Retrovirus genome replication

[ponyo]

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TPR MCAT Biology Chapter 4 Freestanding Practice Q #5, p 129

Which of the following statements concerning viruses is true?

(D) A virus with an RNA genome must code for an RNA-dependent RNA polymerase.

My question: how do retroviruses replicate their genomes? I thought that they replicated from the DNA that they encoded using RT, but this seems to suggest that they only use the DNA intermediate for making actual gene products and that the replication of the genome is conducted by RNA replicase.

Thanks in advance.

 

[whiteshadodw]

TPR MCAT Biology Chapter 4 Freestanding Practice Q #5, p 129



My question: how do retroviruses replicate their genomes? I thought that they replicated from the DNA that they encoded using RT, but this seems to suggest that they only use the DNA intermediate for making actual gene products and that the replication of the genome is conducted by RNA replicase.

Thanks in advance.

A virus doesn't need to convert its genome into DNA. think about it. RNA is sufficient to make new proteins (all proteins necessary for reproduction/replication), and if all the genome fits into RNA, then why do you need to make DNA? it just makes things longer, and decreases proliferation. if you want to proliferate super fast, you don't want to spend extra time converting to DNA if you don't have to. thus, some viruses only use RNA, and never convert any of their genome to DNA.

 

[MD Odyssey]

A virus doesn't need to convert its genome into DNA. think about it. RNA is sufficient to make new proteins (all proteins necessary for reproduction/replication), and if all the genome fits into RNA, then why do you need to make DNA? it just makes things longer, and decreases proliferation. if you want to proliferate super fast, you don't want to spend extra time converting to DNA if you don't have to. thus, some viruses only use RNA, and never convert any of their genome to DNA.

The lifetime of RNA in a cell isn't that long, so retroviruses do need to replicate their DNA. There are a lot of exceptions in the case of viruses - what can happen usually does happen - but in general, this is what happens. The RNA that enters the cell may encode a gene for a reverse transcriptase enzyme or the virus may actually bring the enzyme along with it. The reverse transcriptase enzyme then works backwards to produce a DNA strand which then gets integrated into the host genome by another enzyme called an integrase, which is also coded for by the viral RNA. Once the viral DNA is inserted into the host genome, it then gets transcribed and translated just like any other bit of DNA. So, the central dogma becomes:

RNA -> DNA -> RNA -> proteins

This whole bit about reverse transcription of an RNA is partially why retroviruses are notoriously prone to mutation. There are a bunch of proofreading enzymes involved in DNA replication which reverse transcription enzymes don't have, so the mutation rates for that process are higher. HIV is a good example of this whole process.

Edit: Meant to refer to retroviruses in the last paragraph, instead of RNA viruses
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[whiteshadodw]

The lifetime of RNA in a cell isn't that long, so retroviruses do need to replicate their DNA. There are a lot of exceptions in the case of viruses - what can happen usually does happen - but in general, this is what happens. The RNA that enters the cell may encode a gene for a reverse transcriptase enzyme or the virus may actually bring the enzyme along with it. The reverse transcriptase enzyme then works backwards to produce a DNA strand which then gets integrated into the host genome by another enzyme called an integrase, which is also coded for by the viral RNA. Once the viral DNA is inserted into the host genome, it then gets transcribed and translated just like any other bit of DNA. So, the central dogma becomes:

RNA -> DNA -> RNA -> proteins

This whole bit about reverse transcription of an RNA is partially why RNA viruses are notoriously prone to mutation. There are a bunch of proofreading enzymes involved in DNA replication which reverse transcription enzymes don't have, so the mutation rates for that process are higher. HIV is a good example of this whole process.
​

I think you're over generalizing. there are + and - strand RNA viruses, + strand are ready to be translated, minus strand need to be transcribed into + strand RNA to be translated.

+ strand RNA --> proteins --> more RNA or more particles needed for replication

the bottom line is, if a virus wants to enter lysogeny it needs proteins/enzymes to do this. it doesn't have to enter lysogeny, and thus it doesn't even need DNA to replicate.

the central dogma doesn't fully apply to viruses. there are viruses which have + sense RNA that codes for things like extra RNA replicases to replicate the RNA genome and then you can just get capsids and things like that from the + sense RNA. remember, virsuses replicate, have a genome, are made out of organic matter, but some people don't classify them as living. you can't really group viruses in the same group as eukaryotes or prokaryotes since they are neither.

also, its perfectly plausible that RNA viruses are remnants of an RNA world in which there was NO dna. but that's another topic.

 

[MD Odyssey]

I think you're over generalizing. there are + and - strand RNA viruses, + strand are ready to be translated, minus strand need to be transcribed into + strand RNA to be translated.

As I said, there are lots of exceptions, so some generalization is probably useful. Also, note that the OP was asking specifically about retroviruses.

+ strand RNA --> proteins --> more RNA or more particles needed for replication

the bottom line is, if a virus wants to enter lysogeny it needs proteins/enzymes to do this. it doesn't have to enter lysogeny, and thus it doesn't even need DNA to replicate.

All probably beyond the scope of the MCAT and again, not sure that's really what the OP was asking.

the central dogma doesn't fully apply to viruses.

I wasn't arguing that it did - I was simply pointing out that, in the case of retroviruses, the flow of genetic information follows an altered central dogma.

there are viruses which have + sense RNA that codes for things like extra RNA replicases to replicate the RNA genome and then you can just get capsids and things like that from the + sense RNA. remember, virsuses replicate, have a genome, are made out of organic matter, but some people don't classify them as living. you can't really group viruses in the same group as eukaryotes or prokaryotes since they are neither.

I don't really want to get into a debate about whether viruses are living or non-living, but I should point out that viruses don't replicate. They get replicated by a host cell, which is a big difference. They don't have any of the necessary machinery to replicate their genetic material and they don't have any of the machinery necessary to fabricate their own proteins.

My intent wasn't to say that you were wrong - just thought you weren't giving a complete or pertinent answer to the OP's question.

 

[BilalL]

I personally make sure to separate retroviruses like HIV from + strand RNA viruses like polio. NOTE: They both have + RNA strands

The + strand Polio Virus needs to pack an RNA dependent RNA pol so that it can make - strand RNA( for use in replication to make more + strands).

The infected cell only has a DNA dependent RNA Pol which makes mRNA from DNA. If the + strand virus doesn't have its own RNA dependent pol than it won't be able to make - strand RNA (the template to make the new RNA for the new viruses).

Now this changes with a retro virus which can make DNA from RNA and thus only needs the host cell RNA polymerase which can use the the new integrated virus DNA in the host cell DNA to make mRNA.