1. Chapter 19 - Viruses Ms. Whipple – Brethren Christian High School

2. 1. What is a virus?  A virus is a genome enclosed in a protective coat. It is not alive!!!  Viruses lead “a kind of borrowed life” between life-forms and chemicals  The origins of molecular biology lie in early studies of viruses that infect bacteria

3. 2. Describe the discovery of the Tobacco Mosaic Virus?  The story of how viruses were discovered begins in 1883 with research on the cause of tobacco mosaic disease by Adolf Mayer.  This disease stunts tobacco plant growth and mottles plant leaves.  Mayer concluded that the disease was infectious when he found that he could transmit the disease by rubbing sap from diseased leaves onto healthy plants.  He concluded that the disease must be caused by an extremely small bacterium.  Ten years later, Dimitri Ivanovsky demonstrated that the sap was still infectious even after passing through a filter designed to remove bacteria.

4. 2. Describe the discovery of the Tobacco Mosaic Virus?  In 1897, Martinus Beijerinck ruled out the possibility that the disease was due to a filterable toxin produced by a bacterium by demonstrating that the infectious agent could reproduce.  The sap from one generation of infected plants could be used to infect a second generation of plants that could infect subsequent generations.  Beijerinck also determined that the pathogen could reproduce only within the host, could not be cultivated on nutrient media, and was not inactivated by alcohol, generally lethal to bacteria.  In 1935, Wendell Stanley crystallized the pathogen, the tobacco mosaic virus (TMV).

5. Fig. 19-2 RESULTS 12 3 Extracted sap from tobacco plant with tobacco mosaic disease Passed sap through a porcelain filter known to trap bacteria Rubbed filtered sap on healthy tobacco plants 4 Healthy plants became infected

6. 3. What is the structural components of a virus?  Stanley’s discovery that some viruses could be crystallized was puzzling because not even the simplest cells can aggregate into regular crystals.  So viruses are not cells.  Viruses are infectious particles consisting of nucleic acid encased in a protein coat called a Capsid and, in some cases, a membranous envelope.  The tiniest viruses are only 20 nm in diameter—smaller than a ribosome.  The genome of viruses may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, depending on the kind of virus.

7. 3. What is the structural components of a virus?  A capsid is the protein shell that encloses the viral genome  Capsids are built from protein subunits called capsomeres  A capsid can have various structures  Some viruses have membranous envelopes that help them infect hosts  These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals  Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules

8. Fig. 19-3 RNA Capsomere of capsid DNA Glycoprotein 18  250 nm 70–90 nm (diameter) Glycoproteins 80–200 nm (diameter) 80  225 nm Membranous envelope RNA Capsid Head DNA Tail sheath Tail fiber 50 nm 20 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

9. 4. Viruses that infect Bacteria are called  Bacteriophages!!!

10. 5. How do Viruses replicate themselves? What is the Host Range of a Virus?  Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell  Each virus has a host range, a limited number of host cells that it can infect  Once a viral genome has entered a cell, the cell begins to manufacture viral proteins  The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules  Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses

11. Transcription and manufacture of capsid proteins Self-assembly of new virus particles and their exit from the cell Entry and uncoating Fig. 19-4 VIRUS 1 2 3 DNA Capsid 4 Replication HOST CELL Viral DNA mRNA Capsid proteins Viral DNA

12. 6. Describe the Lytic Cycle of a Viruses life. Use the terms Virulent Phage & Restriction Enzymes in your answer.  The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell  The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses  A phage that reproduces only by the lytic cycle is called a virulent phage  Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA

13. Fig. 19-5-1 Attachment 1

14. Fig. 19-5-2 Entry of phage DNA and degradation of host DNA Attachment 1 2

15. Fig. 19-5-3 Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment 1 2 3

16. Fig. 19-5-4 Phage assembly Assembly Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment 1 2 4 HeadTailTail fibers 3

17. Fig. 19-5-5 Phage assembly HeadTailTail fibers Assembly Release Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment 1 2 4 5 3

18. 7. Describe the Lysogenic Cycle of a Viruses life. Use the terms Temperate Phage & Prophage in your answer.  The lysogenic cycle replicates the phage genome without destroying the host  A virus that can use both the Lytic & Lysogenic cycles is called a Temperate Phage.  The viral DNA molecule is incorporated into the host cell’s chromosome  This integrated viral DNA is known as a prophage  Every time the host divides, it copies the phage DNA and passes the copies to daughter cells

19. 8. What can cause a prophages to initiate a Lytic cycle?  An environmental signal such as stress or a weakened immune system can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode

20. Fig. 19-6 Phage DNA Phage The phage injects its DNA. Bacterial chromosome Phage DNA circularizes. Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce population of bacteria infected with the prophage. The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Lysogenic cycle Prophage The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages.

21. 9. What is a RetroVirus? How does it infect cells? What type of Virus is HIV?  The broadest variety of RNA genomes is found in viruses that infect animals  Retroviruses use reverse transcriptase to copy their RNA genome into DNA  HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

22. Fig. 19-8a Glycoprotein Reverse transcriptase HIV RNA (two identical strands) Capsid Viral envelope HOST CELL Reverse transcriptase Viral RNA RNA-DNA hybrid DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New virus

23. Fig. 19-8b HIV Membrane of white blood cell HIV entering a cell 0.25 µm New HIV leaving a cell

24. 10. Briefly describe the theorized evolution of viruses. Do Viruses attack all life forms? Is there some debate as to when viruses evolved? Why?  Viruses do not fit our definition of living organisms  Most molecular biologists favor the hypothesis that viruses originated from fragments of cellular nucleic acids that could move from one cell to another.  A viral genome usually has more in common with the genome of its host than with those of viruses infecting other hosts.  However, some viruses have genetic sequences that are quite similar to seemingly distantly related viruses.  This genetic similarity may reflect the persistence of groups of viral genes that were evolutionarily successful during the early evolution of viruses and their eukaryotic host cells.  Mimivirus, a double-stranded DNA virus, is the largest virus yet discovered  There is controversy about whether this virus evolved before or after cells