INVADERS

Chapter 24: Viruses Objectives: Summarize the discovery of viruses Describe why viruses are not considered living organisms Describe the basic structure of viruses Compare the lytic and lysogenic cycles of virus replication Summarize the origin of viruses

Virus nonliving particle that can infect both prokaryotic and eukaryotic cells

Virus Structure-vary in shape and size All viruses contain two main parts: 1) Nucleic Acid (Genome) – Either DNA or RNA 2) Capsid – protein coat or lipid protein coat (many different shapes) Helical – Tobacco Mosaic Virus (TMV), Rabies, measles Icosohedreon – Adenovirus, Herpes Simplex, Chicken Pox, Polio Spherical – Influenza Virus 3) Envelope – bilipid membrane that surrounds the capsid (only some viruses) Formed from nuclear membrane or cell membrane as it leave the host cell Proteins in the envelope helps new viruses recognize host cell. Examples: Influenza, chicken pox (varicola), HIV

Function of Viruses 1) Cause Disease by infecting the host cell 2) Used in Genetic Research and biotechnology

Characteristics of Viruses Cells are nonliving! Are not made of cells Do not have organelles or cytoplasm Can’t carry out metabolic processes such as metabolism and homeostasis Do not grow through Cell Division Can’t reproduce outside their host cell (Need host cells structures to reproduce) Wendell Stanley was the first scientist to crystallize a virus. This is evidence that viruses are not made of cells and are not alive.

Classification of Viruses A. Genetic Material: 1) RNA or DNA 2) Single Stranded or Double Stranded 3) Linear or Circular Genetic Information B. Capsid: 1)Shape 2)Presence or absence of an envelope Example: SARS – RNA, Single Stranded, Linear, lollipop-shaped capsid and enveloped (chart pg485)

Other ways viruses are classified: Different viruses infect bacteria, plant and animal hosts Example: Bacteriophages – viruses that infect bacteria -due to receptor site forming to specific proteins on cell wall or membrane e-coli being wiped out by T4 bacteriophages

How do Viruses Spread Air Water Food Bodily Fluids Contact

Obligate intracellular parasites (Viruses) Viruses replicate only by using the host cells enzymes and organelles to make more viruses. Protein synthesis is controlled by the viral genome and the host cell becomes a virus making factory. Protein Synthesis review Transcription: DNA -> mRNA Translation: mRNA -> proteins

Lytic Cycle vs. Lysogenic Cycle Reproduction of viruses Lytic Cycle: Reproduction of Virulent Virus Destroy the cell they infect Stages: 1) Adsorption 2) Penetration 3) Replication 4) Maturation (Assembly) Release Lysogenic Cycle: Reproduction of Temperate Virus Lays dormant in the host cell before it destroys it When triggered will go into the lytic cycle

Lytic Cycle – followed when virus is virulent (active) ADSORPTION- virus particle attaches to a host cell. ENTRY The particle injects its genetic instructions (DNA or RNA) into the host Injected genetic material ‘hi-jacks’ the cell’s machinery and recruits the host’s enzymes. 3. REPLICATION Enzymes make parts for the new virus particles 4. ASSEMBLY -new particles assemble the parts into new viruses 5. RELEASE -Cell explodes (lyses) releasing new viruses which search for a new host cell

The steps of the lysogenic cycle: 1) Viral nucleic acid enters cell Lysogenic Cycle - followed when virus is in latent or temperate state The steps of the lysogenic cycle: 1) Viral nucleic acid enters cell 2) Viral nucleic acid attaches to host cell nucleic acid, creating a prophage 3) Host cell enzyme copies viral nucleic acid 4) Cell divides, and virus nucleic acid is in daughter cells 5) At any moment when the virus is "triggered", the viral nucleic acid detaches from the host cell's DNA and enters stage 2 of the lytic cycle. Common symptoms that appear to "trigger" the viral DNA are hormones, high stress levels (adrenaline), and free energy within the infected cell.

Replication in DNA viruses Two methods: vDNA->mRNA –> make new viruses (in cytoplasm) DNA – Provirus – (Gets incorporated into Host Cell’s DNA) – Makes new viruses

Replication in DNA Viruses Method 1 1. Virus enters the host cells cytoplasm and takes control of the host cells protein synthesis pathway 2. Enzymes transcribe mRNA from the viral DNA 3. Host ribosomes translate Viral mRNA into viral proteins and enzymes replicate the viral DNA 4. New viral particles are assembled (Capsids and nucleic acids)

Replication in DNA Viruses Method 2 Viral DNA is injected into the host cell Viral DNA moves into the nucleus and becomes incorporated into the host cells DNA as a provirus Virus takes over the cell and makes newe viruses as a provirus

Replication in RNA viruses The vRNA serves right away as mRNA and then is translated into proteins (New Viruses) Viral RNA is transcribed into mRNA and then is translated into proteins and new viruses Retro RNA Viruses Contain Reverse Transcriptase which uses RNA as a template to create DNA, which is then inserted into the host cells DNA (ex:HIV) VRNA - > VDNA - > VRNA - > Proteins (Viruses)

Viral Diseases Vector – intermediate host that transfers a pathogen or parasite to another organism Ex: humans, mosquitos, ticks, fleas. Human viral diseases - chickenpox, shingles, viral hepatitis, AIDS, etc… Protease inhibitors – drugs that block virus reproduction Oncogenes – viral genes that cause cancer by messing with cell division checkpoints Proto-oncogene – controls cell growth Vaccination and vector control are the two most effective ways to control viral diseases

Prevention and Treatment Vaccine – weakened sample of virus or virus parts which triggers your body’s immune system Attenuated virus – weakened virus that cannot cause disease Inactivated virus – unable to replicate in host Natural immunity - antibodies are specialized proteins formed in B cells of immune system - antibodies block attachment sites of viruses http://www.pbs.org/wgbh/nova/bioterror/vaccines.html

Disease causing particles even smaller than viruses Viroids – short single strand of RNA without a capsid - interfere with cell processes and cause new viroids to be made - are found only in plants Prions - glycoprotein particle - able to reproduce in mammalian cells - Prion diseases: mad cow disease; Creutzfeldt-Jakob disease & kuru (degenerative nerve diseases of the brain)

Genetic engineering Genetic engineers use viruses to carry desirable genes from one cell to another Improved agriculture – herbicide tolerant soybeans; rot resistant tomatoes; fast growing fish; meatier chickens Correcting genetic disorders - experimental; only done in animal with success; not allowed in humans Pest control – insert genes of plants that create chemical to resist insects into crop plants such as corn Manufacturing of medicine - human insulin &clotting factors 8 & 9 http://www.pbs.org/wgbh/harvest/engineer/transgen.html