The lytic viral cycle

Welcome to post #2 about viruses! Remember the last one? During that post I told you about how viruses are the underdog of the living world (being that no one knows if they are actually living or not), and are composed soley of a protein coat and an inner genome. Today's question revolves around the cycle of infection the virus undergoes in order to initiate reproduction:

Successful reproduction of a lytic virus requires that all of the following processes occur EXCEPT

A) incorporation of viral DNA into host cell DNA
B) translation of viral mRNA
C) binding of the virus to the host cell's surface
D) penetration of the viral genome into the host cell
E) replication of the viral genome

First off, there are two types of cycles that viruses can undergo to take over a cell: they lytic and the lysogenic cycle. The lysogenic cycle is interesting; during this cycle, the viral DNA is integrated into the host cell's DNA for an indefinite period of time. Basically, the viral DNA just moves in and lives in a new cell until it wants a change of scene. This may be in a day, or it may be in 1000 years...there's no real way to tell from our perspective.

The question we're worried about today involves that other cycle--the lytic cycle. During the lytic cycle, the virus takes over a host cell, utilizes the host cell's ability to make ATP, then bursts the cell open. This doesn't take long at all. The lytic cycle has four major stages: Adsorption, Penetration, Biosynthesis of viral products, and Release.

The viral cell is formed kinda like a hypodermic needle. The virus comes across an appropriate host cell (due to the intimacy of viral reproduction, viral cells are closely matched with their host cells. This is why most animal viruses can't jump from species to species, and when they do it is due to a massive mutation) and attaches to proteins found on the host cell's membrane. This adsorption period takes a bit of time and usually requires a slightly elevated temperature to happen effectively.

Once the virus is attached to the outside of the host cell, it then injects its genome into the host cell. Can you guess what this stage of the cycle is called? Yup. Penetration. The protein coat is left on the outside of the cell while the DNA/RNA of the virus does its dirty work inside.

The viral DNA must then figure out how to take over the cell (it's like an evil mastermind!). So, it follows normal DNA replication protocol--first it unzips, and then it translates messenger RNA to send a memo to the cell saying 'Hey! Replicate me!" Which the cell does, no questions asked. Silly minions!

Once that memo gets sent, the cell stops what it was doing, and begins to synthesize the viral products during the "biosynthesis of viral products" phase. The cell reproduces new, baby viruses until all the ATP and other cell resources are gone, and the cell is just PACKED full of new viruses waiting for the chance to infect a cell of their very own.

After the host cell is tapped out--oh you viruses! It's all wham bam thank you host cell--then the host cell bursts open, releasing all the viralings into the big bad world. Release!

So, four major stages in the lytic cycle. Now, back to our question:

Successful reproduction of a lytic virus requires that all of the following processes occur EXCEPT

A) incorporation of viral DNA into host cell DNA
B) translation of viral mRNA
C) binding of the virus to the host cell's surface
D) penetration of the viral genome into the host cell
E) replication of the viral genome

This question is testing your knowledge of the lytic cycle and its differences with the lysogenic cycle. We just learned the 4 stages of the lytic cycle: Adsorption, penetration, biosynthesis, and release. Looking at the 5 answers to this question, which one isn't included in those 4 stages? Translation of viral mRNA is the first step in biosynthesis; binding of the virus to the host cell's surface is the definition of adsorption; penetration of the viral genome into the host cell actually has the word "penetration" right in the answer; and replication of the viral genome is just another way of saying biosynthesis (it's just that biosynthesis sounds more sciencey, so I teach my students to use that word. Impress your friends and family!) The only answer that is not included in the four stages is "A", incorporation of viral DNA into the host cell DNA. Remember that this is the hallmark of the lysogenic cycle--where the viral DNA is incorporated into the host cell's DNA for an indefinite amount of time. Answer: A!

Viruses and ATP

The past couple of posts have been about human anatomy and physiology (which is what I'm currently teaching at SJSU) so I decided to branch out just a tiny bit in today's post--today I'm going to give a brief introduction to viruses. Here's today's question:

Members of which of the following groups CANNOT produce their own ATP?

A) Lichens
B) Bacteria
C) Viruses
D) Diatoms
E) Protozoa

This question is testing two things: your knowledge of vocabulary and your knowledge of organismal groups. First off, the vocab. ATP is the big work in this question. ATP stands for Adenosine Triphospate, and is the energy source for cells (well, it's quite a bit more complex than that, but I'm not going to go into it here. If you would like a very, very in depth discussion on the chemical basis of ATP and its exact function, do a search on Wikipedia. The ATP article there is fabulous). All cells use ATP to carry out essential functions such as growth, repair, and reproduction. Most organisms produce their own ATP--they have to, or they die. So, which of the above groups doesn't? Let's look at the groups and what they are.

Lichens: Lichens are symbiotic associations of (usually) an algae and a fungus. Without getting into the varieties of lichens, or the controversy on their relationship, the particular algae and fungus cannot live alone. However, once together the lichen is able to live, grow, and reproduce all on its own, and therefore produces its own ATP.

Bacteria: Bacteria are microscopic, single celled (for the most part) organisms. These are considered one of the smallest free-living organisms we know about. Bacteria have the ability to function apart from any other organism, although many thrive when in a symbiotic or parasitic relationship with something else. Bacteria produce their own ATP.

Diatoms: Diatoms are algae with cells walls made of silica (ever heard of diatomaceous earth? Yep, that's these guys). Being algae, these singe celled organisms are able to live freely, and do so in bodies of water. A certain type of diatoms is what is responsible for red tide. Neat! Anyhow, since they are able to live freely, they produce their own ATP.

Protozoa: Protozoa are single celled, eukaryotic (have a membrane bound nucleus) organisms that are, for the most part, motile. They are much larger than bacteria, and differ in many other ways that I won't get into here. Once again, however, single celled organisms capable of moving/growing/reproducing without other organisms, so they must produce their own ATP.

Viruses: Ah, viruses. Viruses are the bane of many a biologist. There have been whole summits on if a virus is alive or not, and the latest answer to come from the top minds in the field is "um...dunno." Viruses simple beasts--they consists solely of a protein coat and an inner genome (either DNA or RNA, but not both) and are unable to carry out basic processes such as growth or reproduction without the assistance of another cell. This is where the controversy comes in--if they are unable to grow and reproduce on their own, are they really alive? Well, that's neither here nor there for the moment. What we're worried about is how viruses work. Viruses must hijack another cell and take over its ATP producing capabilities in order to do anything. It does this by injecting its genome into the host cell's genome, and telling the host cell what to do from there. The host cell is sometimes destroyed during this process, and the virus goes on to infect another host.

So, the answer we're looking for is "C" virus. Viruses must hijack another cell for basic functions, and therfore don't produce their own ATP.