19 Viruses BIOLOGY. Outline. Structural Features and Characteristics. The Good the Bad and the Ugly. Structural Features and Characteristics

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1 9 Viruses CAMPBELL BIOLOGY TENTH EDITION Reece Urry Cain Wasserman Minorsky Jackson Outline I. Viruses A. Structure of viruses B. Common Characteristics of Viruses C. Viral replication D. HIV Lecture Presentation by Dr Burns NVC Biol 0 Copyright 04 Pearson 009 Education, Pearson Education, Inc. Inc. The Good the Bad and the Ugly Viruses fit into the bad category Viruses are not like other living organisms. They are acellular, which means they don t have their own cells they hijack other cells and use them to reproduce themselves. Structural Features and Characteristics Non cellular, viral particles = virions Virions are very small Viruses may remain inactive or latent in the host for many years. Viruses lack ribosomes Structural Features and Characteristics Viruses usually have:. Genetic material ( or RNA). Protein coat = capsid 3. Some may have a fatty membrane = envelope Viral Genomes Viral genomes may consist of either Double- or single-stranded, or Double- or single-stranded RNA Depending on its type of nucleic acid, a virus is called a virus or an RNA virus

2 Figure 9.3 RNA capsid capsid Membranous envelope RNA Capsid Head Tail sheath Viruses vary in size, as well as in shape Tail fiber Glycoprotein Glycoproteins 8 50 nm nm (diameter) nm (diameter) 80 5 nm 0 nm 50 nm 50 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4 Bacteriophage Virus Figure mm

3 Are Viruses Alive? They are acellular They can t perform metabolic activity and reproduction without a host Contains either or RNA Do not contain ribosomes, lack the enzymes needed to transcribe Characteristics of Living Organisms. Contain biological molecules including: Proteins, nucleic acids, carbohydrates and lipids. Cellular 3. Reproduce 4. Acquire and use energy - Metabolism 5. Growth and Development 6. Respond to environment 7. Maintain Homeostasis 8. Populations of living organisms evolve and have adaptive traits Types of viruses - Retrovirus Types of viruses - Bacteriophages Retroviruses contain RNA and an enzyme reverse transcriptase. Bacteriophages viruses that infect bacteria Reverse transcriptase turns RNA into Figure 9.4 Entry and uncoating Replication Capsid HOST CELL Viral VIRUS 3 Transcription and manufacture of capsid proteins Viral mrna Capsid proteins Animation: Simplified Viral Reproductive Cycle Right-click slide / select Play 4 Self-assembly of new virus particles and their exit from the cell 3

4 Replicative Cycles of Phages Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle The Lytic Cycle The lytic cycle is a phage replicative cycle that culminates in the death of the host cell The lytic cycle produces new phages and lyses (breaks open) 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 0 Pearson Education, Inc. Figure 9.5- Animation: Phage T4 Lytic Cycle Right-click slide / select Play Figure 9.5- Figure Entry of phage and degradation of host Entry of phage and degradation of host 3 Synthesis of viral genomes and proteins 4

5 Figure Figure Entry of phage and degradation of host 5 Release Entry of phage and degradation of host Phage assembly Phage assembly Head Tail Tail fibers 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers 4 Assembly 3 Synthesis of viral genomes and proteins Lysogenic cycle Some viruses don t immediately kill their hosts They integrate their into the host When the viral is integrated into the host, the genetic material it is called a prophage, the infected cell is the lysogen Induction: The virus will switch to the lytic phase 0.5 µm Figure 9.6b Daughter cell with prophage Phage circularizes. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce a population of bacteria infected with the prophage. lytic cycle is induced Certain factors determine whether or lysogenic cycle is entered Lysogenic cycle Prophage The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Animation: Phage Lambda Lysogenic and Lytic Cycles Right-click slide / select Play Phage integrates into the bacterial chromosome, becoming a prophage. 5

