agent that spreads a disease

Prions – infectious proteins that cause degenerative brain diseases

Mad cow disease/Creutzfeldt-Jakob disease

Act slowly (incubate for at least 10 years)

Virtually indestructible

Misfolded form of protein normally found in brain cells

Summary!! Aka THE END

A virus consists of a nucleic acid surrounded by a protein coat… generally considered non-living

Viruses replicate only in host cells – need to infect to spread

Viruses, viroids, and prions are formidable pathogens in animals and plants, as well as humans

GET VACCINATED!!!!