16: Viruses, Cancer, and the Immune System

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At this point, you should be fairly comfortable with the basic concepts of cell biology. The purpose of this chapter is to build on that basic knowledge and put it together into more complex systems. In addition, we will introduce some more advanced variations on some of the mechanisms and structures that were discussed in earlier chapters. The three topics, viruses, cancer, and immunity, are not only relevant as current news topics, but relate to one another through multiple pathways, which is why they are lumped together.

16.1: Viruses
A virus has both genetic material and protein components, but it is not a living organism. It cannot self-replicate and is completely reliant on the cellular biochemistry of whatever host cell it has infected. The minimal definition of a virus is a nucleic acid genome inside of a protein shell, or capsid. There are variations of this, such as virions that have a membrane coat outside of the capsid, or some that have enzymes inside of the capsid alongside the genome.
16.2: Viral Life Cycles
Viruses can interact with their hosts in two distinct ways: the lytic pathway and the lysogenic pathway. Some viruses are able to switch between the two pathways while others only use one. The distinguishing characteristic of the lytic life cycle is catastrophic death of the host cell by lysis and simultaneous release of viral particles.
16.3: Cancer
Cancer encompasses a set of genetic diseases that lead to uncontrolled cell proliferation in multicellular organisms. The discussion of cancer also happens to be useful in a cell biology course, because it ties together many of the concepts that you just spent most of the semester learning. Although it can be caused in part by an outside agent, the development of cancer is essentially a series of uncorrected mistakes by a cell’s regular processes.
16.4: Oncogenes
Oncogenes are generally dominant gain-of-function mutations of normal cellular genes called protooncogenes. These protooncogenes are themselves positive regulators of the cell cycle, but they are regulated by other factors, either extracellular signals or intracellular mechanisms. Mutations that turn them into oncogenes specifically remove all or some of this regulation. They thus become overactive, and try to push the cell cycle forward leading to increased proliferation.
16.5: Tumor Suppressor Genes
Tumor suppressor genes normally do what would be expected from their name. Whereas the oncogenes mostly drive the cell cycle forward, the tumor suppressor genes’ primary functions are to temporarily stall the cell cycle so that DNA repair mechanisms can have time to work. However, if repair is unsuccessful after a few attempts, the tumor suppressor gene product may then trigger apoptosis rather than allow a damaged cell to replicate and potentially create another genetically damaged cell.
16.6: Human Cancers
Although some oncogenes and tumor suppressor genes have a restricted distribution that hints at likely tumor locations, many of the genes are widespread and even ubiquitous. It is presently unclear, therefore, why certain types of cancer are linked to particular mutated genes, but there are a number of strongly correlated cases.
16.7: Metastasis
The onset of metastasis signals a drastic change in the prognosis of a cancer patient. While pre-metastatic tumors can certainly be dangerous or painful, treatment can be fairly localized, e.g. surgical excision and directed radiation therapy. Once the tumor metastasizes it must be treated systemically due to the potential for secondary tumors literally anywhere in the body. This presents a problem because the tumor cells are derived from the body’s cells and are mostly indistinguishable.
16.8: The Immune System
At its core, immunology is about adaptation. That is, since an animal has no preconception of the various potential infections it may be subject to, it must have a system in place that is flexible enough to deal with almost anything that comes along. Obviously, the systems are not perfect, but considering the wide range of pathogens, immune systems are remarkably efficient. In humans, there are two types of immune response to infection: the innate response and the adaptive (acquired) response.
16.9: DNA Rearrangement
One of the central assumptions throughout our study of the cell has been that although the RNA and proteins in any cell may differ, any cell of a given organism other than the gametes should have the same DNA. This is not the case with B cells or T cells. In these cells, part of the maturation process is to create a unique arrangement of different domains to form a specific antibody (or T-cell receptor).

Thumbnail: Ebola virus. (Public Domain; CDC).