Viral Genomes. Genomes may consist of: 1. Double Stranded DNA 2. Double Stranded RNA 3. Single-stranded RNA 4. Single-stranded DNA

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1 Chapter 19

2 Viral Genomes Genomes may consist of: 1. Double Stranded DNA 2. Double Stranded RNA 3. Single-stranded RNA 4. Single-stranded DNA Genome is usually organized as a single linear or circular molecule of nucleic acids

3 Capsids and Envelopes The protein shell enclosing the viral genome is the CAPSID Different shapes depending on the type of virus Built from several protein subunits called capsomeres Types of proteins within a capsid is usually small Some viruses have accessory structures that help them infect hosts Viral Envelopes Membranous envelope that surrounds capsid Derived from the membranes of the host cell

4 Capsids and Envelopes Many of the most complex capsids are found in bacteriophages Also called phages viruses that infect bacteria T2, T4, T6 are all similar in structure Infect E. Coli (used in several experiments) Protein tail with fibers to attach to the bacterium

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6 Viruses Reproduce only in host cells Viruses lack metabolic enzymes Viruses lack organelles such as ribosomes that enable protein synthesis Each virus can infect cells of only a limited variety HOST RANGE Results from evolution of recognition systems lock-and-key fit between viral surface proteins and specific receptor molecules Host range can be very broad or narrow Viral infections of multicellular eukaryotes is usually limited to particular tissues

7 General Features of Viral Reproductive Cycle 1. Viral infection begins when a virus binds to a host cell and the viral genome makes its way inside T-even phages inject DNA from tail apparatus Could be endocytosis or fusion of the viral envolope 2. Proteins it encodes program the cell to copy the viral nucleic acids and produce viral proteins 3. The host provides all the necessary mechanisms to make nucleic acids and enzymes 4. Viral genomes and capsid proteins self-assemble into new virus particles, which exit the cell

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9 Reproductive Cycle of Phages Phages are the best understood of all viruses Research on phages led to the discovery that some double-stranded DNA viruses can reproduce by two alternative mechanisms: 1. Lytic Cycle 2. Lysogenic Cycle KNOW THESE CYCLES

10 Lytic Cycle A phage reproductive cycle that culminates in death of the host cell During the last stage of infection, the bacterium lyses and releases the phages that were produced Each of these phages can then infect a healthy cell A few successive cycles of the lytic cycle can destroy a bacteria population A phage that reproduces only by a lytic cycle is a virulent phage

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12 Why haven t bacteria populations been destroyed? Bacteria are not defenseless Natural selection favors bacterial mutants with receptors that are no longer recognized by a particular type of phage When phage DNA successfully enters a bacterium, DNA often is identified as foreign and cut by restriction enzymes This restricts the ability of the phage to infect the bacterium This is an example of co-evolution: virus and host both evolve in response to eachother

13 The Lysogenic Cycle Allows replication of the phage genome WITHOUT destroying the host Phages capable of using both modes of reproduction within a bacterium are called temperate phages. One example used in research if phage Infection of the host begins with the phage binds to the surface and injects DNA Within the host the DNA molecule forms a circle DNA molecule is incorporated into a specific site on the E. coli chromosome When integrated this way the viral DNA is known as a prophage

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15 Prophage One prophage gene codes for a protein that prevents transcription of most other prophage genes The phage genome is mostly silent within the bacterium However, some may change the hosts phenotype may produce toxins Every time E. coli divides it then replicates the phage DNA along with its own and passes it to its daughter cells This allows the virus to reproduce/spread without killing the host in which it lives and depends Lysogenic implies that prophages are capable of creative active phages that lyse host cells this usually occurs due to an environmental signal such as chemical or high-energy radiation

16 Reproductive Cycles of Animal Viruses The nature of the genome is the basis for classification of viruses Single Stranded RNA viruses are further classified into three classes based on how they function in the host cell Nearly all animal viruses with RNA genomes have a viral envelope as do some DNA genomes

17 Viral Envelopes Viral envelopes are used to enter the host cell Glycoproteins (important for cell to cell recognition) bind to specific receptor cells on the surface of the host cell Once it enters the cell these glycoproteins are produced by ribosomes and transported the cell surface where they exit and are able to infect other cells This reproductive cycle does not necessarily kill the host cell like the lytic cycle

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19 RNA as Viral Genetic Material The broadest variety of RNA genomes is found among viruses that infect animals Some RNA viruses directly serve as mrna and can be translated immediately after infection Other RNA viruses serve as template for mrna synthesis and is transcribed into complementary RNA strands All RNA RNA synthesis use a viral enzyme to do so

20 Retroviruses RNA animal viruses with the most complicated reproductive cycles Equipped with an enzyme called reverse transcriptase Transcribes and RNA template into DNA HIV is a retrovirus that causes AIDS HIV and other retroviruses are enveloped viruses that contain 2 identical molecules of single-stranded RNA and 2 molecules of reverse transcriptase

21 Retrovirus Reproduction HIV 1. HIV enters the host cell 2. Reverse transcriptase molecules are released into the cytoplasm 3. Catalyze the synthesis of viral DNA 4. Viral DNA enters the cells nucleus and integrates into the DNA of a chromosome 5. Called a provirus 6. Never leaves the hosts genome

22 Evolution of Viruses Viruses have been found that infect every form of life Do you think they evolved before or after the first cells appeared? Scientists hypothesize that they evolved from loose nucleic acid that moved from cell to cell via injured cell surfaces Evolution of genes coding for capsid proteins may have helped the infection of uninjured cells Original source of viral genomes includes Plasmids small, circular DNA found in bacteria and yeasts Transposons DNA segments that can move from one location to another within a cell s genome Most importantly both plasmids and transposons are mobile genetic elements

23 Viral Diseases in Animals May damage or kill cells May produce toxins Damage of a virus depends on the ability of the infected tissue to regenerate (mitosis) How is it different between colds (respiratory cells) and polio (mature nerve cells)? Vaccines Harmless variant or derivative of a pathogen that stimulates the immune system to mount defenses Antibiotics cannot treat viruses

24 Emerging Viruses Viruses that appear suddenly or are new to medical scientists HIV is a classic example (San Francisco1980 s) SARS Severe Acute Respiratory Syndrome (China 2002) Three processes contribute to the emergence of viral diseases 1. RNA viruses have a high rate of mutation (forms new strains flu) which causes epidemics 2. Viruses can spread within a population very quickly technology, social factors, travel etc. 3. Spread of existing viruses from other animals

25 Viral Diseases in Plants Bleached or brown spots on leaves/fruit Stunted growth damaged flowers or roots Diminishes production and quality of crops Most plant viruses have a RNA genome Spread through two major routes: 1. Horizontal Transmission plant is infected from an external source 2. Vertical Transmission A plant inherits a viral infection from parent Genomes/proteins spread throughout the plant through plasmodesmata Lack cures scientists focus on developing resistant varieties

26 Viroids and Prions: Viroids Circular RNA molecules that infect plants Do not encode proteins but can replicate in host plants with use of the host cell enzymes Cause errors in the regulatory systems of plants that control growth Prions Infectious PROTEINS Cause degenerative brain disease in various animal species (includes mad cow) Most likely transmitted in food Act very slowly with long incubation periods Leading model shows prions as misfolded form of proteins normally present in brain cells