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

Page history last edited by Kathryn Addabbo 13 years, 2 months ago

A. Chapter Blog


Chapter 18 focuses on viral and bacterial genomes and how they carry out division. Viruses have several different structures that carry out their functions. They are classified into three categories: helical, polyhedral and complex. An example of a helical virus is tobacco mosaic virus (TMV), found in plants. Adenovirus is an example of a polyhedral virus, this virus causes respiratory symptoms and diarrhea. The replication cycles of these viruses can be split: the lysogenic and lytic cycles. The lysogenic cycle is when the host cell isn't destroyed. It is used as a complex to replicate the viral DNA, which the host cell's daughter cells inherit. The lytic cycle is when the cell is destroyed. The cell is used as a manufacturing point for phages, which after cell lysis, go to infect other cells. Viruses insert their DNA into host cells by first attaching to the cell membrane through receptors. A conformational change then occurs, which lets the virus enter the cell. Insertion of the viral DNA occurs into the cell's DNA via the enzyme integrase. Depending which cycle the cell follows, the host cell then replicates viral proteins and DNA or create phages which cause cell lysis.


HIV is short for human immunodeficiency virus, which is a stepping stone towards AIDS (acquired immune deficiency syndrome). HIV is usually transmitted sexually, but can also be transmitted through sharing needles. Once integrated into the person's DNA, it can remain either active or dormant until the conditions are right for it to become active. HIV carries out its course by attacking helper T cells, compromising the immune system. AIDS is the complete destruction of helper T cells, in which any infection to the person can be fatal. Drugs have been developed to reduce viral proliferation, which has caused a decrease in the amount of AIDS cases worldwide. Common drugs can lose their effectiveness over time due to the mutations the virus displays. It evolves so it does not react to the drug the same way it did before.


Bacterial chromosomes are different from those of eukaryotic individuals. They are a single type, in which there are many copies (typically 1-4 identical chromosomes). These are found in the nucleoid region, which is not separate from the cytoplasm. Unlike eukaryotic chromosomes, they are shorter and circular. Structural and protein sequences account for the largest amount of genes, followed by replication influence, gene expression and chromosome structure. They have an origin of replication, which is the initiation site for replication. When compacted, they are not coiled around histones; they form loops and the DNA supercoils around those loops. Plasmids are also a key part in the genetic makeup of bacteria. They are separate from chromosomes, and they have different roles. There are many types: resistance (R factors) contain genes that fight antibiotics and toxins, degradative enable bacteria to digest and utilize an unusual substance, col-plasmids encode colicins (proteins that kill other bacteria), virulence turn bacterium into pathogenic strains, and fertility (F factors) allow bacteria to mate with each other.


There are three methods of asexual reproduction in bacteria: conjugation, transformation and transduction. Conjugation is when the cells create sex pili and come into direct contact with each other. There is a genetic donor and receiver, in which only one cell changes its genetic makeup. Transformation is when DNA is taken up from the environment. This means that it first had to be ejected from another cell (typically dead). The DNA is taken up via competency factors, and inserted into the cell's DNA. Transduction is when a virus infects a bacterial cell and transfers that cell's DNA into another cell via phages. This can be seen as an error in the lytic cycle; a phage is produced carrying the bacterial chromosome. These three methods can be used to react against antibiotics. Once one cell becomes immune to the drug's effects, it can share its genetic makeup with other cells, which increases the population of immune cells. 


B. Materials


This picture differentiates between the three ways a cell can transfer genetic material. Transformation is defined as the intake of DNA from the environment surrounding the cell, while transduction is when a virus infects the cell and takes genetic material from the virus and inserts it into the cell's DNA. Lastly, conjugation is when physical links (called sex pili) form and transfer genetic material from one cell to another. In this case, the genetic material of only one cell is changed.



This image displays the difference between the lytic and lysogenic cycles. The lytic cycle is when viral DNA is inserted into the cell and that DNA is used to create more phages. To expose these new phages, the host cell must lyse in order for them to escape from the cytoplasm into the outside environment. The lysogenic cycle is when (after the injection of viral DNA), the viral DNA is incorporated into the host cell's DNA. When the cell divides, the daughter cells each carry the viral DNA. This leads to the production of many altered cells. 


Although the paragraph from this link is very short, it talks about HIV superinfection. This is when a person who already has HIV gets infected by other means. At this time, it is a controversial subject and is not well understood. This is extremely rare, and usually never seen. 

HIV superinfection

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