Viruses

SB3. Students will derive the relationship between single-celled and multi-celled organisms and the

increasing complexity of systems. d. Compare and contrast viruses with living

organisms.

Essential Question

How do viruses compare to living

organisms?

What is a virus?A virus is a non-living infective agent that replicates within the cells of living

hosts.Common characteristics of all viruses include:

1 Not a living organism2 Must attack a living cell in order

to reproduce3 Can have a genetic material

made of DNA or RNA

• The tiniest viruses are 20 nm in diameter. (smaller than a ribosome).

• The largest virus known is the Megavirus, having a capsid diameter of 440 nm (10-20 times larger than most viruses)

The Structure Of a Virus• Viruses are

composed of a core of nucleic acid

• The Nucleic acid core is surrounded by a protein coat called a capsid

• The nucleic core is either made up of DNA or RNA but never both

Viral Structures

How do viruses compare to living

organisms?

Reproduction• Viruses are obligate

intracellular parasites that can reproduce only within a host cell.

• They DO NOT have– Enzymes for metabolism– Do not have ribosomes– Do not have the equipment

to make proteins

Reproduction occurs using Lytic or Lysogenic cycles

• The Lytic Cycle– Culminates in

the death of the host cell

– Virulent viruses reproduce only by lytic cyle.

–Natural selection favors bacterial mutations with receptor sites that are resistant to a particular phage or that have restriction enzymes to destroy the phages.

• The Lysogenic Cycle– Replication of

the viral genome without destroying the host cell.

– A temperate virus may reproduce by either cycle.

– Lambda virus: resembles T4 but only has a single short tail fiber

Common VirusesVaricella zoster

Chicken Pox/Shingles InfluenzaThe Flu

Common VirusesFlavivirus

West Nile VirusLyssavirus

Rabies Virus

Common VirusesHepadinavirus

Hepatitis BPicornaviridae

Polio Virus

Deadly VirusesEbola sudan Ebola zaire

Deadly Viruses

Marburg Virus Variola VirusSmall Pox

RetrovirusesRetroviruses are viruses composed, not of DNA but of RNA. Retroviruses

have an enzyme, called reverse transcriptase, that gives them the unique property of transcribing their RNA into DNA after entering a cell.

The retroviral DNA can then integrate into the chromosomal DNA of the host cell, to be expressed there.

Retroviruses are the only viruses in the world that can transform RNA

back into DNA. HIV (human immunodeficiency virus) is a lentivirus

that causes AIDS (acquired immunodeficiency virus).

This failure of the immune system allows life threatening

opportunistic infections and cancers to thrive.

The Hot ZoneThe Hot Zone: A Terrifying True Story is a best selling, 1994 non-fiction bio-thriller by Richard

Preston about the origins and incidents involving viral hemorrhagic fevers, particularly ebolaviruses and

marburgviruses.

The filoviruses, Ebola virus, Sudan virus, Marburg virus, and Ravn virus are Biosafety Level 4 agents. Biosafety

level 4 agents are extremely dangerous to human because they are very infections, have a high case-fatality rate, and there are no known treatments or

cures.

We will be reading an excerpt from the first section of the book, regarding the Marburg virus. The excerpt details Marburg’s symptoms as manifested in the

character “Charles Monet.” We will discover the origin of the virus as well as its gruesome symptoms.