Viruses and

Bacteria© Amy Brown – Science Stuff

What is a virus?What is a virus?Virus: An infectious particle

that is nonliving.

The word virus comes from the Latin word meaning “________”. poison

All viruses are parasites. All viruses require a host.All viruses are parasites. All viruses require a host.

Parasites live in or on other living organisms, causing them harm.

Parasites:

Host:The host is the living organism the parasite lives on. 

Martinus BeijerinckDutch Scientist

1898

Beijerinck is considered the founder of _________.

Virology is:

The study of viruses.

virology

In 1898, he used filtration

experiments to prove that:

an agent smaller than a bacterium

was causing tobacco mosaic

disease.

He was the first to name these

very small particles

“________”.

He was the first to name these

very small particles

“________”. viruses

Wendell StanleyAmerican biochemist

1904 - 1971

In 1935, Stanley was able to isolate crystals of the tobacco mosaic virus.

Living organisms do not crystallize, so Stanley inferred that viruses were not “_____”. alive

Stanley was awarded the Nobel Prize in chemistry.

Characteristics of VirusesViruses are _____________. Most can be seen only with an _________________.

extremely small

electron microscope

A viruses is active only when ______________. inside a living cell

When removed from a living cell, it _______ all activities, but retains its ability to _____________.

ceases

infect the cell

They may be crystallized and stored indefinitely, but even after longs periods of time, they retain:their ability to infect a living cell.

Viruses vary widely in terms of size and structure, but they all have one thing in common:

They enter living cells and use the machinery of the cell to produce more viruses.

Viruses are non-cellular.Viruses are non-cellular.1 – Head2 – Capsid3 – DNA or RNA4 – Tail fibers5 – Base plate6 – Sheath

1. They are not made of cells and have no cell parts.

2. Viruses consist of two parts: DNA or RNA surrounded by a protein coat.

3. Capsid – The protein

coat that surrounds the DNA or

RNA .

The Viral

Capsid

The Viral

Capsid

The capsid is made of _______ that enable the virus to enter a host cell. proteins

The capsid has a particular _____ that must match ________ on the surface of a _________.

The capsid has a particular _____ that must match ________ on the surface of a _________.

shapereceptorshost cell

When the virus attaches to these receptors, the cell is “tricked” into:letting the virus inside.

Viruses can reproduce, but only _______________.

They reproduce

inside a cell by getting the cell

to produce viral parts

instead of cell parts.

inside a living cell

Since viruses must bind precisely to ________ on the __________, they are highly specific to the cells they infect.

proteinscell surface

Plant viruses can only infect plant cells.

Animal viruses can only infect animal cells.

Viruses of eukaryotes are usually tissue specific. Example: Human cold viruses infect only the cells lining the upper respiratory system, ignoring all other tissues.

Bacteriophages are viruses that infect only certain types of bacteria.

Viruses are not affected by any known _______. antibiotic

Anything that will kill the

virus will also kill the host.

Living Characteristics of Viruses:1. They can reproduce--but only

inside a living cell.2. They can mutate or change.3. They have DNA or RNA.

Their genome may consist of only four genes, or up to a hundred genes.

Non-living Characteristics of Viruses:1. They are non-cellular.2. They have no metabolism.

They have no food or energy requirements.

3. They can be crystallized and dehydrated and stored indefinitely. They come to "life" only when injected inside a living cell.

1. Since viruses have no enzymes and no cell parts, they force the host cell to: ___________________.

2. A viral infection begins when:the genetic material (DNA or RNA) of a virus makes its way into a host cell.

3. Once inside, the virus _______ the cell, reprogramming the cell.

start making viral parts

hijacks

4. The viral genome takes over the ________ and makes the host cell start producing _________.host cell

viral parts

5. The host cell will begin to make copies of the _________ and producing the ______________.

viral DNAprotein capsids

6. The host cell assembles the parts into viruses.

7. The reproductive cycle ends with:the exit of hundreds or thousands of viruses from the infected host cell.

8. This often ________ the host cell. Each of these viral progeny has the capacity to infect neighboring cells thereby spreading the infection.

destroys

The Two Reproductive Possibilities:The Two Reproductive Possibilities:

1. Once a virus is inside a host cell, two different processes may occur.

2. Some viruses replicate themselves immediately, killing the host cell.

3. Other viruses replicate themselves in a way that does not destroy the host cell.

4. These two processes are called:a) The lytic cycleb) The lysogenic cycle

4. These two processes are called:a) The lytic cycleb) The lysogenic cycle

In a lytic infection, a virus:enters a cell, makes copies of itself, and causes the cell to burst.

