EK bio lecture 3, Question 52-viruses

[MShopes]

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Hi everyone- this question look rather simple but I believe that two answers of it are absolutely correct while EK chose one of them.

The question is :
Most viruses that infect animals:
A)enter the cell via endocytosis
B)do not require a receptor protein to recognize the host cell
C)leave their capsid outside the host cell
D)can reproduce independently of the host cell.

I can Eliminiate choice B and D right away. B is wrong because any virus would require a chemical receptor on the cell wall of the host cell to be able to recognize it. I can also eliminate D because viruses always need the reproductive machinary of the host cells to reproduce and copy their own DNA or RNA. Indeed, there are some infectious agents called prions and they are naked protein agents (without the RNA or DNA) that can cause infections in animal cells and they don't require DNA or RNA but they would still need the cell to copy themselves.

Now I'm down to choice A and C. EK chooses A which is absolutely correct as endocytosis is the main mechanism for which viruses are englufed inside the host cell to start replicating. But C should be also correct because capsids stay outside while the nucleic acids go inside the host cells to be replicated and grow to mature virions until the cell lyse or they can be incorporated into the host chromosome (be inactive) and stay there for a while. Why C wouldn't be correct as well?

 

[MT Headed]

Most viruses that infect animals:

...But capsids stay outside while the nucleic acids go inside the host cells to be replicated and grow to mature virions until the cell lyse or they can be incorporated into the host chromosome (be inactive) and stay there for a while. Why C wouldn't be correct as well?

No they don't. You are confusing prokaryote viruses (i.e. bacteriophages) with animal viruses. Go look at that beautiful picture again. It only applies to prokaryotic viruses. Go reread the description of how animal viruses infect their host.

The thing about EK Bio is that they removed all extraneous non-MCAT information (every wonder why their books are so thin?), and every sentence in the lesson is a gold nugget. Read and know them all.

 

[MShopes]

No they don't. You are confusing prokaryote viruses (i.e. bacteriophages) with animal viruses. Go look at that beautiful picture again. It only applies to prokaryotic viruses. Go reread the description of how animal viruses infect their host.

The thing about EK Bio is that they removed all extraneous non-MCAT information (every wonder why their books are so thin?), and every sentence in the lesson is a gold nugget. Read and know them all.

The picture indeed refers to bacteriophages but that doesn't mean that the opposite is true that capsids enter the host cells for animal viruses' case. The EK does not mention any specific mechanism for how animal cells get infected with the virus. EK authors only provided one mechanism of infection which most likely refer to bacteriophages because of the picture. Where does it state that in animal viruses, capsids does not stay outside the cell? I read in my microbiology book that it depends on the virus. The minimum is the viral information (nucleic acids) but some viruses require certain proteins (capsid) to enter the cell as well. So in a summary, capsids sometimes go inside and some times does not in general. But the question said most viruses leave their capsid outside which I kind of agreed about.

BTW, your input is highly appreciated and I'm not saying you are wrong. I'm not sure myself if I'm right but tell me where does it say what you just said?

 

[Arctangent]

Hi everyone- this question look rather simple but I believe that two answers of it are absolutely correct while EK chose one of them.

The question is :
Most viruses that infect animals:
A)enter the cell via endocytosis
B)do not require a receptor protein to recognize the host cell
C)leave their capsid outside the host cell
D)can reproduce independently of the host cell.

I can Eliminiate choice B and D right away. B is wrong because any virus would require a chemical receptor on the cell wall of the host cell to be able to recognize it. I can also eliminate D because viruses always need the reproductive machinary of the host cells to reproduce and copy their own DNA or RNA. Indeed, there are some infectious agents called prions and they are naked protein agents (without the RNA or DNA) that can cause infections in animal cells and they don't require DNA or RNA but they would still need the cell to copy themselves.

Now I'm down to choice A and C. EK chooses A which is absolutely correct as endocytosis is the main mechanism for which viruses are englufed inside the host cell to start replicating. But C should be also correct because capsids stay outside while the nucleic acids go inside the host cells to be replicated and grow to mature virions until the cell lyse or they can be incorporated into the host chromosome (be inactive) and stay there for a while. Why C wouldn't be correct as well?

The way I look at it is that pretend instead you have the fusion between a host's plasma membrane and the same membrane surrounding a protein. As the membranes fuse, the protein is taken into the cytosol. Similarly, when an enveloped virus is taken in, the protein and the internalized nucleic acid enter the host cell, and afterwards the capsid releases the nucleic acid.

 

[MShopes]

The way I look at it is that pretend instead you have the fusion between a host's plasma membrane and the same membrane surrounding a protein. As the membranes fuse, the protein is taken into the cytosol. Similarly, when an enveloped virus is taken in, the protein and the internalized nucleic acid enter the host cell, and afterwards the capsid releases the nucleic acid.

This case is absolutely correct and that what happens when the whole virus including a capsid is inside the host cell. However I was concerned that some viruses just inject their nucleic acids inside the cell without the capsid going inside. What I came to after a long search is that in bacteriophages, the capsid does not go inside with the virus' nucleic acids. In animal viruses, the capsid does indeed go in by the process of endocytosis which involves fusion of membranes like you mentioned. Even EK mentioned that most viruses that infect eukaryotes are engulfed in by endocytosis so I'm assuming they mean the virus with the capsid, otherwise they would have said most virus' nucleic acids are engulfed in only.

 

[MT Headed]

Where does it state that in animal viruses, capsids does not stay outside the cell?

EK Bio top of page Page 54 "Most viruses that infect eukaryotes are engulfed by an endocytotic process."

They talk about the process a little more in EK Audio Osmosis too. The endosome merges with a lysosome, but before the enzymes can dissolve the viral contents, the virus breaks through the lysosome's membrane and into the cytosol.

 

[MShopes]

EK Bio top of page Page 54 "Most viruses that infect eukaryotes are engulfed by an endocytotic process."

They talk about the process a little more in EK Audio Osmosis too. The endosome merges with a lysosome, but before the enzymes can dissolve the viral contents, the virus breaks through the lysosome's membrane and into the cytosol.

Yea I just stated that sentence about the engulfment in my above comment. You are absolutely right. I just always thought it refers to the viral nucleic acids not the virus as a whole. But your point is clear now. So to sum it up, Eukaryotic including animal and plant viruses don't leave their capsids outside while the bacteriophages do leave their capsids outside right?

 

[BilalL]

Well viriods which infect plants only do not have capsids at all.

 

[MShopes]

Well viriods which infect plants only do not have capsids at all.

Yea thats a good point. They are more or less the opposite of prions which have proteins but don't have RNA or DNA...they are called the naked proteins.