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  #1  
Old 01-07-2010
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DNA DNA-dependent DNA polymerase

What exactly does these sentence from First Aid mean?

(Under HBV)
"Cellular RNA polyerase transcribes RNA from DNA template. Reverse transcriptase transcribes DNA genome from RNA intermediate. However, the virion enzyme is a DNA-dependent DNA polymerase."

Okay, the first sentence (correct me if I am wrong) means that our cell's RNA polymerase makes RNA from HBV's DNA. The 2nd sentence means that HBV has reverse transcriptase that makes back its DNA from the RNA that our RNA polymerase has just made?

What would the 3rd sentence mean? Has it got any relation with the fact that HBV can incorprate its DNA into our cells?
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  #2  
Old 01-07-2010
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Yes exactly,
All what you said makes sense and is correct
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  #3  
Old 01-08-2010
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That statement is referring to how HBV makes it's DNA copies inside the host cell.

Since HBV is a DNA virus, it uses DNA-Dep-DNA polymerase. All this means is this:

1. HBV starts with a parental Genome that is dsDNA
2. It then forms an intermediate that is ssRNA
3. Lastly it ends up with a dsDNA progeny

So all that sentence is saying is that although there is a RNA intermediate, overall the HBV polyermase enzyme is DNA dep.

Hope that helped .
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  #4  
Old 01-08-2010
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So does that mean DNA-dependent DNA polymerase is actually another additional enzyme (which means HBV got 2 enzyme which is reverse transcriptase and this DNA-dependent DNA polymerase),

or does that mean our liver cell's RNA polymerase and HBV's reverse transcriptase are collectively called "DNA-dependent DNA polymerase, because RNA polymerase change DNA to RNA, and reverse transcriptase change RNA back to DNA?
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Old 01-08-2010
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the statement "DNA dependent DNA polymerase" is wrong. There's no such thing in HBV. The only virion associated enzyme is reverse transcriptase which makes DNA from the RNA intermediate. However, it's called reverse transcriptase in retroviruses while in HBV they call it DNA polymerase.

So as housewannabe told you and you have figured out;
First you have double stranded DNA viral genome this will enter the nucleus after uncoating and then after repair it transforms into closed circular dsDNA and then the host's RNA polymerase II makes mRNA from it. This mRNA is transcribed in the nucleus making viral capsid proteins (which go out to the cytoplasm) then the mRNA itself goes out to the cytoplasm where "the virion associated DNA polymerase" will make from it the progeny viral DNA.

HBV does not integrate it's genome into the host DNA, it stays episomal.

I include the Wikipedia image here for clarification

DNA-dependent DNA polymerase-hbv_replication.jpg
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  #6  
Old 01-08-2010
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Ahh, I see! Thanks Sabio for the explanation

But one question though,
Quote:
Originally Posted by Sabio View Post

HBV does not integrate it's genome into the host DNA, it stays episomal.

Attachment 407
I always thought the mechanism by which HBV can cause Hepatocellular carcinoma is by integrating its DNA into hepatocyte's DNA. Am I wrong or have I misunderstood you sentence?
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  #7  
Old 01-08-2010
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Show Teeth

1)HBV ---> high incidence rate of oncogenicity
by integration of its DNA genom to host cell ( and not host DNA )-->
* transcription then translation into proteins with oncogenic effects : e.g X protein
* mutaional inactivation of tumor suppressor genes
2)HCV ---> less incidence of oncogenicity and must pass with cirrhosis then HCC but HBV can induce HCC without cirrhosis
The chronic repeated cell injury ---> high rate of proliferation with more liability to mutaion ---> incotrolable proliferation which in normal cells contolled by appotosis but these new cells are immoratal
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Old 01-08-2010
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Thank you very much tomymajor for the great explanation
I'd like to bring the attention of the reader to this thread
Oncogenic Viruses
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Old 01-08-2010
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Smile

I also found that mechanism of HCC in HBV either
* integration of Its DNA into host cell genom
or * integration into the host cell without inegration into the genom

Netresult is + of oncogenic pathway and - of suppressor pathway
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Old 02-18-2012
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Default sorry but you guys were wrong

HBV carries its own DNA-dependent DNA polymerase. it's different from reverse transcriptase, AKA RNA-dependent DNA polymerase

becacuse HBV virus is partially double stranded circular DNA... that means partially single strand part of DNA must be elongated to be complete circular double stranded DNA...

In this process, they use their own DNA-dependent DNA polymerase. They carry that enzyme inside the virus, and not make it inside the human cell. This is the reason why purified HBV virion itself is not infectious.. they need DNA-dependent DNA polymerase, which is already carried by the virus.

After completing making a double strand DNA, they use cellular(human's) RNA polymerase to make mRNA from complete dsDNA.

They also make reverse transcriptase from this mRNA. and use RT to make viral dsDNA from mRNA they made.

newly made dsDNA is enclosed in the envelope and exit the cell

To sum up...

DNA-dependent DNA polymerase: carried as enzyme itself by HBV virus to elongate its incomplete dsDNA to make complete dsDNA after entry to the human cell.

Reverse transcriptase: carried as gene inside the HBV gemone, translated from HBV mRNA which is made from HBV genome by human's RNA polymerase.
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  #11  
Old 02-18-2012
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Default continue..

to help understanding.....

partial dsDNA -------> complete dsDNA ------> mRNA --------> dsDNA
1 2 3

1. by DNA-dep DNA polymerase(HBV's) : HBV carries this enzyme.. so the purified genome of HBV itself would not be infectious (they need this enzyme in the viral particle)


2. by DNA-dep RNA polymerase(host's) : host enzyme is needed, located in the nucleus.

3. by reverse transcriptase(= RNA-dep DNA polymerase) : translated from HBV's mRNA which is made from HBV genome by host's RNA polymerase(#2)
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  #12  
Old 02-25-2012
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Default HBV DNA Polymerase has dual activity

@barota
You are mostly right and have explained it well.

Just want to add,
The HBV DNA polymerase is a dual-function enzyme that carries out not only DNA-dependent DNA replication(to synthesize missing portion of DNA) but also RNA-dependent DNA replication (reverse transcriptase) activity.
So it is the same enzyme!

References:
http://www.medscape.com/viewarticle/440517_2
and
Review of medical microbiology and immunology By Warren Levinson. Page 294
http://books.google.com/books?id=Q_8...merase&f=false

Last edited by MSC_; 02-25-2012 at 02:23 AM.
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  #13  
Old 02-28-2012
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Default thanks

Quote:
Originally Posted by MSC_ View Post
@barota
You are mostly right and have explained it well.

Just want to add,
The HBV DNA polymerase is a dual-function enzyme that carries out not only DNA-dependent DNA replication(to synthesize missing portion of DNA) but also RNA-dependent DNA replication (reverse transcriptase) activity.
So it is the same enzyme!

References:
http://www.medscape.com/viewarticle/440517_2
and
Review of medical microbiology and immunology By Warren Levinson. Page 294
http://books.google.com/books?id=Q_8...merase&f=false
@MSC
Hi~
thanks for your comment.
Good luck
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  #14  
Old 01-12-2013
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Good point explained ....
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