Viruses

Living or Not

Virus• Virus – small

particle made of 1. Nucleic acids

either DNA or RNA

2. Surrounded by a protein coat

Virus • Viruses are smaller than the tiniest bacterium.

• They are not considered to be alive:

1. Don’t grow or develop

2. Don’t carry on respiration

3. They can’t reproduce on their own

(Because they’re not alive, they cannot be “killed”. )

Virus Structure

Capsid DNA

Attachment Protein

Virus• The outer protein coat is called the capsid.

• This capsid gives the virus its shape.

• The protein arrangement determines what cell can be infected.

Attachment protein

Virus • Every virus has a specially shaped device called an attachment protein

• These proteins anchor the virus to the cell

• Each virus can only attach to a specific kind of cell

• This can be an animal, plant, fungus, protist or bacteria cell

Attachment Protein

Viral Replication Cycles• A virus must

infect a host cell to replicate.

• Once inside, the virus takes over the cell metabolism.

Viral Replication Cycles• 2 ways for a virus

to enter a cell.

1. Virus nucleic acid is injected into the cell.

Viral Replication Cycles2. Virus is

surrounded by plasma membrane and taken into the cell cytoplasm. This vacuole will burst releasing the virus.

Lytic Cycle• Once inside, the

virus takes over the cell processes.

• The cell enzymes and energy are used to make more virus.

• The cell will burst releasing the viruses.

• Lysis – bursting of a cell.

Lytic Cycle• The new viruses

infect more cells.• This active

process of making more viruses is called a lytic cycle

Lysogenic Cycle• Lysogenic cycle – replication cycle

where the viral nucleic acid is incorporated in the cell DNA.

• This new DNA is called a provirus

Lysogenic Cycle• The cell carries out its own functions. • Every time the cell replicates, it copies

the viral DNA.• This resting phase can last for many

years.

Lysogenic cycle

• At any time, the provirus can be activated and enter a lytic cycle.

• When this happens, the virus will replicate and kill the cell.

• Examples: Herpes simplex I Hepatitis B Chicken pox

Retrovirus• Retrovirus – a

virus with RNA as its nucleic acid.

• A retrovirus produces a DNA copy of its RNA

• It must work backwards making DNA from RNA

• HIV is a retrovirus

Vaccines• Viruses cannot be

“killed” with antibiotics.

• They are controlled by vaccines.

• Vaccine- weakened form of a virus used to produce an immune response.

Illnesses caused by viruses

• Cold• Flu • Chicken Pox• Measles • Mumps• Polio• HIV Aids

BREAKING NEWS !• Papiloma virus

causes cancer!!!• HPV(human

papiloma virus) has been shown to cause cervical cancer

• Other viruses are being studied as possible causes of cancer.(that means tumors)

Spreading Disease• Disease causing

agents like viruses and bacteria are easily spread from person to person.

• Typhoid Mary was a cook who infected 53 people with the bacterial disease typhoid fever.