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Chapter 18 Blog: Genetics of Viruses and Bacteria (Aarti)

Page history last edited by Aarti Patel 13 years ago

On this page you are required to add two items (link to a website, video, animation, student-created slide show, student-created PowerPoint presentation) and one journal article pertaining to a topic in this chapter.  A one-paragraph summary must accompany each item describing the main idea and how it applies to the lecture topic.  Please see the PBWorks help guide for assistance embedding video and other items directly in the page.  I will also produce a how-to video on using tables to wrap text around items and other useful tips.  Please see the syllabus for organization and grading details.


A. Daily Blog:


     Chapter 18 explained how viruses and bacteria worked.  A virus is an infectious particle consisting of nucleotides closed in a protein coat.  There are many types of viruses.  Viruses differ in there host range.  The host range is the number and type of species or cells a virus can infect.  Viruses also have different structures.  The protein coat around the nucleotides is structurally different in viruses (see picture below).  The genome of viruses is also varied.  Some viruses carry DNA and others have RNA.  Some virus genomes are single stranded and some are double stranded.  The genetic material could be linear or circular.  Since a virus is not living, it uses a host cell to replicate and reproduce.  In the lytic cycle it follows the 5 steps of the viral reproductive cycle:  1) Attachment of the virus to the host cell, 2) Entry of the viral genome into the host cell, 3) Integration, the viral genome is inserted into the hot cell genome by an enzyme called integrase, 4) Synthesis of viral components, the viral genome is transcribed and translated, 5) Viral assembly, the components are put together and the virus is recreated, and 6) Release of the created viruses, by either lysing the cell or exiting using exosytosis.  The lysogenic cycle, on the other hand, is when, after integration of the genome, the viral genome is not copied.  The viral genome becomes a part of the host cell DNA.  As the host cell divides, the viral DNA is divided as well.

     HIV remains latent in the host cell for long periods of time.  It is either integrated into the host DNA or it exists as a episome , or plasmid.  HIV invades and destroys the helper T cells of the immune system.  HIV causes AIDS.  When one has AIDS, the helper T cells are destroyed, the function of the immune is severely compromised.  The individual is more susceptible to infections.  Scientists have created drugs fro antiviral treatments.  One way is to create drugs that specifically bind to proteins found in the virus.  This will inhibit protein function, thus inhibiting viral DNA replication.  Another approach is to use antiviral drugs that inhibit the function of enzymes needed to assemble HIV capsids.  Another approach is to give vaccinations of HIV.  This will allow the immune system to create antibodies against it when it invades.

     Bacteria do not contain a nucleus.  Their genetic material is located in the nucleoid region.  The genetic material is circular, double-stranded DNA.  The regions DNA contains structural genes that encode for proteins.  The rest of the DNA is nucleotide sequences that aid in replication, gene expression, and structure.  One of these sequences is the origin of replication.  This is where replication is initiated.  For the genetic material to fit inside the bacterium, the circular DNA is looped and the loops are supercoiled.  Plasmids are small, circular pieces of DNA, separate from the chromosomes.  They divide independently of

the bacterial DNA. Plasmids help with the growth of the bacterium or aid in survival.  There are five types of plasmids: 1) resistance plasmids, carry genes that have resistance towards toxins; 2) degradative plasmids, carry genes for digestion and utilization; 3) col-plasmids, carry genes that code for colicins; 4) virulence plasmids, carry genes that turn the bacterium into a pathologenic strain; and 5) fertility plasmids, carry gene that are needed in mating.

     Bacteria are able to transfer genetic material to another bacterium.  Conjugation is when there is direct contact between two bacterium.  The genetic material is passed from one bacterium to another via a pilus between the two.  Transformation is when the donor cell is lysed and the genetic material is released to the outside.  Another bacterium takes up those DNA fragment and incorporates is with its own DNA.  Transduction is DNA transfer occurs via a phage.   When a phage is created in a host cell, is may contain fragments of the bacterial DNA.  When this phage causes the host cell to lyse, it then infects another cell.  It injects whatever DNA is contains, into the other cell.  Now this cell has a fragment of DNA from the previous cell.   Due to gene transfer, bacteria have acquired antibiotic resistance.  If one bacteria contains an antibiotic resistance gene, it can transfer that gene to other bacterial cells.  This means that now, rather than just one bacterium with the resistance gene, other have the gene as well. 


B. Useful Information:



  • http://www.ncbi.nlm.nih.gov/pubmed/21394574 
    • This article explains the structure of HIV-I protease.  HIV-I protease has 9 receptor sites.  These receptors are needed for HIV to bind to host cells.  These proteases were crystallized.  Researchers found that the binding constant is weaker.  This helps to find an antiviral drug against HIV.








Viruses have different protein coat structures.  The protein coat can be assembled in a helical structure.  The protein coat may be polyhedral.  Some polyhedral viruses have a viral envelope surrounding it.  Another structure of viruses is complex.  These viruses have multiple protein coat structures.




Viruses can replicate in two ways: through the lytic cycle  or the lysogenic cycle.  In the lytic cycle, after the viral DNA is integrated in the bacterial DNA, it is expressed.  New viruses are synthesized and assembled.   Those viruses cause the cell to lyse and then go and infect other bacteria.  In the lysogenic cycle, after the viral DNA is integrated, it is not expressed.  It remains in the bacterial DNA for a prolonged time.  The viral DNA replicates as the bacterial cell replicates. 



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