Diversity of Living Things

1. Diversity of Living Things 2.2 Viruses p. 54-59 Nelson Biology 11

2. 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

3. 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

4. Structure of Viruses • 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

5. 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 on p. 55) http://www.emc.maricopa.edu/faculty/farabee/biobk/T2phage.gif

6. Importance of viruses • Cause many human diseases • Differ in terms of their ability to spread from person to person • Epidemic: large-scale outbreak of disease in a particular region • Pandemic: epidemic occurring on global scale http://nursing-resource.com/influenza/

7. Importance of viruses cont’d • Small number of viruses have been linked to cancer. • Viruses also cause disease in animals and plants • They can be useful in ecosystems by controlling populations of certain organisms http://www.topnews.in/trials-start-potent-new-hepatitis-c-drug-developed-cardiff-2261786

8. Classification • 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

9. Phylogeny • 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

10. Viral Diversity and Specificity • Most viruses are selective and host-specific • Bacteriophages have a very restricted host range, while most plant viruses can infect a wide range of host plants • Host range: limited range of host cells that a virus can infect. http://micro.magnet.fsu.edu/cells/viruses/images/bacteriophage.jpg

11. Viral Diversity and Specificity • Some animal viruses have a broad range • ex. Swine flu virus (hogs and humans) • Rabies (many mammals) http://www.miamidade.gov/asd/images/rabies.jpg

12. Viral Diversity and Specificity • Other animal viruses have a very narrow host range • Ex. Cold virus only infects cells in the human upper respiratory tract • AIDS affects certain types of white blood cells http://www.teenaids.org/Portals/0/images/whatIsAIDS-pic3.gif

13. Viral Replication • 4 basic steps in the “lytic cycle” • 1. Attachment • 2. Synthesis • 3. Assembly • 4. 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.

14. Lytic Cycle

15. Viral Replication- Attachment and Synthesis Attachment: • Virus attaches to a specific receptor site on the host cell membrane • Lock and key model • Whole virus or just DNA/RNA enters the host cell Synthesis: • Viral DNA/RNA directs host cell in replicating virus parts http://www.pickens.k12.sc.us/phsteachers/edmunds/lytic%20cycle%201%20animated.gif

16. Viral Replication- Assembly and Release Assembly: Viral parts are assembled into new virus particles Release: • New virus particles are released from host cell • Lysis (destruction/bursting) of host cell http://www.microphage.com/images/lysis2.jpg

17. 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

18. Lysogenic Cycle-Viral Replication • 4 basic steps in the “lysogenic cycle” • 1. Attachment- same as lytic cycle • 2. Synthesis • 3. Assembly • 4. Release

19. Temperate Phages • Temperate Phages are bacteriophages that do not destroy host cell. • They inject their nucleic acid into host cell and integrate it into the host DNA • Bacteria cells will replicate with the viral DNA incorporated into the daughter cell eventually lytic cycle can begin due to some kind of environmental trigger

20. 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

21. 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.

22. 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 • As capsules to deliver drugs to target cells in the body such as cancerous tumour cells

23. 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/