Viruses

Viruses are not exactly alive. Well, many scientists are having a heated debate on whether they should be considered living or not. However, it is quite clear that they are mysterious, and only appear to be "alive" when they are within their hosts. They can have single or double stranded DNA or RNA. The shell that surrounds them, made of capsomere proteins, is called a capsid. Capsomere proteins typically don't vary too much.
Capsids vary in shape and size between different viruses. 

Some viruses have viral envelopes that are created from the membranes of the virus's previous host cells. They contain phospholipids, membrane proteins and glycoproteins. They aid in infecting host cells. Capsids can also be more complex, but those are typically found in bacteriophages (viruses that only infect bacteria).

Viruses have ranges of organisms that they can infect. Some are highly specialized, and they can only infect one type of tissue in one species. Others can infect species that are completely different from each other like birds and horses.

Viruses also have different kinds of replicative cycles, but for now, let's talk about two different kinds of phage cycles: the lyctic cycle and the lysogenic cycle.

Phages that replicate only by the lyctic cycle are known as virulent phages. Here is a picture of the lyctic cycle, as drawn by me. It's ugly, I know, but I take great pride in it anyway.

In this picture, we can see that the phage latching onto the host and then injecting its DNA. Afterwards, the DNA is used to create parts of the new viruses. These virus components are then assembled into proper viruses. After the cell fills up with these viruses, it lyses, or kind of explodes, releasing thousands of new viruses. 

However, this doesn't always work, as restriction enzymes can recognize the viral DNA as foreign and cut it up. Also, natural selection ends up favoring bacteria that don't respond to these viruses.

The lysogenic cycle, on the other hand, does not kill its hosts. Some phages are capable of reproducing by both cycles. These are called temperate phages. 

On this note, let's talk about the lysogenic cycle next. Here is another ugly picture:

In this cycle, we can see that phage DNA becomes integrated into the bacteria's DNA. This is known as a prophage.

When the bacteria reproduces, the prophage is copied and inherited by the cells created. Afterward, environmental signals turns the cell from lysogenic to lyctic and replication starts. These environmental signals can include radiation or some kind of chemical.

Actually, now that I mention it, I still haven't told you about how RNA viruses (retroviruses) reproduce. These have the most complex cycles. Here's a picture below of a replicative cycle of an enveloped retrovirus. The cycle of an unenveloped virus is relatively similar, so I didn't explain it in his page. However, if you would like to know, email me - my email is at the bottom of this page.

Let's go over everything in depth.

(1) The virus binds to the receptors on the host cell with its glycoproteins.

(2) The virus enters the cell.

(3) Its capsid is taken apart with enzymes, letting out the genome.

(4) The RNA genome allows for creation of a complementary RNA strand.

(5) The RNA strands can function as mRNA.

(6) The mRNA is translated into glycoproteins for the viral envelope. This is done in the golgi apparatus and endoplasmic reticulum.

(7) Vesicles move these glycoproteins to the cell membrane.

(8) The mRNA is also translated into capsid. This is done in the cytosol.

(9) Going back to the RNA genome of the virus, new copies are made using the first strands.

(10) The virus is assembled.

(11) The virus leaves the cell with its new envelope. 

Retroviruses have the most complicated replicative cycles. They even have an enxyme called reverse transcriptase that can transcribe RNA to DNA. Viral RNA turns into DNA with this enzyme.

The viral DNA integrates into chromosomal DNA and becomes a provirus that never leaves the cell. DNA is then turned into RNA again, which is used to rebuild more viruses as template strands and mRNA.

An example of this kind of virus is HIV, which causes AIDS (acquired immunodeficiency syndrome). 

Now that we have discussed this, it is also important to discuss how viruses evolved in the first placed. Scientists think they were created from bits of cells that broke off and moved between cells, likely via injured cell surfaces. Evolving genes for capsid proteins may then have created the switch to uninjured cell surfaces by allowing the virus to bind to receptors.

We think viral genomes could be from:

After all, viruses have more genes in common with their hosts than they do with other viruses - some of these genes being almost identical. The Mimivirus and Pandoravirus support this hypothesis. They are absolutely massive and have as much as 1000-2000 genes. Many of these are for DNA repair, protein folding, translation, and polysaccharide synthesis; they are completely useless to an average virus.

Unforunately, we are still not sure if viruses evolved before the first cells and exploited them or if they evolved after and scavenged genes from them.

What we do know, though, is that viral infections affect us and our food (crops and livestock). And since we are all rather obsessed with eating, you cannot mess with the human race's food supply. Unfortunately, we are under attack by more than just viruses. There are other, less complex things out there as well.

Viroids and prions are also very annoying, but also very fascinating. Viroids cause plant disease while prions cause animal disease. But wait, can we pause and discuss the fact that I just mentioned that plants can get viral infections? 

They actually can! Plants can get sick from horizontal and vertical transmission.

Back to our topic before, let's discuss viroids and prions. Viroids are RNA mlecules that are circular and only a few hundred nucleotides long. They replicate from host enzymes. However, they don't translate into proteins.

Prions are more complicated - they're infectious proteins! They actually cause brain disease in various animal species, and they act very slowly, taking at least 10 years to show effects. They're practically indestestructible, and can't be destroyed even if you heat them to cooking temperatures. Prions are normal proteins that have been misfolded, and when they contact normal proteins, those proteins can take the same shape as the prion, which causes those proteins to group up and eventually cause errors in the cell. In total, this can degrade the entire brain. 

Amazing, isn't it?

Unfortunately, that is all I have for you today. As always, feel free to email me at twisha.sharma30@gmail.com if you have any questions! Soon, I will be planning to move on to other aspects of biology as well, so be prepared for that! Also, all pictures on this site were drawn by myself, but I copied them from the Campbell Biology textbook cited on the Welcome page. Please check it out! Thank you so much for reading and have an amazing day!