VirusesChp 21

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Viruses

Viruses possess only a portion of the properties of “living” organisms– Segments of DNA or RNA wrapped in a

protein coat called a capsid• No Metabolism • Must reproduce within cells

Key Concepts

Non-cellular infectious agents Composed of nucleic acid, DNA or RNA and protein

Characteristic of Life

Virus Cell

Growth No Yes

Homeostasis No Yes

Metabolism No Yes

Mutation Yes Yes

Nucleic Acid DNA or RNA DNA

Reproduction Only within host cell Independently by cell division

Structure Nucleic acid core, protein covering, some have envelope

Cytoplasm, cell membrane, etc.

All viruses have 2 main parts:

1. DNA or RNA - genetic info

2. Capsid - a protein encasement

Viral Shapes

General Structure:– Helical- rod-shaped protein coat wound

helically around nucleic acid core– Polyhedral- many-sided coat– Enveloped- envelope around virus is made

mostly of membrane remnants from previously infected cell

Helical Virus

http://www.mcb.uct.ac.za/tutorial/virovirion.jpg, http://textbookofbacteriology.net/themicrobialworld/hsv1struc.jpg, http://www.iayork.com/Images/2008/3-10-08/AdenovirusModel.jpg

Enveloped VirusAdenovirus (Polyhedral)

http://2.bp.blogspot.com/_aPfF45kK_Bg/RyT7ZGjngFI/AAAAAAAAAM8/eR6I9tdzafM/s320/BacteriophageCartoon.jpg

http://www.brooklyn.cuny.edu/bc/ahp/LAD/C4b/graphics/C4b_TMV.GIF

http://www.apositivelife.com/images/graphs/bio.jpg

The influenza virus has an envelope derived from the host cell. It consists of proteins, lipids and glycoproteins. The envelope helps the virus evade the host cell’s immune system.

Virus leaving host cell enveloped by host cell membrane

Grouping Viruses Grouped according to:

– Presence of capsid and envelope - shape

– RNA or DNA, single or double stranded - structure

Viral Group Nucleic Acid

Shape and Structure Example

Papovaviruses DNA Icosohedral, non-env Warts, cancer

Adenoviruses DNA Icosohedral, non-env Resp. & intestinal infections

Herpesviruses DNA Icosohedral, enveloped Herpes simplex, chicken ox, mono, shingles

Poxviruses DNA Complex brick, enveloped Small pox, cow pox

Picornaviruses RNA Icosohedral, non-env Polio, hepatitis, cancer

Myxoviruses RNA Helical, enveloped Influenza A B C

Rhabdoviruses RNA Helical, enveloped Rabies

Retroviruses RNA Icosahedral, enveloped HIV

Viral Multiplication Cycles

5 Steps– Attachment– Penetration– Replication– Assembly– Release

•Lytic pathway•Host cell lysis relatively immediate

•Lysogenic pathway•Viral DNA integrates into bacterial chromosomes, there is a period of dormancy.

http://www.sciencegateway.org/resources/biologytext/cb/virus/phagerep.gif

http://www.accessexcellence.org/RC/VL/GG/images/examples_of_viral.gif

Disease Causing Viruses

Emerging Viruses– Viruses that evolve in a geographically

isolated area and are pathogenic (disease causing) to humans

Viruses and Cancer– Viruses are capable of altering growth

properties of humans cells they infect by triggering oncogene expression (HPV)

Prions and Viroids

Prions– Infectious protein conformations containing no

genetic material. They can cause normal proteins to assume an abnormal shape preventing proper function (Mad Cow Disease)

Viroids– Tiny, circular molecules of RNA

• No protein or membrane coat• Important infectious disease agents in plants