To start things off, chapter 18 incorporates information about not only viral but also bacterial genetics. A virus is a small infectious particle that consists of nucleic acid enclosed in a protein coat. There are over 4,000 different types of viruses out there! Viruses do share some of the same qualities with one another however they vary greatly in their characteristics. These characteristics include in their host range, the structure, and the genome composition.
What is host range you may ask? Well before you can define what a host range is, you must identify the meaning of a host first. A host is a cell that is infected by a virus, fungus, or a bacterium. It can also be classified as a eukaryotic cell that contains photosynthetic or nonphotosynthetic endosymbionts. A host range on the other hand is the number of species and cell types that a virus or a bacterium can affect.
Another trait that varies is the structural differences. For starters, viruses are usually smaller than the wavelength of visible light. And that is EXTREMELY tiny, and they can range in size from about 20–400 nm in diameter. Before we get to the structural differences of the viruses, it is crucial that we understand the similarities in their structure first. All viruses have a protein coat known as a capsid. The capsid encloses a genome consisting of one or more molecules of nucleic acid. These capsids have a variety of shapes! These shapes include helical and polyhedral.
Another part of the virus is the viral envelope. The viral envelope encloses a viral capsid that consists of a membrane derived from the plasma membrane of the host cell. It is embedded usually with virally encoded spike glycoproteins. However some of the differences in the viruses are their structure like we said before. All viruses contain nucleic acid surrounded by a protein capsid. However, not all viruses have an outer envelope surrounding the capsid.
Both the capsid AND the envelope enable viruses to infect their host (in the beginning I was confused about this topic). How do they do this you may ask? Well they have specialized proteins that help them bind to the surface of the host cell. An important term to become familiar with is a bacteriophage, or also known as a phage. A phage is a virus that happens to infect bacteria. These phages may also vary in structure from another, depending on how complex their protein coats are.
The genetic material in a virus is called a viral genome. The composition itself as well as the size overall varies among different types of viruses. For example, the nucleic acid of some viruses is DNA, whereas in others it is RNA. The genome can either be linear or circular as well, however this all really depends on the type of virus.
When a virus infects a host cell it essentially goes through a series of steps that result in the production of new viruses during a viral infection. These series of steps are more commonly known as the viral reproductive cycle. This process can be completed in fewer than 5 to 6 steps.
Attachment- the virus most of course attach to the surface of the host cell. The attachment process is usually specific to one or a couple types of cells due to the fact that the proteins in the virus bind to only specific molecules on the cell surface.
Entry- the viral genome in this stage enters the cell. Attachment of phage λ stimulates a conformational change in the phage coat proteins. Because of that action, the shaft (also called the sheath) contracts. The change then injects its DNA into the bacterial cytoplasm. When the viral genome enters the cell, one or several genes are expressed immediately. This is a result of the actions from the host cell enzymes and ribosomes.
Integration- viruses that are capable of integration carry a gene that encodes for a certain enzyme. This enzyme is known as integrase. In order for integration to essentially occur, the gene must be expressed soon after entry so that the integrase protein can be made. Integrase then can essentially cut the host’s chromosomal DNA and insert the viral genome into the chromosome. Once this has been integrated, the phage DNA in the bacteria can now be called a prophage. When this is in the prophage stage still, it can undergo the lysogenic cycle. Then, it undergoes reverse transcriptase, which in turn is a viral enzyme that catalyzes the synthesis of viral DNA starting with viral RNA as a template. The viral double-stranded DNA enters the host cell nucleus and is inserted into a host chromosome via integrase. When they are integrated, the viral DNA in a eukaryotic cell is known as a provirus. Retroviruses are viruses that follow this mechanism.
Synthesis of Viral Components- this is the step in which synthesis occurs. An enzyme called excisionase is required for this process. When the host cell ribosomes translate the viral mRNA into viral proteins. This expression of the phage then eventually leads to the degradation of the host cells.
Viral Assembly- once all of the necessary components have been synthesized, they have to be assorted and assembled into new viruses in this stage. They use the help of phages and certain capsid proteins to do the job if they cannot self –assemble on their own.
Release- bacteria are surrounded by a rigid wall. The phage must burst or lyse in this stage in order for it to escape. The lytic cycle occurs.
