Viruses

What are they?How do they work?

Where do they come from?And… What good are they?

Viruses

What is a virus?

An infectious agent made up of:

a core of nucleic acid – DNA or RNA

(ONLY ABOUT 5 GENES)

a protein coat (capsid)

How Big are viruses?

Polio virus = 20 nanometers

20nm

1 nm – 0ne billionth of a meter!

3000 polio viruses fit across the diameter of a period at the end of a sentence in your book.

Some viruses are naked!They have no envelopeSome viruses have no envelope –

they’re naked!

Some viruses have an envelope to cover them!

Envelope – came from host’s cell membrane when virus budded out of host

Viruses don’t reproduce…..They Replicate.

Once inside a cell, the cell does all the work and produces new

viruses using the inserted Dna or Rna as a

Guide.

Lytic CycleAttachment

injection

replication

a

assembly

Lysis

The lytic cycle

Sometimes the virus doesn’t kill the cell right away and it becomes part of

the cell’s genes.If this happens, the virus DNA becomes a prophage and can become activated

at any time (like a time bomb).In the meantime, the prophage is

passed on to all the offspring of that cell …. Maybe for many generations.

Lysogenic cycle

Lytic cycle Attachment

Integration – virus

DNA joins cell’s DNA

Cell’s DNA (along with viral dna is replicated

Each daughter cell contains both bacterial and viral dna

Viral dna becomes activated

Replication

Assembly

lysis

Are viruses living or nonliving?

What are some characteristics of life?

Do viruses exhibit these characteristics?

When were viruses discovered?Viruses have apparently always been around.

However, it wasn’t until 1897 that a Dutch scientist named Beijerinck called an invisible agent that was smaller than bacteria a virus (Latin for poison). He was studying tobacco leaves that had been infected with what we now know as tobacco mosaic virus.

Classification of Viruses By Shape Host type function

Retroviruses attack a certain way.

DNA viruses attack another way.

Animal viruses

Plant viruses

Bacteria viruses

RetrovirusesContain RNA

When infecting a cell, these viruses have to transcribe the RNA to DNA before the viral code can be read.

This requires an enzyme, reverse transcriptase, to Change RNA to DNA then the viral code can be added to the cell’s DNA.

Once part of the cell’s Dna, the viral code can cause the cell to make more viruses.

HIV IS A RETROVIRUS

REVERSE TRANSCRIPTASE

rna

CAPSID

Nonviral infectious agents

• Prions– PIECE OF PROTEIN– CAUSE OF MAD-COW

DISEASE – CAN INFECT ANIMALS

– INCLUDING HUMANS

• vIROIDS– Single strand of

RNA– Causes plant

diseases

Human diseases caused by viruses

• Common cold• Influenza (flu)• Chickenpox• Polio• HIV• Some pneumonia• Some meningitis• herpes

Viruses in the biosphere

• Animal diseases– Distemper– Rabies– pneumonia

• Plant Diseases– Discolor leaves– Stunt growth– Kill plants

Uses for viruses

• Vaccines – dead or weakened form that stimulates the immune system to fight the virus when exposed to it.

• Genetic engineering – use a virus carrier to insert genes into diseased cells.

• Agriculture – Pest control– Colorful variations in some plants - flowers

interesting viruses

• Ebola virus – kills quickly; requires close contact, such as when preparing a body for burial or using dirty needles; since it kills so fast, it generally dies out quickly.

• Human sarcoma virus – causes tumors

• Adenovirus – causesThe Common cold

Bacteriophages – infect bacteria; very well studied.

Smallpox – once wiped out entire nations, now a memory –

Most successful vaccination program in world.

Smallpox virus

Aren’t you glad there’s no smallpox anymore?

Vaccines – dead or weakened form of virus

injected to provide immunity.

First vaccine

• Chinese had been using the idea for centuries, but didn’t market it.

• Dr. Edward Jenner noticed that milkmaids who had had cowpox rarely got smallpox.– Injected a boy with cowpox pus – he got

cowpox– When injected with smallpox pus, he did not

get it!

The end