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NO T ES :

2 0 .1 - V IRU S ES

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Vocabulary

– Virus

– Capsid

– Lytic cycle

– Lysogenic cycle

– Retrovirus

Key Concepts

– What is a virus?

– How do viruses

reproduce?

– How do viruses

make us sick?

– Importance of

viruses

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What is a virus?

viruses are non-

cellular particles

made up of genetic

material (DNA or

RNA… not both) and

protein that invade

living cells

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Are Viruses Alive?

like living things, viruses have genetic material and they can pass this on to future generations

BUT, unlike living things, viruses are NOT made of cells

outside of a host cell, they don't reproduce, feed, or grow

therefore, most biologists consider viruses NONLIVING

What do viruses do? viruses can invade the cells of plants,

animals, and bacteria

viruses cause illnesses like colds, flu,

chicken pox, herpes, AIDS, polio, rabies,

measles, mumps…

Chicken pox

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How are viruses named?

● since viruses are non-living they are not

named in the same way that living things are

named

● many viruses are named for the disease that

they cause or the organ/tissue they infect

● EX: rabies virus; poliovirus

adenovirus

(infects adenoid tonsils)

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What do viruses look like?

a virus is made of a core of genetic material

surrounded by a protein coat (CAPSID)

viruses are SMALL!! (20-400 nanometers)

Bar = 100nm

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viruses come in a variety shapes: rod-

shaped; tadpole-shaped; helical;

cubelike (see pg 575)

T-4 bacteriophageHuman Influenza

virus particles

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How does a virus get into a host cell?

● the proteins on the outer coat (capsid) of

the virus attach to a receptor site on the host cell’s plasma membrane

● Analogy: the 2 proteins fit together like 2

puzzle pieces

● this attachment process is SPECIFIC!

each virus can usually only attach to a

few kinds of cells

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How do viruses reproduce?

1) LYTIC CYCLE:

viruses insert their genetic material into a

host cell

once inside, the viral genes take over the

cell’s machinery (ribosomes, etc.)

the “hijacked” cell transcribes the viral genes

-uses the host cells own enzymes (e.g. DNA

polymerase) & raw materials to make

thousands of new viruses

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1) LYTIC CYCLE

the cells LYSE (burst) when the concentration

of new viral particles is high

the lytic cycle has its name because the

host cell lyses and allows viral particles to

escape

the new viruses can then infect other host

cells

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2) LYSOGENIC CYCLE:

● begins like the lytic cycle (virus injects its

DNA into host cell);

● the viral DNA is integrated into the host

cell’s chromosome – now called a

PROVIRUS.

● the provirus may remain dormant, but every

time the host cell reproduces, the provirus

is also replicated and passed along to the

new daughter cells.

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2) LYSOGENIC CYCLE:

● the lysogenic phase may continue for many

years;

● at any time, the provirus may be activated

and enter a LYTIC CYCLE.

EX: cold sores caused by herpes simplex I

virus (the provirus remains in your cells;

when it enters a lytic cycle, a cold sore

forms)…WHY does this happen?

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Examples of Lysogenic Viruses:

● herpes simplex I and II

● hepatitis B virus

● chicken pox (may erupt later in life as

shingles)

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RETROVIRUSES:

contain RNA as their genetic material

retroviruses infect a cell and produce a DNA copy of

their RNA

-(retro = backward; RNA is copied in to DNA)

this DNA is inserted into the host cell's DNA

example of a retrovirus:

HIV

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Retrovirus example: HIV

● HIV = human immunodeficiency virus

● infects white blood cells (helper T cells)

● the viral genetic material becomes a

provirus – infected person may not know

it, but can still transmit the virus

● eventually, white blood cell count drops

too low and AIDS develops

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How Can You Protect Yourself

From Viral Infections?

• The best way: use of

VACCINES

-preparation of weakened or

killed virus or viral proteins

-when injected, the vaccine

stimulates the immune

system

-sometimes produces

permanent immunity

How Can You Protect Yourself

From Viral Infections?

• Protect yourself

-stay away from known sick people

-wash your hands often

-safe practices (i.e. coughing /

sneezing into a tissue or your

sleeve; don’t share food or drink

with sick people)

Viral Diseases:● common cold

● influenza

● AIDS (HIV)

● chicken pox

● hepatitis B

● West Nile Virus

● human papillomavirus

(HPV) – linked to forms

of cancer