Viruses

Warts are a skin virus!

Herpes mouth virus:

Other Viral Diseases

• Measles

• Polio

• Smallpox

• Influenza

Viruses & Cancer • Hepatitis B Virus • Human Papilloma

Virus

• HPV

Tree Man - HPV

Is a Virus a Living Organism?

Properties of life – Cellular Respiration

– Reproduction

– Metabolism

– Homeostasis

– Heredity

– Responsiveness

– Growth and development

Are viruses living or non-living?

Nonliving

• Not cells

• No respiration

• No growth

• No development

• No metabolism

• Can be

crystalline,

dormant

Living

• Replicate using a

host

• Contain DNA,RNA

• Contain protein

coat

Virus Characteristics

• Non-living:

– Do not grow

– Do not develop

– Do not carry out

respiration

• All they can do is

replicate but they

can’t to that without a

host cell

Viruses are very small, even

compared to bacteria, and they

can only be viewed with the

electron microscope. The

smallest viruses are about 20nm

and the largest about 300nm.

Virus Structure

• Virus means Poison in Latin

• Composed of nucleic acids (DNA or RNA) wrapped in a protein coat called a capsid

• Some viruses have an extra layer of protection called an envelope

Viral Structure

Nucleic Acid – RNA or DNA

Capsid – protein coat that surrounds the DNA

or RNA in a virus

Lipid Membrane – a membrane around the

capsid in many kinds of viruses; helps the

virus enter cells (“enveloped” viruses;

without the membrane, the virus is “naked”)

– Made of proteins, lipids, and glycoproteins

Origin of Viruses

• Scientists suggest viruses might have

originated from their host cells

• May be nucleic acids that broke away

from their host but maintain the ability

to replicate parasitically within the host

cell

Viruses are host cell specific.

Most viruses are restricted to certain kinds

of cells (those that infect plants cannot

infect animal cells).

Why?

Scientists think that viruses originated from

escaped genetic material from host cells.

Plant peach virus

Viruses can

infect

• Plants

• Animals

• Bacteria

Differences in Viruses

Species Specific Viruses:

• Ex. Smallpox

(eradicated)

• Non-species specific:

Flu & West Nile

Cell-type Specific:

• Ex. Polio – only

infects intestinal &

nerve cells

Naming Viruses

• Viruses are named

after the diseases

they cause or the

tissues or organs they

infect

• Ex. Bacteriophage –

Infects a bacterium

Virus Categories • DNA viruses – stable, do not mutate rapidly

– Single-stranded or double-stranded

– Smallpox, Hepatitis B

– Warts

– Chickenpox

– mononucleosis

• RNA viruses – mutate rapidly, unstable – Single-stranded or double-stranded

– Rhinovirus

– Human immunodeficiency virus (HIV)

– Influenza viruses

– Rabies

Virus Shapes

Helical

Rodlike with capsid proteins winding around the core in a spiral

Tobacco Mosaic Virus

Virus Shapes

Polyhedral

Has many sides

Most polyhedral capsids have 20 sides

and 12 corners

Virus Shapes

Polyhedral capsid attached to a helical tail.

Chicken Pox & Polio Virus

Rabies virus Hepatitis B

(Liver)

How do viruses replicate?

2 methods of replication:

1. Lytic Cycle – the virus enters the cell, replicates itself hundreds of times, and then bursts out of the cell, destroying it.

2. Lysogenic Cycle – the virus DNA integrates with the host DNA and the host’s cell helps create more virus DNA when it divides. An environmental change may cause the virus to enter the Lytic Cycle.

Attachment

• Attachment proteins on virus recognize & attach to the receptor site on the plasma membrane of the host cell

• Puzzle pieces

• http://www.slic2.wsu.edu:82/hurlbert/micro101/images/101VirusFusionAn.gif

Viruses Enter Living Cells

Viruses enter bacterial cells by punching a

hole in the cells wall and injecting its DNA

Viruses Enter Living Cells

Viruses enter plant cells through tiny rips in

the cell wall.

Viruses enter animal cells by endocytosis.

Virus Cycles

• Once a virus enters a cell

it either enters into the

Lytic or Lyosogenic Cycle

• Lytic Cycle – takes over

host cell immediately &

kills cells by (bursting or

lysis)

• http://whyfiles.org/132aid

s2/images/virus_movie.gif

Viral Replication: Lytic Cycle

Lysogenic Cycle

• Viral DNA becomes

incorporated into host

cells DNA & becomes

a Provirus

• Provirus is replicated

with host cells

chromosome

• http://goldiesroom.org

/video_archive.htm

• At any time a provirus

can be activated &

enter into a lytic cycle

– Physical Stress

– Sunburn

– Emotional Stress-

anxiety

– Immuno-depressed

Provirus Examples

• Cold sores (Herpes simplex I)

• Genital Herpes (Herpes simplex II)

• Hepatitis B

• Chicken Pox (Varicella zoster)

• Shingles – painful infection of some nerve cells

Also important…

Any agent (not just viruses) that causes disease is a pathogen.

When a virus inserts its genetic material into a host’s DNA, it is called a provirus.

Some viruses replicate very slowly and only cause damage when the conditions are “right”. (cold sores)

Mutating viruses

Viruses can mutate when they copy the

genetic material

– Copy something wrong

– Mistake proves useful

– More powerful virus (more infectious)

Viruses don’t mutate often, except…

– Influenza

– HIV

Retroviruses

• Have RNA as their

nucleic acid – most

complex replication

cycle

• Ex. HIV

• Make DNA from RNA

using the enzyme

reverse transcriptase

HIV

• Double-stranded

DNA enters into

host cells

chromosome &

becomes a Provirus

• Can test for the

enzyme reverse

transcriptase to see

if you have the virus

The spread of West Nile virus (1999 –

2002) – bird, horse, mosquito or human

Viruses can be beneficial…

Bacteriophages – attack & destroy bacteria

Baculovirus – ebola-like virus that attacks

insects

– Could use for pest control in crops

• Cabbage loopers eat

cabbage crops

• Virus can kill pests in days

– (it’s really gross)

… and then there are those that are not so good….

