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Viruses and Bacteria

Viruses and Bacteria. How does its size compare to that of various cells?. Viruses are smaller than prokaryotic cells (like bacteria) and significantly smaller than eukaryotic cells (like human cells) http://www.cellsalive.com/howbig.htm. Is a virus a living organism?.

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Viruses and Bacteria

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  1. Viruses and Bacteria

  2. How does its size compare to that of various cells? • Viruses are smaller than prokaryotic cells (like bacteria) and significantly smaller than eukaryotic cells (like human cells) • http://www.cellsalive.com/howbig.htm

  3. Is a virus a living organism? • Most scientists describe viruses asnon-living • Don’t consist of cells • Lack organelles: use “machinery” of the “host” cell to carry out necessary cell functions

  4. Viruses: Structure • They are much less complex than cells and lack the enzymes and organelles to carry out cellular functions. • Tiny, non-living particlesthat can infect cells and cause disease. • Have Genetic material (DNA or RNA) • Have a protein “shell” (capsid) which may be surrounded by an envelope(lipid bilayer).

  5. Virus Structure Yellow Fever Virus Adenovirus Smallpox virus

  6. Viruses: Replication • Must gain access into a host cell • cannot reproduce outside a cell; must be cultured in living tissue. • Virus binds to a receptor on the membrane of the host cell. • Virus surface proteins, like glycoproteins, have a complementary shape to a cell receptor.

  7. “Host range”: each cell type has a unique collection of receptors at its surface. • HIV only infects white blood cells because other cells don’t have the right receptor for HIV to bind to. • Bird flu used to only infect certain bird species but a mutation allowed the virus to “jump species” because now it can bind to human cells.

  8. Virus Replication-continued • “Hijacks” the host cell’s enzymes and organelles; copies are made of the viral genome, proteins for capsids, etc. • New viruses are assembled and leave the cell. • Lysis: host cell bursts open and is destroyed. • Budding: viruses bud from the membrane; may or may not destroy the cell.

  9. Virus Release

  10. Virus Replication-continued • Lytic cycle: virus immediately begins replicating and soon the host cell is destroyed. (Virulent) • Lysogenic cycle: virus “hides” in the cell, incorporates its DNA into a host cell chromosome. (Temperate) • At some point, the temperate virus turns virulent.

  11. Lytic Lysogenic What happens to the viral genome determines the next step. 1 5 2 5 4 3 3 4 http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter17/animation_quiz_2.html

  12. Retroviruses • Retroviruses = RNA virus. • Example: HIV • Have reverse transcriptase; an enzyme that turns the RNA  DNA so it can incorporate into host DNA. • So these viruses carry out a lysogenic cycle of replication. http://www.whfreeman.com/kuby/content/anm/kb03an01.htm

  13. Reverse Transcriptase RNA virus DNA (single strand) DNA (double strand) A U G C C A T A C G G T T A C G G T A T G C C A Single DNA strand used as template for complementary DNA strand Reverse transcriptase

  14. Host Chromosome

  15. Preventing Viral Infection • Vaccines exist for many viruses • Vaccines contain a pathogen (like a virus) in a form that will trigger the immune system, but not make the person sick • “Memory” white blood cells will “recognize” the pathogen if the person is ever exposed again and will be able to fight it quickly • Prevention, not treatment

  16. Vaccine contains an altered (aka not harmful) form of the virus. Antibodies made in the body Immune system is triggered. White blood cells produce antibodies specific to the proteins on the virus. Virus from the vaccine If the virus invades the body in the future, memory cells “recognize” it. Antibodies are produced and “attack and destroy” the virus before symptoms appear. Vaccines Pathogenic virus

  17. Vaccines and Immunity Memory cells remain in the body for years and become activated if the same antigen enters the body. Antibodies are produced in response to antigens- the surface proteins on the virus in the vaccine or the pathogenic virus.

  18. Treating Viral Infections • Very few treatments are available for viral infections. The immune system must fight it off. Why? • Viruses mutate very quickly • They’re not living; antibiotics inhibit cell functions that viruses don’t carry out. • Some antiviral drugs exist that interfere with protein production (in the infected cell).

  19. Bacteria: Structure • Simplest cells: prokaryotes • Can be cultured on agar; are able to live and reproduce on their own, don’t need to infect a host cell. • Have a cell wall, cell membrane, cytoplasm, ribosomes, enzymes, etc. • Contain DNA (not in a nucleus) • Single chromosome-loop shape. • May also have DNA in a plasmid.

  20. Additional Bacterial Structures • Some bacteria may have the following structures: • Capsule (covering outside of cell wall that protects bacterial cell) • Pili (hair-like structures used to transfer genetic material from one bacterium to another) • Endospore (Thick covering around DNA; developed under harsh conditions) • Plasmid (small, additional loop of DNA) • Flagella (Used for motion)

  21. Bacteria Structure

  22. Bacteria Reproduction • Binary Fission: one cell copies its DNA and divides to form 2 genetically identical cells.

  23. Genetic Variation • Prior to cell division, DNA is replicated. If a mistake is made a mutation occurs. • Mutation is a source of genetic diversity. • Natural Selection acts on diversity in a population. • Some bacteria are more likely to survive than others, if they have more favorable genes. • Although reproduction does not lead to genetic recombination, other processes such as conjugation or transformation do.

  24. Conjugation-transfer of plasmid http://www.hhmi.org/biointeractive/animations/conjugation/conj_frames.htm

  25. Bacteria and Disease • Bacteria can harm the body by releasing toxins. • They don’t often invade cells. • Many bacterial diseases can be prevented with vaccination. Antibiotics are used for treatment.

  26. Bacteria and Antibiotics • Antibiotics: Chemicals that interfere with cell functions; inhibit bacteria growth. • Bacteria are sensitive if they are harmed and don’t grow in the presence of the antibiotic. • Bacteria are resistant if they avoid harm and do grow in the presence of the antibiotic.

  27. Plate on Day 1: no visible bacteria Plate on Day 2: bacteria growth around the plate. Clear areas = areas of no growth = zone of inhibition. Bacteria Growth and Antibiotics http://www.sumanasinc.com/scienceinfocus/sif_antibiotics.html

  28. Evolution of Resistant Bacteria • Natural genetic variation (caused by mutation) results in some bacteria in a population being resistant to an antibiotic. • After a course of antibiotics, the resistant bacteria are the only ones left to survive and reproduce. The population of bacteria becomes composed of mostly the resistant type. • The resistance genes can be passed on through conjugation as well, even to other bacteria species. • Over time, many bacteria populations are resistant to the antibiotic.

  29. Fighting Infection • The immune system also produces antibodies against invading bacteria in order to fight the infection.

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