Question

1. What defining feature characterizes the lytic replication cycle?
2. Can viruses be cultured? If so, how is this "culturing" different from culturing a bacteria?
3. How does the lysogenic replication cycle differ from the lytic replication cycle?
4. Provide a brief and general summary of how virus infections are treated.
5. What are the 4 different types of animal virus infections?
6. How might the process of restriction help protect a cell from a viral infection?
7. Is the process of restriction a guaranteed defense against viruses? Why or why not?

Answer 1

1. The cycle typically leads to the lysis of the host cell. There is attachment: the virus attach to bacteria (host): can attach on cell wall, flagella or pili. The virus inserts its DNA into host and it takes over host cell's machinery.The virus reproduces itself and self-assembles. (induction occurs). Lysozyme weakens cell wall enough for newly assembled virions to break out; lysis occurs.

2.Viruses can be grown both in vivo (within a whole living organism, plant, or animal) or in vitro (outside a living organism in cells in an artificial environment, such as a test tube, cell culture flask, or agar plate). Bacteriophages can be grown in the presence of a dense layer of bacteria grown in a 0.7 % soft agar in a Petri dish or flat (horizontal) flask. The agar concentration is decreased from the 1.5% usually used in culturing bacteria. The soft 0.7% agar allows the bacteriophages to easily diffuse through the medium. For lytic bacteriophages, lysing of the bacterial hosts can then be readily observed when a clear zone called a plaque is detected. As the phage kills the bacteria, many plaques are observed among the cloudy bacterial lawn.

For in vitro studies, various types of cells can support the growth of viruses. A primary cell culture is freshly prepared from animal organs or tissues. Cells are extracted from tissues by mechanical scraping to release cells or by an enzymatic method using trypsin or collagenase to break up tissue and release single cells into suspension. Because of anchorage-dependence requirements, primary cell cultures require a liquid culture medium in a Petri dish or tissue-culture flask so cells have a solid surface such as glass or plastic for attachment and growth. Primary cultures usually have a limited life span. When cells in a primary culture undergo mitosis and a sufficient density of cells is produced, cells come in contact with other cells. When this cell-to-cell-contact occurs, mitosis is triggered to stop. This is called contact inhibition and it prevents the density of the cells from becoming too high. To prevent contact inhibition, cells from the primary cell culture must be transferred to another vessel with fresh growth medium. This is called a secondary cell culture. Periodically, cell density must be reduced by pouring off some cells and adding fresh medium to provide space and nutrients to maintain cell growth.

3.In a lyosgenic cycle, the virus is inserted into th bacterial chromosome, and it is replicated and passed on to all daughter cells it is induced to leave the chromosome. However, a lytic cycle is a replication cycle that results in cell death.

4. For any virus, treatment is for the symptoms, such as using over-the-counter pain relievers to ease pain and reduce fever, rest for fatigue, etc. As such it is not easy to treat viruses. In case of hepatitis C, a liver disease, involves a strict medication regimen that can take from several weeks up to a few months before the virus is cleared from your body. Other viruses don’t have a cure, but there are medications that may speed healing if they are taken early enough after your exposure to the virus. These include medications for influenza and shingles (herpes zoster). Medications for other viruses, such as HIV, keep the virus in check, but aren’t a cure. They can prevent the virus from replicating and causing more damage. Most people can rely on their immune system to fight off the majority of viral infections. You can follow certain measure like:

• resting at home
• sipping water for hydration
• drinking lemon and honey drinks for coughs and colds
• taking paracetamol or ibuprofen to relieve pain and fever.

5.There are several types of animal viruses. They are commonly grouped into families according to the type of genetic material present in the virus.

a. Double-stranded DNA viruses usually have a polyhedral or complex structure. Examples include: Papilloma (cervical cancer and warts), Herpes (simplex I and II), Epstein-Barr virus (mononucleosis) and Variola (smallpox).

b. Single-stranded DNA viruses usually have a polyhedral structure and depend on adenoviruses for parts of their growth.

c.Double-stranded RNA viruses usually have a polyhedral structure with the diarrhea viruses being a common example.

d. Single-stranded RNA viruses are usually of two subtypes: those that can serve as messenger RNA (mRNA) and those that serve as a template for mRNA. Examples include: Ebola viruses, Rhinovirus (common cold), HIV, rabies virus and influenza viruses.

6. The bacterial cell uses the restriction enzyme to cut the invading DNA of the virus at the specific recognition site of the enzyme. This prevents the virus from taking over the cellular metabolism for its own replication. But bacterial DNA will also contain sites that could be cleaved by the restriction enzyme.

7.Yes. As the sequences recognized by the restriction enzymes are very short, the bacterium itself will almost certainly contain some within its genome. In order to prevent destruction of its own DNA by the restriction enzymes, methyl groups are added. These modifications must not interfere with the DNA base-pairing, and therefore, usually only a few specific bases are modified on each strand.