A.Chapter 18 Summary

          A virus is a small infectious particle that consists of nucleic acid enclosed in a protein coat. Although they sound quite simple there are over 4,000 different types. Viruses can be categorized by their structure. There are three different kinds of virus structures: helical, polyhedral, and phages. The structure affects the way the different viruses replicate. There are six overall steps of reproduction. 1) Attachment. Viruses generally bind to specific cells because they recognize and bind to specific molecules on cell surfaces. 2) Entry. In phages, DNA, the genetic material, is injected into the cell. In polyhedral viruses, the capsids fuse with the membrane of the host cell, releasing all of its contents into the cytosol. 3) Integration. The enzyme integrase is expressed which cuts the host’s DNA and inserts the viral DNA. The lysogenic cycle stops here. The viral DNA is replicated everytime the cell replicates. When the genetic material remains inactive for a long time it is considered latent. However if the cycle was to continue and lead to the lysis of the cell then it would be called the lytic cycle. 4) Synthesis of Viral Components. In this step the viral DNA is transcribed and translated to make many proteins that are necessary for the creation of viral components and degradation of the host cell’s DNA. 5) Viral Assembly. All the viral components are assembled to create new viruses. 6) Release.  All of the new viruses are released. Bacteriaphages lyse the host cell but polyhedral viruses bud off from the host cell’s membrane.

            Human immunodeficiency virus (HIV), is spread through sexual contact and needles. The virus causes AIDS, short for acquired immune deficiency syndrome, which is caused from the viral destruction of helper T cells, a type of white blood cell. Helper T cells function as messengers in the immunes system and without them, the immune system stops working, leaving the individual very vulnerable to diseases that would normally not phase a healthy person. There has been a lot of research in the prevention of the HIV virus. One way is by creating drugs that bind to reverse transcriptase. The HIV virus’s genome is made up of single-stranded RNA. In order for it to be integrated into the double stranded DNA of the host cell’s chromosome, reverse transcriptase builds a double stranded DNA molecule, using the RNA as a template. Without reverse tanscriptase the viral genome would not be able to be integrated into any of the host cell’s DNA.  Another way to stop the spread of the HIV virus is to use a drug that inhibits proteases, enzymes that help assemble the HIV capsid. If the capsid cannot be created than no new viruses can be made. Another way of stopping the proliferatin of AIDS is to use an HIV vaccine. However in order for vaccines to be affective they must be inexpensive, easy to assemble, easy to administer, easy to store and long lasting.

          Bacteria store their genetic material much differently than eukaryotes. Bacteria tend to only have one chromosome, but there can be between 1-4 copies of  it in the cell. Bacteria chromosomes are often circular and short (a few million base pairs long.) They have one origin of replication. In order to be  packed into the tiny bacteria cell the chromosome is first looped and then the loops are supercoiled. Aside from the chromosome, bacteria cells also have plasmids, which are small circular pieces of DNA. They often carry only a few genes but their size can vary greatly. Plasmids have their own origins of replication. Plasmids help with survival and provide growth advantages. Plasmids are grouped into five different categories

1. Resistence- also known as R factors, resistant against antibiotics and toxins
2. Degradative-digest substances
3. Col-plasmids-encode Colicins, proteins that kill other bacteria
4. Virulence- carry genes that turn bacteria into a pathogenic strain
5. Fertility- also known as F factors, allow bacteria to mate with each other

          There are three different types of gene transfer: conjugation, transformation, and transduction. Conjugation is when a strand of F factor DNA is transferred through the a sex pilus which connects both the cells. Transformation is when a fragment of DNA is taken up from the environment and incorporated into the DNA. Transduction is when a virus cell takes up a fragment of the chromosome from the bacteria it was in and incorporates it into the virus particle and than transfers the DNA fragment to its host. Horizontal gene transfer is the transfer of genes from an organism that is not in the same generation. Examples are conjugation, transformation, and transduction. Horizontal gene transfer is often the cause of acquired antibiotic resistance, which is when the gene that is resistant to an antibiotic is transferred to different strains of a virus.

B. Useful Materials

Reverse Transcriptase and the Generation of Retroviral DNA.

This article is all about revers transcriptase. Reverse transcription was first discovered by Howard Temin. Before this discovery he claimed that the RNA was integrated into DNA but no one believed him because they didn't understand how that was possible. It changed the what was thought to be irreversible sequence of DNA to RNA to protein.The AIDS pandemic, which is also discussed in our book in addition to RT, made it even more of an interest to scientists because it played such a significant role in the replication of the virus. This article focuses on the role of reverse transcriptase in the virus replication cycle.

Source: PubMed