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Chapter 18: Viral and Bacterial Genetics

Page history last edited by Derek Weber 12 years, 9 months ago


Learning Objectives  

18.1 Genetic Properties of Viruses 

  • Describe the different structures found in viruses.
  • Understand the basic mechanism of viral replication.
  • Describe the disease AIDS.
  • Compare and contrast a lytic viral reproductive cycle with a lysogenic viral reproductive cycle.
  • Describe how viruses can contribute DNA to their hosts.
  • Explain how the HIV virus compromises the immune system.
  • Describe potential sources of drug activity against HIV, and explain why common drug treatments lose their efficiency over time.

18.3 Genetic Properties of Bacteria

  • Outline the key features of bacterial chromosome.
  • Describe bacterial plasmids and list their functions.

18.4 Gene transfer Between Bacteria

  • Distinguish the various methods of genetic transfer in bacteria.
  • Describe how gene transfer contributes to the spread of antibiotic resistance.

 

Chapter Summary

Viruses are non-living, intracellular parasitic particles, made of nucleic acid, either DNA or RNA, but not both.  A protein coat called a capsid surrounds the nucleic core.   Still another coating, a lipid-rich envelope encloses many viruses.  Viruses are able to replicate only by using the host cell’s DNA and machinery that leads to the production of more viruses.  Replication is slightly different for a virus with DNA rather than a virus with RNA genome.  They vary greatly in appearance and size and most are so small they can only be viewed through electron microscopy.  Viruses infect nearly all forms of life and cause numerous plant, fungal, and animal diseases.

 

Bacteriophages are large, double-stranded DNA viruses that infect bacteria. They are composed of proteins that make up the head, capsid, tail, base plate, and tail fibers. Great diversity exists among the phages with some belonging to a T-series and others given different names.  These bacterial viruses exhibit two types of reproductive cycles:  the lytic cycle and the lysogenic cycle.  The lytic cycle occurs when the virus immediately kills the infected host cell in which it is replicating.  The virus injects its head, mostly DNA, into the host cell’s cytoplasm, which turns on the host cell’s replicating machinery, allowing the virus to multiply within infected host cells. Newly produced viruses are released when the host cells lyses. The lysogenic cycle does not immediately kill the infected host cells.  Lysogeny occurs when the viral head is injected into the host cell’s cytoplasm and combines with the host cell’s DNA producing a prophase.  The expression of the prophase genome may not immediately be expressed, but is replicated during normal mitotic division. However, at a later time, the prophase may initiate viral replication.   Viruses may infect cells with damaging effects, transforming benign bacteria into disease-causing forms as in cholera.

 

The human immunodeficiency virus (HIV) is an example of a complex animal retrovirus containing a single-stranded RNA and reverse transcriptase, an enzyme needed to convert viral RNA into viral DNA.  HIV infects and kills CD4+ cells, T cells that are importantcomponentsinthehumanimmunesystem.Clinicalsymptomsofacquired immunodeficiency syndrome (AIDS)developonlyafteralongperiodoflatencyduringwhichtheimmunesystemsuppressestheongoingHIVinfection.  First, each HIV particle possesses a glycoprotein on its surface that fits into a receptor on the CD4 cell’s surface andundergoesaconformationalchangeandbindstoCCR5.Thevirusthenentersthecellviaendocytosis. Viral RNA is synthesized into a double strand of viral DNA wherein it combines with the host cell’s DNA, directing the host cell machinery to produce more HIV copies.   Rupturing and killing the T cells now release the replicated viruses.  ThedecreaseinTcellsdebilitatesthebody’simmunesystemandthepatientgenerallysuccumbstootherwisenonfatalinfectionsandcancers.Newkindsofcombinationdrugtherapies, such as AZT other nucleoside analogs, and protease inhibitorsarepromisinginthefightagainstAIDS.  DiscoveryofHIV-infectedpatientswithdefectivegeneshavehelpeddiscovernewwaystofighttheHIVinfection.  Chemicals called chemokines appear to inhibit HIV infection and the search for HIV-inhibiting chemokines is intense. Still others possessing a mutation in the gene that codes for the CCR5 receptor appears to block HIV infection. 

 

Viruses are nonliving infectious agents that infect all living organisms, including bacteria, protists, fungus, plants, and animals.   Some human viruses include influenza, smallpox, infectious hepatitis, yellow fever, polio, rabies, AIDS, cancers, leukemias.  They cause major losses in agriculture, forestry and productivity of natural ecosystems.  Emergingvirusesareonesthat originateinonespeciesandtransfertoa different species.HIVisanexample,asareinfluenzaandtheEbolavirus.Epidemiologicalstudieslinkanumberofviruseswithcertainformsofcancerandarenowestimatedtocontributetofifteenpercentofhumancancers.

 

Prokaryotic organisms differ from eukaryotic organisms in the following way:  unicellular, cell size, chromosomes organization, cell division and genetic recombination, internal compartmentalization, flagella structure, and metabolic diversity.  Bacteria are single-celled organisms, but can exist singularly, in colonies or in filamentous organizations. There are few integrated activities between prokaryotic cells and no true specialization of cells as found in even the most primitive multicellular organisms. Bacteria do not possess chromosomes like eukaryotes as their genes are contained in a single, double-stranded ring of DNA found in the nucleoid region of the cell. They lack internal compartmentalization and do not have any membrane-bound organelles. Internally, they have a complex membrane system formed from invaginations of the plasma membrane. Photosynthetic and/or respiratory enzymes may be associated with these membranes. Like eukaryotes, they have ribosomes, but they are distinctly different in protein and RNA content. Genetic variation in bacteria results from exchange of DNA fragments or by mutation. A high rate of mutation coupled with a very short generation time can rapidly change the characteristics of a bacterial population.

 

Bacteria are serious plant and human pathogens. Most plant pathogens are rod-shaped pseudomonads, while animal pathogens are extremely diverse. In addition, bacteria cause a wide variety of human diseases including anthrax, botulism, Chlamydia, cholera, dental caries, diphtheria, gonorrhea, leprosy, lyme disease, peptic ulcers, plague, pneumonia, tuberculosis, typhoid fever, and typhus.  Tuberculosis has been around for thousands of years with millions of new cases reported worldwide each year.  Dental caries are caused by a wide variety of bacteria, exacerbated by high sugar diets. The many sexually transmitted diseases (STDs) are causing widespread problems throughout society with many becoming resistant to antibiotic treatment.

 

Virtual Lectures  

 

Animations

18.1 Genetic Properties of Viruses

  1. Viruses: Virulence
  2. Viruses: Temperate
  3. Viruses: Bacteriophage replication
  4. Viruses: Animal

18.3 Genetic Properties of Bacteria

  1. Bacteria: Growth
  2. Bacteria: Binary_Fission

 

18.4 Gene transfer Between Bacteria

  1. Horizontal_Gene_Transfer
  2. Conjugation
  3. Transformation
  4. Transduction
  5. Origins of Antibiotic Resistance

 

PowerPoint Presentations (click link to download)

 

Reading Assignments and Homework

Please access the ConnectPlus site for Health Science Academy to access our reading assignments and homework.

 

Miscellaneous

 

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