Chapter 19 Viruses

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings I. Discovery Tobacco mosaic disease - stunts growth of tobacco plants, gives leaves mosaic color Researchers hypothesized that a particle smaller than bacteria caused the disease (1800s) In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, tobacco mosaic virus (TMV)

Fig 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

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings II. Structure Very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope NOT cells!

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings A. Viral Genomes Viral genomes may consist of either – Double- or single-stranded DNA, or – Double- or single-stranded RNA Virus is called a DNA virus or an RNA virus

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings B. Capsids and Envelopes Capsid = protein shell that encloses the viral genome Capsids are built from protein subunits (capsomeres)

Fig 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

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Some have membranous envelopes that help them infect hosts Viral envelopes = surround the capsids of influenza viruses and many other viruses found in animals – derived from the host cell’s membrane, contain a combination of viral and host cell molecules

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Bacteriophages (also called phages) infect bacteria – most complex capsids found among viruses – Have an elongated capsid head that encloses their DNA – A protein tail piece attaches the phage to the host and injects DNA inside

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Concept 19.2: Viruses reproduce only in host cells 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

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings III. General Reproductive Cycles Viral genome enters cell, cell begins to manufacture viral proteins Virus uses host molecules Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses

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

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings IV. Reproductive Cycles of Phages Phages have two reproductive mechanisms:

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings A. The Lytic Cycle Reproductive cycle that culminates in death of host cell Produces new phages and digests the host’s cell wall, releasing the progeny viruses Virulent phage = reproduces only by the lytic cycle Bacteria have defenses against phages (ex. restriction enzymes that recognize and cut up certain phage DNA)

Fig Phage assembly HeadTailTail fibers Assembly Release Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings B. The Lysogenic Cycle Replicates the phage genome without destroying the host Viral DNA is incorporated into the host cell’s chromosome Prophage = integrated viral DNA When host divides, it copies the phage DNA and passes copies to daughter cells

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode Temperate phages = use both lytic and lysogenic cycles

Fig 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.

Fig. 19-UN1 Phage DNA Bacterial chromosome The phage attaches to a host cell and injects its DNA Prophage Lysogenic cycle Temperate phage only Genome integrates into bacterial chromosome as prophage, which (1) is replicated and passed on to daughter cells and (2) can be induced to leave the chromosome and initiate a lytic cycle Lytic cycle Virulent or temperate phage Destruction of host DNA Production of new phages Lysis of host cell causes release of progeny phages

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings V. Repro Cycles of Animal Viruses Two key variables to classify viruses: – DNA or RNA? – Single-stranded or double-stranded?

Table 19-1a

Table 19-1b

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings A. Viral Envelopes Viral glycoproteins on envelope bind to receptor molecules on surface of host cell Some are formed from host cell’s plasma membrane as the viral capsids exit

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Others form from the host’s nuclear envelope and are replaced by an envelope made from Golgi apparatus membrane

Fig Capsid RNA Envelope (with glycoproteins) Capsid and viral genome enter the cell HOST CELL Viral genome (RNA) Template mRNA ER Glyco- proteins Capsid proteins Copy of genome (RNA) New virus

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings B. RNA as Genetic Material Retroviruses use reverse transcriptase to copy their RNA genome into DNA Discussion: HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

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

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

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Provirus = viral DNA that is integrated into the host genome – Unlike prophage, it remains a permanent resident of host cell – Host’s RNA polymerase transcribes the proviral DNA into RNA molecules RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from cell

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings VI. Evolution Viruses do not fit life definition Can reproduce only within cells, probably evolved as bits of cellular nucleic acid Plasmids – circular DNA, copy on own, can be moved between cells Transposons – DNA segments that move within a genome These and viruses are mobile genetic elements (maybe source of viral genomes)

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings You should now be able to: 1.Explain how capsids and envelopes are formed 2.Distinguish between the lytic and lysogenic reproductive cycles 3.Explain why viruses are obligate intracellular parasites 4.Describe the reproductive cycle of an HIV retrovirus