(Transcribed from Dr. Meissienburg’s lecture, 3 April 2000 by Brian Buschman)
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This is a very loose summary of the material presented. It is written primarily for my personal use.
Viruses are obligatory parasites because they are unable to duplicate their own DNA or transcribe/translate their genes. They must use the cellular mechanisms of a host cell to do this. Viruses have a capsid, the capsule that surrounds it’s nucleic acid, and the domain responsible for binding to the host cell.
The virus replicates by inserting it’s nucleic acid into a host cell. The nucleic acid may be DNA or RNA and may be single stranded or double stranded. It then uses the cell machinery to reproduce it’s nucleic acid and to make more virus “parts.” The host cell will assemble more virus particles.
The viral reproduction can proceed by either the lytic or the lysogenic pathways. The classis example of the lytic pathway is when the T4-phage binds to the host, injects it’s DNA and hijacks the cellular machinery. It uses the cell to produce it’s DNA and to synthesize it’s proteins. The proteins are then assembled into the new virons. The host cell is finally lysed and about 200 virons are released. The cycle takes about 25 min.
The l-phage is the classic example in this case. It binds to the host and injects it’s DNA which is incorporated into the host genome by integrase. As the cell goes through it’s cycle it reproduces it’s DNA with the phage DNA and passes on the phage genome to it’s daughter cells. In the host cell the only part of the phage DNA that is transcribed or translated is the phage repressor. The repressor keeps the other genes from being expressed. If the level of the repressor falls below a certain level then the phage DNA will be removed from the host DNA molecule and will be transcribed and translated. At this point the cell enters the lytic pathway.
The body is able to fight viruses or virally infected cells by recognizing viral identifying proteins on the cell or capsid membrane called “spike” proteins. This functions through CD8 killer T-cells as discussed in histology and previous biochem lectures.
When a virus inserts it’s RNA into a cell it can be either + or – RNA. The + RNA is able to function as mRNA and the – RNA is only a template for + RNA. In the host cell the virus does not necessarily use reverse transcriptase to integrate the genes into the host genome. Instead it may chose to reproduce the RNA. If it injected + RNA then it will be copied to – RNA which will then be copied to make more + RNA. If there is enough + RNA then the + RNA will act as mRNA and will be translated to produce the viral proteins.
RNA viruses can work by using an enzyme, reverse transcriptase, that is contained in the capsid with the RNA to convert the RNA into DNA. It has a long terminal repeat that appears on either end which contains the viral promoter. Reverse transcriptase has a very high error rate. This can be bad for the virus or can save it. The high error rate can be useful because it can cause it to change fast enough to avoid being killed by drugs, antibodies and CD8 cells.
Plasmids are small circular pieces of DNA that are not essential to the functioning of the cell but can be inserted into the cell. They can just be junk genes or may code for valuable stuff such as antibiotic resistance.
Bacteria have three mechanisms classified as parasexual processes to transfer genetic material to another cell.
1) Transformation is when a cell, on it’s own, takes up pieces of DNA. It may take up random pieces of DNA or it may only take up DNA made for it’s specific species. The DNA will be inserted into it’s chromosome by homologous recombination.
2) Transduction is when bacteria will use a bacteriophage to transfer material from one cell to another.
3) Conjugation is when there is a mechanism of directly transferring genetic material from one cell to another. The classic example is when a cell has the “F-factor” which allows it to grow a sex-pili. The sex-pili will contact another cell and will inject the F-factor into the new cell. When it does that it may also pass other genes with it. The recipient cell will not grow a sex-pili and begin to fertilize more cells.
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