Lecture #23—Viruses
So far in the semester we have focused on discussing the principles of evolution. For the rest of the semester we will discuss the results of evolution—the organisms that have evolved on this planet. We start with the simplest entity: viruses. They are different from traditional organisms because they are not composed of cells.
Viruses have various shapes:
The Baltimore Classification puts viruses into 7 families. In this system we see that the virus particle either starts out as single stranded DNA or RNA (ssDNA or ssRNA) or double stranded DNA or RNA (dsDNA or dsRNA). From there, it either forms mRNA directly or via another step. Once viral mRNA is made it directs the host’s ribosomes to start making viral protein.
Let’s see how the process works by looking at the steps involved when a phage virus successfully attacks and kills a bacterial cell. The virus starts as a single strand DNA wrapped in a protein coat. The virus protein coat connects with the host cell membrane’s protein and then pierces it. The DNA is injected inside. Note, it has 2 jobs to do: 1) it must make more virus DNA by using the nucleic acid bases of the host and 2) it must make virus protein to cover the virus DNA. Once this is done, the virus particles cause the bacterial cell to split open and release the virions. This is called lysis.
Lysogenic life cycle: Some viruses do not kill the host cell immediately. Once the DNA is inside the host nucleus it sneaks itself into the host DNA and lies there indefinitely. Each time the host cell divides and makes new DNA in preparation for the division, the virus DNA is replicated also; each new generation of host cell is carrying the hidden viruses. Only much latter does the virus become active and attack the cell making numerous copies.
Four major ways viruses get into host cells:
1. Injection—this is the way that bacterial viruses (phages) do it.
2. Endocytosis—animal viruses like Ebola do this. They have an envelope surrounding them and this fuses with the host cell which then envelopes the virus and draws it inside.
3. Fusion—animal viruses like HIV do this. They have an envelope surrounding the them and this fuses with the host cell membrane and this particle is drawn inside the host cell.
4. Wound entry--Plant viruses enter their hosts via wounds or passed by biting insects
Two major ways viruses escape host cell:
1) Bursting (=lysis)—the host cell splits open and releases virions. Seen with phage viruses.
2) Budding—the host cell pinches off a bit of its cell membrane with a virus particle inside. This is seen primarily with plants and animals hosts.
Examples of viruses:
Ebola is a single stranded RNA virus (-ssRNA) and it enters the cell via endocytosis. The RNA replicates directly into RNA. The RNA acts as mRNA and engages the ribosome to make Ebola protein capsids. Budding occurs to release virions.
HIV (Human Immunodeficiency Virus) is called a retro virus because the single stranded RNA virus first makes a double stranded DNA molecule (dsDNA). This then in turn makes mRNA. It is an envelope virus that enters via fusion and leaves the host via budding. You need to know the details of the life cycle.
Viruses act as vectors to transfer genes from host to host.
1. Transformation—this is where naked DNA in the water gets picked up by the host cell (common in bacteria) and incorporated into their genome.
2. Recombination—this occurs when two or more different viruses invade the same host cell and their DNA or RNA combines in new ways.
3. Transduction—this occurs when a virus picks up the DNA of one host cell (e.g. bacterial species 1) and transfers it to different host species (e.g. bacterial species 2) which combines the new DNA into its own genome.
Where did viruses originally come from? 3 major hypotheses:
Terms/Concepts to Define:
Virus
Virion
Capsid
Capsomere
Baltimore Classification System
Phage
Lytic Life Cycle
Lysogenic Life Cycle
Phage Therapy
Endocytosis
Ebola
HIV
Macrophage
T cell
ssDNA
dsDNA
ssRNA
Retro virus
Transformation
Recombination
Transduction
Can you answer these questions?
1. How does a virus differ from a bacterium?
2. Sketch a picture of a T4 bacteriophage.
3. What is the general principle used by the Baltimore classification system?
4. Describe the life cycle of a bacteriophage in detail.
5. What is the fundamental principle involved in phage therapy?
6. What is the key difference between the lytic and lysogenic life cycle?
7. How do animal viruses enter and leave host cells?
8. Describe the steps of the HIV life cycle?
9. AZT blocks reverse transcriptase enzyme in the HIV life cycle, what effect would this have?
10. AZT does have an effect to reduce the number of circulating HIV virions in the blood of a HIV patient but that effect stops working after a while; the virus has evolved. Think of this from an evolutionary viewpoint; what exactly has happened? It is not enough to say the virus has gained resistance.
11. Patients who have been treated successfully with anti-HIV drugs will have cleared all of the HIV virions from their blood. But if they stop treatment the RNA viruses reappear in their blood. How can this happen?
12. How can viruses affect the evolution of other organisms?
13. Which of the various hypotheses for the origin of viruses seems most credible to you and why?