Diversity of Living Things

2.2 Viruses

What is a Virus?

• Microscopic particles capable of reproducing only within living cells

• Lifeless outside of living cells, in living cells (host cell) they can reproduce.

• Virus means poison in Latin

http://www.humanillnesses.com/original/images/hdc_0001_0003_0_img0280.jpg

Viruses• Classified as non-living matter• But has many characteristics

of living matter• 1934: early electron

microscope allowed scientist to first see viruses

• Less than 0.1 micrometers in diameter (1 micrometer= 10 -6 m)

• 5000 flu viruses fit on the head of a pin

Structure of Virus

• Basic structure: Nucleic acid (DNA or RNA) in core, surrounded by capsid (protein coat)

• Some viruses have lipid membrane around capsid (HIV)

http://www.microscopy.fsu.edu/cells/viruses/images/virus.jpg

Structure of Viruses

• Bacteriophages or phages (category of viruses that invade and destroy bacteria cells) have unique shape and distinct head and tail regions

• The capsid can display various shapes (See Figure 3 and 4 on p. 55) http://www.emc.maricopa.edu/faculty/farabee/biobk/T2phage.gif

Viral Diversity and Specificity• Most viruses are selective and host-specific

- Bacteriophages have a very restricted host range- Most plant viruses can infect a wide range of host plants- Some animal viruses have a broad range while others have a very narrow host range

swine flu - hogs & humansrabies - many mammals and birdscommon cold - only cells in the upper respiratory

tract of humansHIV - only certain types of human white blood cells

Classification and Phylogeny

• Classified into orders, families, genera and species

• Classified based on size, shape and type of genetic material

http://www.goalfinder.com/product.asp?productid=99

Origin of Virus

• Many theories about origins of viruses• Could have been parasitic organisms that

depended less and less on their own cell components

• Could have come from fragments of genetic material of other organisms

• Another hypothesis is that virus-like particles existed before the first cells

Importance of viruses• Cause many human diseases – Mild: common cold, chicken pox– Serious: AIDs, cholera and

rabies• Ability to spread from person to

person. Ex. Influenza can infect millions of people in a short amount of time.– Epidemic: large-scale outbreak

of disease in a particular region– Pandemic: epidemic occurring

on global scale http://nursing-resource.com/influenza/

Importance of viruses cont’d• Small number of viruses have been

linked to cancer. This can happen if the virus alters the host cell’s DNA leading to uncontrolled cell division. Hepatitis C has been shown to produce this effect in liver cells.

• Viruses also cause disease in animals and plants

• They can be useful in ecosystems by controlling populations of certain organismshttp://www.topnews.in/trials-start-

potent-new-hepatitis-c-drug-developed-cardiff-2261786

Viral Replication• 4 basic steps in the “lytic cycle”

1. Attachment2. Synthesis3. Assembly4. Release

• See video: http://www.youtube.com/watch?v=wLoslN6d3Ec

• Some viruses have a lysogenic cycle, where host cells are not destroyed

• Cancer-causing viruses can act by adding specific genes to a host cell’s DNA, causing it to become a cancer cell

• Transduction is when a virus transfers DNA from one bacteria cell to another. This only happens on rare occasions.

Lytic Cycle

Lytic Cycle

• Lytic cycle can take as few as 25-45 minutes to produce as many as 300 new viruses

http://www.oralchelation.com/viewpoint/images/virus1.gif

Lysogenic cycle & Transmission

• Herpes virus remains dormant in body cells• During stress, virus can be activated and go through lytic

cycle causing cold sores to form• Virus can then go in dormant stage, therefore person

remains infected with the virus• Virus can be transmitted in many ways such as air,

physical contact, insect bites. See Table 2 on p. 57

Vaccinations and Human Health• Viruses do not respond to

treatment by antibiotics or other drugs, but some can be prevented by vaccines

• Vaccines are mixtures that contain weakened or dead forms of a virus. B-cells retain memory of the disease so immune system can react quickly when exposed to real virus.

http://www.scienceclarified.com/scitech/Bacteria-and-Viruses/Fighting-an-Invisible-Enemy.html

More on vaccinations

• Vaccinations have dramatically improved human health

• Some diseases have been completely eliminated such as small pox

• It is not possible to create vaccines for all viruses. For example, there is no vaccine against HIV due to the virus structure and characteristics of the infection

Applications of viruses

• Viruses are used in genetic engineering:– to treat diseases through gene therapy by

inserting gene into individuals suffering from genetic disorder

– to insert gene from one species to another species (to create GMOs and for genetic engineering of plants)

– as capsules to deliver drugs to target cells in the body such as cancerous tumour cells

Viral Vectors

• Viruses can be used as vectors (carriers) of genes into cells.

• Images from:• http://stemcells.nih.gov/info/2006report/2006Chapter4.htm• http://news.haverford.edu/blogs/nicu/2010/06/20/the-worth-of-viral-infection/