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Chapter 20: Viral and Bacterial Genetics (Erica)

Page history last edited by Erica Choi 13 years ago




A. Daily Blog

     All viruses have DNA or RNA surrounded by a protein capsid, but not all have an outer envelop or capsid but there are four main different structures: helical capsid (where there is a capside of 2,130 identical protein subunits arranged around a strand of RNA), polyhedral capsides that have protein fibers with a knob, an envelope composed of a lipid bilayer and spike glycoproteins, or protein coats that have additional accessory structures that help them invade bacterial cells. Viral replication starts with attachment of the virus to the surface of a host cell.  Then the genome of the virus enters the host cell and attachement of a phage stimulates a conformational change in the phage coat proteins so that after a series of events the phage injects its DNA into the hosts' cytoplasm.  One or several of the viral genes are expressed immediately thanks to the host cell enzymes and ribosomes, and this expression leads to the reproductive cycle, depending on the virus.  The third step is integration, viruses that are able to integrate actually have a gene that encodes for the enzyme integrase.  Integrase cuts the host's chromosomal DNA and inserts the viral genome into it.  After it is integrated, the phage DNA inside the host cell is known as a prophage and can proceed to the lysogenic cycle, or it can be excised from the bacterial chromosome and proceed to the next step. In the next step new viruses are produced by a host cell which involves the replication of the viral genome and the synthesis of viral proteins that compose the protein coat. The virus then exits the host cell via two ways: lysogenic viral cycle and the lytic viral cycle. The lysogenic viral cycle is when the virusexits the cell using a part of the plasma membrane of the host cell, and "bubbles" off of it into the environment.  However, the lytic cycle is when the virus bursts open the cell, lyses it, and the virus spill out of the host cell into the environment.  Viruses can contribute DNA to their hosts by injecting their DNA into the host, and excising the DNA and inserting their DNA into the host's DNA.  The way that the virus AIDS works, since it has RNA instead of DNA genetic material, is that it uses an enzyme called reverse transcriptase to convert the RNA into DNA, which is then injected into the host cell and proceeds to function. The HIV virus compromises the immune system because of the inability to identify errors.  this creates mutant strains of HIV, and undermines the ability of the body to fight HIV since the mutant strains could be resistant to the body's defenses.   Potential sources of drug activity against HIV could be using antiviral treatments the specifically bind to proteins encoded in the viral genome in order to inhibit viral replication. You could also create drugs that inhibit proteases which would prevent the assembly of capsids. 

     A bacterial chromosome usually has a single type of chromosome, but it can have more than one copy of it.  It is located, and tightly packed, within the nucleoid region of the cell.  This is not bound by a membrane, and the DNA is directly contacted by the cytoplasm.  These chromosome are usually circular, and tend to be shorter than eukaryotic chromosomes.  Bacterial DNA forms loop domains, which is one way that this DNA is compacted.  Supercoiling is also another way to compact the chromosome.  Plasmids are small, circular pieces of DNA that is separate from teh chromosome.  They are pretty small, and it even has it's own origin of replication, even though it is not actually an organism.  Plasmids usually code for some sort of growth advantage or survival.  Resistance plasmids contain genes that allow teh resistance against antibiotics and toxins, degradative plasmids carry genes that allow bacteria to digest and use unknown substances, col plasmids make proteins that kill other bacteria, virulence plasmids turn bacterium into a pathogenic strain, and fertility plasmids allow bacteria to mate with each other by touching.  

     Gene transfer can occur in a few ways.  One process, conjugation, allows direct contact between a donor and recipient cell.  The donor cell transfers a strand of DNA to the recipient.  Transformation allows a cell to use a fragment of its DNA from a donor cell to be released into the environment, this usually happens when the cell dies.  This DNA can be taken up by a recipient cell, which can incorporate the DNA into its own chromosome. Transduction is when a virus infects a donor cell and causes the chromosome of a donor cell to break into fragments.  This fragment of bacterial chromosome is incorporated into a new virus, which can transfer this DNA into a recipient cell. Gene transfer contributes to the spread of antibiotic resistance because a strain that could normally be resistant to an antibiotic, could not be after a genetic alteration in the bacteria's own genome.  This could usually be a result of horizontal transfer of resistant genes from a resistant strain, which makes it easier for bacterium strains to resist these antibiotics.



B. Useful Materials

While reviewing this chapter, I wanted a clarification between the lysogenic cycle and lytic cycle and i came across this video describing the lysogenic cycle.  Basically, the lysogenic cycle is when viral genetic material becomes a part of the host cell's DNA. Later, if the cell lysis the whole replication process is known as the lytic cycle; when the cell bursts and releases the new viruses.  
  This video goes into detail about bacterial conjugation and how it allows the exchange of genetic material between bacteria. 
Phenotypic and Genotypic variations  These researchers looked at how deletions and insertions in the genome influence phenotypic properties.  They collected different cultures of the same type of bacteria, and made sure that all the genomes were pretty identical and had similar infections cycles.  The differences were found in their latent periods and the amount of bacteria that were able to lyse which determined how point mutations in tail and spike proteins were enough to change the phenotype. 



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