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!

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