On a scale of one to ten, how done are you with this semester?
Dear Diary, at least we started viruses this week!
I think we can all name some viruses (HIV, Herpes, Influenza) but what IS a virus exactly?
The original definition was something that was small (smaller than bacteria) and could only grow on other living tissue. This definition arose from how viruses were first isolated. Charles Chamberland created a filter with pore sizes so small that bacteria would be filtered out of a medium. Dmitry Ivanovsky and Martinus Beijerinck showed that if you took material from sick organisms (like plants) and put it through the filter, the filtered liquid was still infectious and caused disease to healthy plants. Since the bacteria was “filtered out” the disease must be caused by something smaller than a bacterium. After that, it was thought that all viruses were smaller than bacteria…. what no one knew until recently, was that there are large viruses too! They were getting filtered out along with the bacteria and we never characterized them!
So, the modern definition of a virus: An infectious agent that can only replicate inside a living cell… an OBLIGATE INTRACELLULAR PARASITE! Sometimes there is also a statement about it being a genome and a capsid…. although now viruses have been discovered that lack capsids… so that complicates things too.
The capsid of viruses can come in two distinct shapes…
Icosahedral: a soccer ball shape with the genome in the middle
Helical: a long tube like shape, with the genome in the middle
Everything else which is not shaped like a tube or a soccer ball, is called “complex”
The icosahderal shape has 5 fold, 3 fold and 2 fold axis of symmetry and the proteins (capsomers) are arranged in groups of five (pentamer) and groups of six (hexamers). Can you find them all?
Some viruses have an additional lipid bilayer ENVELOPE outside their capsids. In a surprise move, having an envelope makes a virus LESS STABLE outside the human body! How can having more layers make you less stable?
Hint: the answer to the question above might have something to do with RECEPTORS. These are proteins on the outer most surface of the virus that attach it to host cells, and determine WHICH host cell and WHAT species the virus will infect.
Viruses are classified by the Baltimore classification system, and sometimes the host classification system is added into that, just to make it more colorful.
See the pretty colors?
There have been some pretty nasty viral pandemics throughout human history. The Smallpox and the 1918 Influenza pandemics were discussed. Do you think that influenza will ever get eradicated like smallpox did?
Viruses replicate (make new viruses) by using the lytic (AKA productive) pathway. This includes: Attachment, Entry, Uncoating, Gene Expression, Genome Replication, Assembly and Egress. Spoiler alert! Many students put Genome Replication ahead of Gene Expression. Why couldn’t genome replication happen BEFORE gene expression?
There are two dormancy cycles, in which no new viruses are made: Lysogeny (dormancy with integration) and Latent (dormancy without integration).
Lysogeny happens a lot in bacteriophages (which are FREAKING AWESOME!!) and less often in eukaryotes. Although, when it does happen, its pretty powerful (HIV).
Here are some bacteriophages (can’t see them, too small) killing bacteria by lysis. I wish you could hear the bacteria screaming.
Latency is another style of dormancy, and is marked by recurring lytic replication cycles (and the disease that goes with it) separated by dormancy periods. Herpesviruses are a great example of this.
What to focus on this week:
- Describe all of the important structural parts of a virus and their functions
- Be able to identify the 5-fold, 3-fold and 2-fold axis of symmetry as well as the pentamers and hexamers
- Understand why an envelope is useful, and also makes the virus less stable
- Be able to list and describe all of the steps of the lytic replication cycle. Seriously, I cannot stress this enough.
- Describe the difference between a plus strand and a minus strand
- Describe why viruses might go into a dormancy period and why lysogeny differs from latency
- Describe how latency and lysogeny differ from the lytic cycle
- List the reasons why Smallpox was a good candidate for eradication
Questions or comments? Leave ’em below!