Heidi A. Everett
Viruses may enter a cell through a variety of means. The key to their entrance into the cell is their successful passage through the cell membrane and or cell wall. Cell membranes display specific surface receptors on their extracellular surface that may be composed of more then just proteins. Viruses with surface proteins that are capable of attaching to the host cell's receptors will then be capable of entering the cell (Alberts, 2002). Viral surface receptors have evolved so that their surface receptors mimic various receptors the cell has for necessary substances it would want entering the cell. Viruses are also capable of using more then just one type of surface receptor (Alberts, 2002). Students should not consider cells to have special surface receptors just for viruses; rather, viruses often exploit receptors that are beneficial to the cell for other purposes. This could be a possible area for a misconception to develop during the unit and students should be alerted of this.
Once the virus has successfully attached to cell surface receptor, the next step is entering the cell. There are various ways in which the virus will enter. The virus may fuse with the host cell's membrane, form a pore in the cell membrane, or disrupt the cell membrane's integrity (Alberts, 2002). The tobacco mosaic virus (TMV) infects specifically the tobacco plant cell by looking for small breaches in the cell wall. Animal viruses typically enter the cell through endocytosis or by being engulfed by the cell membrane. Bacteriophages pass through the cell membrane by injecting their nucleic acid into the cell acting like a hypodermic needle poking a hole through the cell membrane (Johnson, 2004). Once the cell membrane has been breached the virus takes over the cell's machinery to make more copies of its nucleic acid. The cell is now ready to produce viral proteins in large quantities. Viral proteins are a force to be reckoned with that will overtake the intracellular space of the host cell until the cell membrane lyses.
The section just reviewed is a critical piece to the unit, as students will be recreating these events using cells and viruses they have constructed. Students will chose the type of cell they want to construct (animal, plant, or prokaryotic) and then chose the type of virus they want to construct based on the categories listed above.
The life cycle of a virus once it has entered the cell can vary greatly, but there are basic steps that most viruses follow. The virus must release its nucleic acid through the disassembly of its capsid and any other components such as an envelope so that the viral nucleic acid can be copied. The next step involves the production of new viral protein particles through transcription and translation of the newly replicated viral nucleic acid. These newly made viral protein particles are then assembled into the correct structure and then released from the host cell (Saltzman, 2007).
Vaccines
An important goal of this unit is for students to understand the biotechnology used in creating a vaccine. Vaccines can be defined as " a solution containing all or part of a harmless version of a pathogen" (Johnson, p.235, 2004). Vaccines may contain genetically engineered viruses, weakened, or killed viruses. The advancements seen in the field of biotechnology have lowered the health risks associated with receiving a modern day vaccine as compared to the first vaccine administered in 1796 by Edward Jenner (Campbell, 2005).
Various different technologies are providing for a whole new type of vaccine called the DNA Vaccine. This type of vaccine removes one of the key steps in the way vaccines are currently made by simply inserting DNA of a virus or microorganism into an individual rather then using protein particles created from the DNA and then inoculating an individual with these proteins that will initiate an immune response (DNA Vaccines, 2003). DNA vaccines show great promise but are still in the beginnings of development.
The more commonly used vaccines that are used today are created through genetic engineering. Viral proteins are produced using techniques that use restriction enzymes that extract DNA sequences from viral genomes that code for their surface protein that allows attachment to specific host cell. This extracted DNA sequence is then inserted into a plasmid using hybridization and DNA ligase. A bacterial cell uptakes the recombinant DNA plasmid and makes many copies of the recombinant DNA as it reproduces. As the bacterial cell clones itself through asexual reproduction, it produces large quantities of the gene of interest, which happens to be the viral surface protein (Johnson, 2004).
This gene is then inserted into a harmless virus that uptakes the gene of interest and starts producing and displaying the surface protein associated with the harmful virus (Johnson, 2004). This modified harmless virus makes up a genetically engineered vaccine. The genital herpes vaccine is an example of a genetically engineered vaccine (Johnson, 2004). The Salk polio vaccine is an example of vaccine that is made from a killed virus rather then using genetic engineering methods. The small pox vaccine is made from naturally occurring, weakened viral particles (Saltzman, 2007).
