1. By, Mackenzie Pabst Viruses;Section 18-1

2. What is a Virus? • disease-causing, nonliving particle composed of an inner core of nucleic acids surrounded by a capsid • A virus replicates inside living cells called host cells

3. Why are Viruses Non-Living? • Viruses are nonliving because: • They don't exhibit all the criteria for life • Criteria for life includes: • Respiration, growth and development • All viruses can do is replicate • Replication is making exact copies of themselves with the help of living cells

4. How are Viruses Named? • Some Viruses are named after the disease they cause • Example: Rabies Virus • Other viruses were named for the organ or tissue they infect. • Example: The Adenovirus effects the adenoid tissue Rabies Virus

5. How does a Virus attach to a Host Cell? • For a virus to replicate, it must enter a host cell • When a virus’ proteins interlock with the receptor site, the virus is attached to its host cell • The attachment process is like two pieces of a jigsaw puzzle fitting together

6. Virus Attachment • Each virus has a specifically shaped attachment protein • Viruses are species specific • Some Viruses are cell-type specific • Example: polio viruses normally infect only intestinal and nerve cells.

7. Lytic Cycle: A Viral Replication Process • Attachment:The Virus attaches to the Host Cell • Entry:The Bacteriophageinjects its nucleic acid into the bacterial cell • Replication :The host’s metabolic machinery makes the virus’ nucleic acid and proteins

8. Lytic Cycle: A Viral Replication Process D. Assembly:New Virus Parts are made E. Lysis and Release:The host cell breaks open and releases new viruses • This results in the killing of the host cell

9. Another Viral Replication Process… Of an Enveloped Virus: • After attachment, the membrane of the host cell surrounds the virus • This produces a virus-filled vacuole inside the host cell's cytoplasm. • The virus bursts out of the vacuole and releases its nucleic acid into the cell.

10. The Lysogenic Cycle

11. The Lysogenic Cycle: • Begins the same way as the Lytic Cycle • The virus attaches to the host cell and its nucleic acid is injected into the cell. • Then the viral DNA is integrated into the host cell's chromosomes • This is done instead of immediately taking over the host's genetic material (As in the Lytic Cycle) • This Cycle does not kill its host cell

12. RNA Viruses: • Many Viruses, including HIV, are RNA viruses • This means their only nucleic acid is RNA • The RNA virus with the most complex replication cycle is the Retrovirus • The Retrovirus - uses reverse transcriptase to make DNA from viral RNA • the retroviral DNA is then integrated into the host cell's chromosome

13. Retrovirus Cycle:

14. Retrovirus Cycle: • Once inside a host cell, the retrovirus makes DNA from its RNA with reverse transcriptase • Reverse transriptase produces double-stranded DNA from the viral RNA. • The double-stranded viral DNA becomes part of the host cell’s chromosome and it becomes a provirus

15. Human Immunodeficiency Virus(HIV) • HIV infects white blood cells in humans • The infected cells still function normally • Because the viral genetic material is a provirus that only produces a small number of new viruses at a time • An infected person will not appear sick • Can infect others with the virus through body fluids

16. Human Immunodeficiency Virus(HIV) • Most people with HIV will get AIDS • Over time, more white blood cells are infected and killed • White Blood cells are essential to our immune system • Their destruction reduces the body’s ability to protect itself from other viruses

17. Virus’ link to Cancer: • Some viruses have been linked to certain cancers in humans and animals • Viruses can disrupt the normal growth and division of cells in a host • This causes abnormal growth and creates tumors

18. Prions: • Are composed of proteins but have no nucleic acid to carry genetic information • Are thought to act by causing other proteins to fold themselves incorrectly, resulting in improper functioning • Responsible for many animal diseases • Example: Mad Cow Disease

19. Viroids: • Composed of a single circular strand of RNA with no protein coat • Shown to cause infectious diseases in several plants. • Have less RNA than most viruses

20. Plant Viruses: • There are more than 400 plant viruses • Plant Viruses can cause stunted growth and yield losses in their host plants. • The first virus to be identified as a plant virus was the tobacco mosaic virus

21. Plant Viruses: • Do not undergo lytic or lysogenic phases • Plant Viruses are named according to their host plant. • Not all viral plant diseases are fatal or even harmful. • The Mosaic virus, as seen in the following picture, cause patterns of color in the flowers of plants.