Ch. 19 Viruses Objective: EK 3.C.3: Viral replication results in genetic variation, and viral infection can introduce genetic variation into the hosts. L.O. 3.29 The student is able to construct an explanation of how viruses introduce genetic variation in host organisms.

19.1 A Virus Consists of a Nucleic Acid Surrounded By a Protein Coat 1st discovered viruses – Tobacco Mosaic Virus Filtered plant sap still infected plants. Something smaller (viruses) was infected them. Extracted sap from tobacco plant with tobacco mosaic disease RESULTS Passed sap through a porcelain filter known to trap bacteria Healthy plants became infected Rubbed filtered sap on healthy tobacco plants 1 2 3 4

Structure of Viruses Nucleic Acid: typically 1 linear strand or circular molecule Double stranded DNA Single stranded DNA Double stranded RNA Single stranded RNA Protein shell (capsid) encloses nucleic acid. These come in many shapes/sizes. Some have accessory structures: Viral envelopes Tail fibers Capsomere of capsid RNA Capsomere DNA Glycoprotein Glycoproteins Membranous envelope Capsid Head Tail sheath Tail fiber 18  250 nm 80  225 nm 70–90 nm (diameter) 80–200 nm (diameter) 20 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

19.2 Viruses Replicate Only in Host Cells Viruses do not have organelles to build proteins, replicate nucleic acids, metabolize, etc. Needs a host cell to do it. Viruses can only infect certain hosts (host range) Only certain species Only specific cells/tissues within species Ex: flu only infects cells of the respiratory system

General Features of Viral Replicative Cycles Infection begins with the insertion of the viral nucleic acid into the host cell: Use tail fibers to inject DNA into host cell (like a needle) Endocytosis of entire virus Viral DNA commandeers cells to make more viruses using the cell’s own machinery. Kills cells when done.

The Lytic Cycle The virus injects DNA into the host cell New phage DNA and proteins are synthesized and assembled into phages. The cell lyses, releasing phages. Phage Phage DNA The phage injects its DNA. Bacterial chromosome Lytic cycle lytic cycle is induced or Phage DNA circularizes. Certain factors determine whether lysogenic cycle is entered The virus injects DNA into the host cell Host cells starts copying viral DNA and making capsid and accessory parts. Once done, the cell lyses (bursts) releasing new viruses to infect other cells.

Animation: Phage T4 Lytic Cycle Right-click slide / select “Play” © 2011 Pearson Education, Inc.

The Lysogenic Cycle Same beginning and end as lytic cycle Insert DNA …… make copies and cell lyses After the DNA is inserted, it is integrated into host cell’s DNA. Prophage; 1 gene codes for a protein that prevents transcription of the rest of its genes. Cell grows and replicates (binary fission) normally with viral DNA still “hiding” in host DNA. lytic cycle is induced or Phage DNA circularizes. Certain factors determine whether lysogenic cycle is entered Lysogenic cycle Prophage Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce a population of bacteria infected with the prophage. The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage.

Animation: Phage Lambda Lysogenic and Lytic Cycles Right-click slide / select “Play” © 2011 Pearson Education, Inc.

Replicative Cycles of Animal Viruses Be aware of (but don’t memorize) that animal viruses are classified according to type of nucleic acid and presence of an envelope.

RNA as Viral Genetic Material Class IV – viral RNA serves as mRNA to immediately make proteins. Class V – viral RNA used to make many mRNAs. Class VI (retroviruses) – RNA + reverse transcriptase  DNA Viral DNA embeds in host’s DNA Viral proteins made with host’s

Figure 19.8 Membrane of white blood cell Glycoprotein Viral envelope HIV Capsid RNA (two identical strands) HOST CELL Reverse transcriptase HIV Reverse transcriptase Viral RNA RNA-DNA hybrid DNA 0.25 m HIV entering a cell NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation Figure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS. mRNA New virus New HIV leaving a cell

Animation: HIV Reproductive Cycle Right-click slide / select “Play” © 2011 Pearson Education, Inc.

Evolution of Viruses Not living but effects all living things. Evolved after life. Naked DNA infected injured cells at first, then evolved to have an envelope and could infect healthy cells. Viral nucleic acids are more similar to host’s nucleic acids but there are some viral genes that are similar for all viruses.