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Chapter 18: Genetics of Viruses and Bacteria (Maria W)

Page history last edited by Maria Waterhouse 13 years, 1 month ago

A.  Daily Blog



18.1: Viruses are nonliving particles with nucleic acid genomes and must take up a cell in order to replicate. It is composed of nucleic acid encased in a protein coat. Viruses differ in host range, structure, and genome composition. Host range is the number of species and cells the virus can infect. Some can only infect one species, while others can infect hundreds. Also, viruses may only be able to infect a specific type of cell in that species, such as brain cell, liver cell, etc. All viruses have a protein coat called a capsid, but they can have a variety of shapes, including helical and polyhedral. They can also have a viral envelope enclosing the capsid. Viruses that infect bacteria, called phages, usually have a more complex structure so that the virus can become anchored against the bacterial wall. Viruses can either have RNA or DNA located inside them, and different amounts of each nucleic acid. 

     The reproductive cycle of viruses include attachment to the surface of the host cell, genome entry into the host cell, and integration, which involves the cutting of the host cell's DNA and insertion of the viral genome into the DNA. Then viral components are synthesized and the virus is assembled. Finally, the newly synthesized virus is released from the cell. 

     The disease AIDS, or acquired immune deficiency syndrome, can be passed between sexual partners, sharing needles with drug users, and from mother to child. AIDS destroys helper T cells, a cell that plays an important role in the immune system. The victim's immunity system is compromised and becomes more susceptible to other diseases. The drugs used to prevent HIV spreading throughout the body help by inhibiting replication of the virus. However, over time, mutant strains of HIV may form that are immune to these drugs

The lyctic cycle involves cell lysis when the virus is removed from the host cell, whereas the lysogenic cycle consists only of replication, integration, and excision. 


18.3: Bacterial chromosome contain a nucleoid region that contains the nucleic acid but is not separate from the cytoplasm of the cell. The chromosomes are circular and shorter, but due to chromosomal loops and DNA supercoiling, the chromosome is much more compact. Plasmids are small, circular pieces of DNA that exist separate from the bacterial chromosome. They help with resistance to toxins, digest unusual substances, kill other bacteria, and any other growth advantages to the cell of survival needs. 


18.4: Bacteria can transfer their genomes in three main ways: conjugation, transformation, and transduction. Conjugation requires direct contact between cells, and the donor cell transfers a strand of DNA into the recipient cell. Transformation takes place when a fragment of the donor cell's DNA is released into the environment, and a recipient cell takes up these fragments. Transformation can take place when the donor cell dies. In transduction, a virus infects a donor cell, takes up some of that bacterial's chromosome, and passes that DNA on to another cell. Since bacteria take up DNA from the environment, they will be able to transfer antibiotic resistance because the cells that survive the antibiotics will be able to be passed on. 

B.  Useful Materials


This video describes the life cycle of HIV, the virus that leads to AIDS. We discussed HIV in class and how it damages the organism's immune system



Virus animation

This website shows an animation of how a virus infects a host cell step by step. The steps are described and each new step is hyperlinked from another page to split up the steps in a simpler way.



This PubMed article describes how studies are being made for viruses that have reversible capsid growth,. The capsid is the protein coat that covers the virus, and though this phenomenon is important to the virus, little is understood about it. 

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