Viroids

• Infectious particles of naked RNA.

• Similar to viruses, but lack a capsid.

• Ex:

– Coconut Blight

– Chrysanthemum Wilt

Viroids

• Composed of a single circular strand of RNA with no protein coat

• Diseases in Plants

• Enter through wounds or insect bites

• Don’t use surface recognition

• Doesn’t undergo lytic or lysogenic phases

• May cause stunted growth & yield losses

• Mosaic viruses are not harmful cause beautiful patterns

What is a prion?

• Prions have recently been implicated in diseases called Encephalopathies which affect the brain. The most common ones that are know today are Bovine Spongiform Encephalopathy or BSE in cattle, Scrapie in sheep and new variant Creutzfeldt-Jakob disease in humans.

• Prions are infectious agents but they are not like bacteria or viruses. Prions are pieces of protein that can transfer the disease from one organism to another.

• IT is simply a protein where the chains have folded incorrectly. (secondary structure) The chains have the same amino acid sequence as the normal protein. It is not known how the malfunction in protein folding occurs

Prion Action

Prions

• Proteins but no

nucleic acids

• Cause proteins to fold

incorrectly

• Animal diseases:

– Mad Cow Disease

– Creutzfeldt-Jacob

disease

Treatments for Viruses

• Vaccines (preventative).

• Some Drugs

– Ex: Ara-A Acyclovin

• Comment - some treatments are working

on the reverse transcriptase.

Cow pox vaccination 1749

• Acquired

Immunization

• Artificial

injection of a

small amount

of virus

• Body’s immune

response

makes

antibodies

Human Immunodeficiency Virus

(HIV)

Acquired Immuno Deficiency

Syndrome (AIDS)

Basic Structure

• Viral envelope – lipid bilayer; glycoproteins protrude from surface

– Glycoproteins enable virus to recognize surface proteins of special immune cells and to enter the cell (like a key to the cell’s door)

• 2 strands RNA – only 9 genes; 3 are found in many viruses (structural proteins)

• Reverse Transcriptase – turns RNA into DNA (this makes HIV a retrovirus); DNA instructs cell to make more viruses

HIV Virus

HIV virus infects T-cells

• HIV virus Weakens the immune system

• AIDS patients die of “common” diseases when T cell (WBC) count falls

AIDS = break down of the immune

system & death due to common

diseases versus death by AIDS virus

B Memory cells……..

Activation of the Immune Response

AIDS Virus on a T Cell

Activation of the Immune Response

AIDS virus destroys Helper T cells !

Loss of T Cells makes the patient

susceptible to common infections

Prevent attachment

Prevent Replication

Prevent Coat formation

How Is HIV Spread?

• Sexual contact

• Sharing contaminated needles

• Blood transfusions

• Breast feeding (mother to baby)

• Mother to baby during pregnancy or birth

Think about it…

• In the US, there is better than a 1/1000 chance of

contracting HIV during unprotected sex

• A person can be contagious for more than 10 years before

any sign of the disease is apparent

• HIV becomes AIDS when the number of immune cells drops

below a predetermined number

• No one dies from HIV or AIDS; people die from secondary

infections (ranging from the common cold to cancer)

• More than 3 million people (size of Chicago) die each year

• There are approx. 14,000 new cases of HIV

worldwide every day

Viruses are not usually included in

classification systems as they are non-cellular

and they are dependent on a host cell for their

replication and metabolic processes.

VIRUSES

Review

What is a

virus?

• Viruses have very simple structures -

• Most of them are simply a protein coat surrounding a core of nucleic acids.

• Could be DNA or RNA

LIVING or NOT LIVING?

• Viruses are not capable of living as free entities; in fact they are generally inert and inactive outside of their host cells, which are needed only for the virus to replicate.

• Viruses can infect all types of cells animals, plants and bacteria, but each different type of virus is specific to its particular type of host cell.

Bacterial Viruses

• Best understood of all viruses.

• May show two types of virus life cycles:

– Lytic Cycle

– Lysogenic Cycle

VIRUS

REPLICATION

Once inside the host cell the virus hijacks the cells genetic machinery and uses it to make more viruses.

The lytic cycle – short, replicates quickly, destroys host cell when releasing thousands of new viruses. Cold, flu, Ebola.

The lysogenic cycle

– virus enters cell,

replicates DNA,

and WAITS.

Herpes, wart, HIV

Your immune system responds

to the infection, and in the

process of fighting, it produces

chemicals called pyrogens that

cause your body temperature to

increase.

Lytic Mode Triggers

• Switches the host from Lysogenic to Lytic

Cycles

• ex: radiation, chemicals, stress

Herpes Viruses

• Use nuclear membrane.

• Viral DNA integrated into Host DNA as

a provirus.

• Shows both lytic and lysogenic life cycles.

RNA Viruses

• Classes III - VI

• Class VI - Retrovirus - use Reverse

Transcriptase to make DNA from an RNA

template.

Human Immunodeficiency Virus

• HIV - causes AIDS.

• Retrovirus from chimps.

• Destroys the body’s immune system,

allowing other diseases to kill.

Plant Viruses

• Can be an important agricultural problem.

• Ex: Tobacco Mosaic Virus

Tulip Flower Breakage