Why doesn't RNA Pol require a primer?

[Deepa100]

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[physics junkie]

Because it doesn't. Why does DNA pol require one? My guess is design issues...but I couldn't begin to speculate as to why.

 

[Foghorn]

Why doesn't RNA Pol require a primer?

Primers are used for DNA replication. RNA polymerase isn't used for DNA synthesis. If I remember correctly, RNA Pol doesn't have exonuclease activity meaning it doesn't correct nucleotide mistakes whereas DNA Pol has this function. This implies DNA fidelity > RNA fidelity.

Why does DNA pol require one?

Most likely DNA Pol, based on its protein folding properties, needs to recognize a short stretch of ds template so it "knows" where to start DNA replication.

stealth edit

 

[physics junkie]

Primers are used for DNA replication. RNA polymerase isn't used for DNA synthesis. If I remember correctly, RNA Pol doesn't have exonuclease activity meaning it doesn't correct nucleotide mistakes whereas DNA Pol has this function. This implies DNA fidelity > RNA fidelity.

Most likely DNA Pol, based on its protein folding properties, needs to recognize a short stretch of ds template so it "knows" where to start DNA replication.

stealth edit

I don't get it. It needs to recognize a short stretch but that doesn't explain why it can't recognize that short stretch without a primer.

 

[Bacchus]

Its based on the design of the molecule, atleast that is my speculation. DNA Polymerase requires a primer whereas RNA Polymerase doesn't. The difference in the replication is that RNA polymerase recognizes the promoter sequence and DNA polymerase does not. My guess this is what causes a primer to be needed so the DNA polymerase knows where to start.

 

[BloodySurgeon]

I think its because while its unwinding the DNA it can only open it into small segments without messing up the coil and need a primer to make sure you are connecting to the right segments (because it opens and closes so many times), which causes okazaki fragments (which are later ligased). DNA polymerase also has two proteins with two subunits each which one is used to copy the DNA and the other is to proof-read. That is why there are very few mutations in replications, nearly 1 in every million times. RNA polymerase is missing this second attachment and therefore cannot proofread.

(going back to the answer) RNA polymerase doesn't need a primer because it's one continuous unwinding and only need a promoter to signal transcription.

 

[Kaustikos]

I was just going to say that RNA polymerase is made of a f**kton of primers and proteins that add to specificity whereas DNA polymerase doesn't and thus needs the primers.

But your answers are more suitable. Not to mention the conservation of sequence.

 

[mmc48]

The answer has to do with the active site geometry of the RNA polymerase complex. RNA pol binds to a promoter sequence, and initiation is heavily dependent on the presence a TAC sequence on the template strand of DNA. Typically, the first base in an RNA transcript is an A. Thus positioning of the RNA polymerase of the TAC stretch of DNA is favorable to the steric environment of the active site of the RNA pol. A-T basepairing is favored inside the active site for the initial placement of the 1st nucleotide. Thus, the answer lies in the steric constraint inside the active site of the polymerase. This makes sense in comparison to DNA polymerase. RNA pol makes several abortive transcripts initially before moving on successfully to elongation and synthesis of the full RNA transcript. DNA polymerase just has to have the 3' OH group of the previous nucleotide on the RNA primer attack the phosphate group of the incoming nucleotide. Thus, DNA polymerase has a higher processivity.

There, there is more information that you ever wanted to know about RNA polymerase.

I just took a test on all of this junk.

 

[TheGreatHunt]

What kind of Primer would RNA Pol use? an... RNA primer?? heh.

But seriously, it synthesizes RNA, it only makes sense that it carries it's own primer so to speak.