During the past few decades many different viruses have risen to the people’s attention. One of these viruses includes AIDS. AIDS, or also known as acquired immune deficiency syndrome is a disease that is caused by the human immunodeficiency virus (HIV). This leads to a defect in the immune system or infected individuals. The amount of deaths caused by this disease is devastating. This disease is the result of a viral destruction of the helper T cells. Helper T cells are a type of white blood cell that plays an essential role in the immune system of mammals. When they are destroyed by HIV, the function of the immune system is compromised.
There are many differences and similarities between the lytic viral reproductive cycle and the lysogenic viral reproductive cycle. The lytic cycle is the growth cycle of a bacteriophage in which the production and release of new viruses lyses the host cell. During the lytic cycle, the phages are made and the bacterial cell is destroyed. However, in the lysogenic stage, the integrated phage DNA is replicated along with the DNA of the host cell. Environmental conditions can influence how long the phage remains in the lysogenic cycle.
Viruses can contribute DNA to their hosts. They do this by integrating their DNA into the host. When the host cell replicates, the DNA that can be contributed by the virus, also gets replicated. This can change the composition of the cell and affect the species in turn.
The HIV virus can compromise the immune system. When there is a defect in the immune system, there is much destruction of the helper T cells (as described above). When this occurs, the immune system is compromised and the body can then get infected with deadly diseases that normally, they would not get infected with. Potential sources of drug activity against HIV are being researched to specifically bind to proteins encoded by the viral genome. Or there will be research that will inhibit protease to help fight as well. The only downfall of many of the medicines and remedies used on humans today is that we essentially and eventually become immune to them over time. Once this occurs, these medicines will no longer work for us.
Moving onto Section 3, this section focuses on the genetic properties of bacteria. A bacteria chromosome consists of a nucleoid region. This is a site in a bacterial cell where the genetic material (DNA) is located. This region is in direct contact with the cytoplasm of the cell. Another key feature is that in most but not ass bacterial species contain circular chromosomal DNA. Another is that a typical chromosome is a few million base pairs in length. Following that, most bacterial species contain a SINGLE type of chromosome. However this chromosome may be present in multiple copies. Several thousand different genes are interspersed throughout the chromosome. One origin of replication is required to initiate DNA replication.
Plasmids are a small circular piece of DNA found naturally in many strains of bacteria and occasionally in eukaryotic cells; can be used as a vector in cloning experiments. The function of the bacteria phages usually provide some type of growth advantage to the cell or may aid in the survival only under certain conditions.
Moving onto the last section of the chapter, we discuss the gene transfer between bacteria. There are various methods of genetic transfer in bacteria. These include conjugation, transformation, and transduction. In conjugation, this requires direct contact between a donor and the recipient cell. The donor cell transfers a strand of DNA to the recipient.
The second method is transformation. In this method, a fragment of its DNA from a donor cell is released into the environment. This usually occurs when a bacterial cell dies. This DNA fragment is taken up by a recipient cell. In return it incorporates the DNA into its chromosome. The last type is transduction. This occurs when a virus infects a donor cell; it causes the bacterial chromosome of the donor cell to break up into fragments. A fragment of bacterial chromosomal DNA is incorporated into a newly made virus particle. The virus then transfers this fragment of DNA to a recipient cell.
Gene transfer contributes to the spread of antibiotic resistance greatly. This occurs when a cell has become almost immune to the medication. When the cell replicates, its genes that are immune to the medication get replicated alongside it. When this occurs, the offspring cells carry this genetic trait and continuously spread it whenever they go through cell division.
B. Useful Materials
This video is extremely informative and descriptive. It shows the HIV life cycle through animation. I felt that this video helped me understand the structure and the deadliness of HIV better. The video is relevant to the chapter because we discussed HIV and it's deadliness. To further your understanding and education on this disease I highly recommend watching this video.
This video shows the viral life cycle. It is informative and uses animations to go through the steps. The narrators voice has a slight accent in it but it is still a helpful tool to this chapter. I felt that this video was necessary because it is short and sweet and gets the point of the cycle across.
This article is about a new cure that scientists are discovering for AIDS. In this method, scientists used an isotope dilution method in combination with matrix- assisted laser and validates the determination of concentrations of the antiretroviral drug tenofovir. There are many advantages to this new method. One of the main advantages are that the method is extremely fast. This method has to be researched more, before it is admitted to anymore test subjects because scientists do not yet know of the harmful effects.
Chapter 18 Blog: Genetics of Viruses and Bacteria (NL)
Comments (1)
Derek Weber said
at 2:14 am on Apr 2, 2011
Nice.
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