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Chapter 18 Blog: Genetics Of Viruses And Bacteria (Siddarth)

Page history last edited by Siddarth Santhebennur 13 years, 4 months ago


A.  Daily Blog

     For this chapter, we decided to take a look at viruses and bacteria and how they survive in this world as opposed to eukaryotic cells. We initially talked about viruses and some of the different kinds of viruses that we can fin around us. Viruses are very small compared to cells in the human body. They can range from about 20-400 nanometers in diameter in length. Even though there may are different viruses that exist around us, there are some common features that can be pointed out for ALL viruses. Every viruses is surrounded by a capsid. A capsid is a protein coat that encloses a virus's genome. Capsids can appear in different forms which thus results in the different shapes that observe when looking at a virus. Two main capsid shapes that can be distinguished are: helical and polyhedral. A helical-shaped protein coat gives the virus a cylindrical look and the genetic material is encompassed in this shape as well. Polyhedral-proteins coats are more complex in structure and have terminal knobs on the ends of it's corners. Another feature seen in many viruses is the presence of a viral envelope. A viral envelope is a  structure enclosing the viral capsid that consists of a membrane derived from the plasma membrane of the host cell; is embedded with virally encoded spike. Some special viruses have terminal knobs that are composed of glycoproteins. These glycoproteins act as anchors to a host cell in order for the virus to transfer it's genetic material into the cell. Such viruses are known as bacteriophages. The main differences that we see among viruses are what composes the genome as well as the structure. Viruses are considered to be nonliving. They are dependent on other cells in order to reproduce. In the case of reproduction, viruses follow a six step process:


  1. Attachment: This step is highly specific since the virus needs to bind to the right receptor on the outside of the host cell. As mentioned before, the viruses use a glycoprotein complex on their outside in order to attach and anchor themselves down
  2. Entry: The step itself is pretty self-explanatory. Once the virus has attached to the host cell, a conformational change occurs on the protein coat. The genetic information is either injected into the cell OR the viruses protein coat could fuse with the cell membrane (bringing the genetic information along with it).
  3. Integration:Viruses have the ability to fuse their genetic material with the host cell's genetic information. The genetic information brought in by the virus could either be RNA or DNA. If it is RNA, the strands are used to synthesize DNA strands with the help of the enzyme Reverse Transcriptase.
  4.  Synthesis of Viral Components:Once the genetic material from the virus has been added to the host cell, the host cell unknowingly produces factors that are required for building more viruses. Examples of this would production of protein coat, replication of viral genetic information, etc.   
  5. Viral Assembly: This step is all about putting all the newly produced viral parts together in order to form new viruses
  6. Release: Once the new viruses have assembled, they can either lyse the cell and leave to find new cells to attach to OR they could reassemble with their membrane that might have been attached to the cell membrane (depending what on what virus) 

This is the overall process of viral replication. It does go into much more detail when we observe different types of viruses, but the is just the basis. A big reason why scientists are so interested in the replication system of viruses is because what some of them result in. For example, a well known virus is the human immunodeficiency virus (HIV). HIV is considered to be an emerging virus. An emerging virus is exactly what is sounds like...it is a new virus! After multiple replications within the human body, this virus can give one acquired immune deficiency syndrome (AIDS). HIV targets T cells which are used to produce antibodies. As HIV starts to slowly destroy these T cells, the immune system deteriorates and results in AIDS. Researchers are currently in the process of finding drugs to inhibit the function of HIV, however they have come across some issues such as disturbing the function of an organism's cell. Researchers have seen that over a prolonged period of time, the drugs tend to lose their effect. The reason behind this is because the virus has the ability to adapt to new conditions and become immune to such drugs. MOVIN ON...a virus can typically go through two different cycles to reproduce: the lytic & lysogenic cycle. The lytic cycle starts off when the virus inserts its genetic information into the host cell. The next step is the formation of new viruses within the host cell. After these new viruses are formed, they lyse the host cell and the whole process starts all over again. The lysogenic cycle starts off with the step as the lytic cycle. Once the genetic information has been inserted into the cell, it is integrated into the host cell's genetic information. Every time the host cell divides, the viral genetic information gets copied as well and brought along with the new cells that are formed. 

     Although we have examined bacterial DNA before, we haven't closely examined it and pointed out some of it's key features. There are many key features that bacterial DNA has which make them unique. One important feature is the chromosomal structure is in a circular form. A typical bacterial chromosome is about a few million base pairs long. Most bacterial chromosomes a single type of chromosome but it may be present in multiple forms. On a bacterial chromosome, there are several thousand genes interspersed throughout the chromosome. Unlike eukaryotic chromosomes, bacterial chromosomes have only one origin of replication. Let's move on shall we? What are plasmids? Plasmids are  small circular pieces of DNA found naturally in many strains of bacteria and occasionally in eukaryotic cells; can be used as a vector in cloning experiments. There are different kinds of plasmids that each have a specific function. 


  1. Resistance Plasmids: assist in resistance against antibiotics and toxins.
  2. Degradative Plasmids: carry genes that enable the bacterium to digest and utilize an unusual substance.
  3. Col-Plasmids: encode for colicins which are proteins that kill other bacteria.
  4. Virulence Plasmids: carry genes that turn bacterium to pathogenic strains.
  5. Fertility Plasmids: allow bateria to mate with each other

There are three major ways that bacteria transfer their genes: transduction, transformation, conjugation. Conjugation is when there are two bacterial cells present. A direct contact occurs between both cells in which a transfer of gene occurs. The end result is that the donor and the recipient both get the same plasmid in the end. When transduction occurs, a virus enter the donor bacterial cell and breaks up the genetic material located there. After this has occurred, the small fragments enter into the virus. The virus takes these pieces of DNA and inserts it into the recipient cell which causes gene transfer. The last method that we have observed before is transformation. This occurs when a lysed bacteria cell has genetic information floating around it. This genetic material is then taken up by a living bacterial cell and incorporated in the genetic material. An example of this would be the Beadle & Tatum experiment. In this experiment they killed S-Bacterium which had a special covering that made them resilient. When they were placed amongst R-Bacterium (lack covering) they formed the covering later on. This is how antibiotics tend to get old and not usable for treating certain sicknesses.  

B. Useful Materials


Useful Video


This is a video showing an example of how a virus can invade a cell. In this case authors of this video are looking at how a virus affects a human. It goes over how it attaches to a human cell and how it replicates (A.K.A. the six steps of replication) 

Useful Website

Bacterial Conjugation  
This is a cool little animation that takes you step by step through one of the methods of gene transfer in bacterial cells. It is an interactive video which allows you to answer questions based on what you have learned from the animation.

Useful Article

Longevity of rAAV vector and plasmid DNA in blood after intramuscular injection in nonhuman primates: implications for gene doping.  
This is a very interesting article where researchers have found a way to track whether a bacteria has gone under a gene transfer. It discusses how one can use PCR microassay's in order to organize the DNA structures. This DNA is extracted from the blood cell's and looked at and compared to a test ruler to see what kind of gene transfer. 




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