Bacteriophage T4 is an example of a bacteriophage that causes a lytic infection.

Now, let’s learn the steps to the lytic cycle!

Attachment. Tail fibers are used to attach to receptor sites on the surface of the host cell.

Entry. Phage DNA is injected. Empty capsid remains outside. Host cell DNA is destroyed.

Synthesis. The host cell is directed to produce viral genomes and protein capsids.Assembly. The viral DNA or

RNA is assembled inside the protein coat.

Release. The cell swells, bursts, and releases 100’s of new viruses.

A phage that reproduces only by a lytic cycle is called a

virulent phage.

1. In this type of viral reproduction:the host cell makes copies of the viral genetic material indefinitely.

2.The virus incorporates its DNA into the DNA of the host cell. The viral DNA is then ________ along with the host cell’s own DNA.

3. Lysogenic viruses do not __________________. A lysogenic virus may remain ________ for some period of time.

replicated

kill the cell right away inactive

Bacteriophage(phage)

Phage attaches and injects its DNA.

Bacterialchromosome

Certain features determine whether:

Lytic cycle is induced

Lysogenic cycle Is entered

LYSOGENICLYTIC

Viral DNA is incorporated into the bacterial chromosome, forming a prophage.

Prophage

Bacterial cell divides by binary fission normally. The viral genome is copied and passed to daughter cells.

Many cell divisions may

occur, producing a large

population of bacteria that are infected with the

prophage.

Daughter cell with prophage

New viruses are produced.

The host cell bursts, releasing the new viruses.

Prophage: The viral DNA embedded into the host cell’s DNA.

The prophage may remain part of the host __________________ before becoming active. for many generations

Eventually, certain environmental conditions (chemicals, radiation) may trigger the switchover from the ________ cycle to the ____ cycle.lysogenic lytic

Retroviruses

Retroviruses have ____ as their genetic information rather than _____. DNA

RNA

• These viruses have an enzyme called __________________, which transcribes their ____ template into ____. The newly made DNA then enters the ___________ and integrates into the DNA of a chromosome.• In this way, the retrovirus may remain dormant for some length of time.

• It will eventually become ______, causing the host cell to make ___________, and causing the _____ of the host cell.• Retroviruses are responsible for some types of cancer.• The AIDS virus is a retrovirus.

reverse transcriptase RNADNA

cell’s nucleus

activenew viruses death

Comparison of Viruses and CellsComparison of Viruses and Cells

Have DNA or RNA and a protein capsid

Only within a host cell

Cell membrane, cytoplasm, nucleus, cytoplasmic organelles

Reproduce independently, either sexually or asexually

DNA or RNA

NoneNo

No

Yes

DNAYes, in multicellular

organismsYes

Yes

Yes

Viral DiseasesThere is little that can be done to cure a viral infection.

Antibiotics are effective against _______, but not against _______.

bacteriaviruses

A few new drugs have been developed that interfere with the reproduction of the virus, but they only seem to slow the effect of the virus.They do not provide a cure.

The battle against viral diseases lies in the use of vaccines.

Vaccines:Contain a harmless variation of the pathogen.

Our immune system launches a response to the harmless form, thereby learning to _________ it the next time that we are _______ to it.

recognizeexposed

When we are exposed to the "real" pathogen, our immune system can respond __________ since it has already _______ to recognize the pathogen.

much fasterlearned

Colds

Hepatitis

Flu

AIDS

Chicken poxWest Nile Virus