6 Figure 9.6a Phage The phage injects its. Replicative Cycles of Animal Viruses Phage Bacterial chromosome Phage circularizes. There are two key variables used to classify viruses that infect animals or RNA? Single-stranded or double-stranded? The cell lyses, releasing phages. Lytic cycle lytic cycle is induced Certain factors determine whether or lysogenic cycle is entered New phage and proteins are synthesized and assembled into phages. HIV - Retrovirus Figure 9.8a Glycoprotein Viral envelope Capsid Retroviruses have RNA which gets changed to a a double stranded Reverse transcriptase HIV RNA (two identical strands) Viral RNA RNA- hybrid HOST CELL Reverse transcriptase Retroviruses have an enzyme called reverse transcriptase to do this Chromosomal NUCLEUS Provirus HIV is a retrovirus, HIV uses the lysogenic cycle then switches to the lytic cycle RNA genome for the next viral generation mrna New virus Figure 9.8b HIV Membrane of white blood cell 0.5 m HIV entering a cell New HIV leaving a cell Animation: HIV Reproductive Cycle Right-click slide / select Play 6

7 Steps in HIV Virus Replication. Attach to a host cell = helper T Cell (CD4 cells) HIV envelope has gp0 fits CD4 receptor on T cell Then binds with coreceptor ie CCR5. Penetrate host cell using endocytosis 3. Viral RNA is turned into viral Using the enzyme reverse transcriptase 4. Integrate the viral into the host Using the enzyme integrase Steps in HIV Virus Replication 5. Transcribe the integrated viral to make viral mrna 6. Protein synthesis 7. Assemble new viral particles Using the enzyme protease 8. Release = new particles bud off Fig. 7.6 HIV Human Immunodeficiency Virus: Cause AIDS HIV is a particularly devastating virus because it attacks the hosts immune system The human immune system has T cells to protect us against foreign bodies like viruses and bacteria. T cells have receptors on their surface that recognize foreign vs our own cells HIV Infection HIV receptors on the surface of the envelope bind with the receptors CD4 and coreceptor CCR5) on the T cells, penetrated host cell Protein coat (capsid) dissolves HIV has three main enzymes: Integrase, reverse transcriptase and protease HIV Infection Reverse transcriptase does the opposite of RNA polymerase: It turns RNA into a double stranded molecule Integrase puts this viral into the T cell s For a period of time the may not produce any protein but if the host cell replicates then the viral is also replicated 7

8 HIV Infection Eventually the viral in the host cell will begin to make proteins needed to make new HIV Proteases help package the new virus components into a new envelope using the host cell membrane The new viruses are released from the host to infect other cells HIV Treatments There is no cure. These treatments can slow the spread of the virus in the body but not completely get rid of it There are four main targets to fight HIV:. Reverse Transcriptase Inhibitors. Protease Inhibitors 3. Integrase Inhibitors 4. Fusion Inhibitors - prevent HIV entry into cells. AZT anti-viral drug AZT is a reverse transcriptase inhibitor. It also inhibits polymerase but has X greater affinity for reverse transcriptase AZT is a thymidine analog Influenza Virus In influenza killed 0-50 million people, more than the numbers of soldiers in WWI The type of influenza that causes the most problem infects both bird, swine and human hosts Influenza virus capsid covered with envelope with protein spikes. Different strains have different types of proteins. Ebola virus Ebola virus causes severe hemorrhagic fever Natural host unknown most likely fruit bats Death rates 5 90% of those infected Viruses can cause cancer Hepatitis B infection liver cancer Papilloma virus cervical cancer 8

9 Important Concepts Know the vocabulary in the lecture Structural features and characteristics of viruses, what are common shapes of the caspids lysogenic cycle and lytic cycle Be able to describe in detail the steps of HIV infection of T Cells What are retroviruses and what enzyme do they have Important Concepts What three enzymes does HIV have, what are their functions What cells are the host cell of HIV What are the main targets of HIV drugs Examples of viruses given in class Know the examples of viruses that cause cancer, and what cancers they cause. 9