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I have went through Lehinger ( biochemistry book) and cooper ( cell Biology) but i didn't find my answer. plese help me

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    $\begingroup$ Your question isn't very clear; could you please elaborate? $\endgroup$ – Astrolamb Aug 6 '18 at 19:37
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The (very) basic answer to your question about in-vitro vs. in-vivo primer usage is:

  1. In-vivo - Although either (DNA or RNA primers) can be used for in-vivo primer extension biochemistry, DNA primers are not available (not made) in living cells. Therefore all natural in-vivo primer chemistries utilize RNA primers.

  2. In-vitro - DNA primers are easily synthesized, inexpensive, functional, and substantially more stable (less likely to be degraded). DNA primers are therefore primarily used for most in-vitro purposes (i.e. PCR and other in-vitro primer extension applications).

Beyond that, all other reasons (complex evolutionary reasons vs. human choices in-vitro); involving subtle differences between RNA and DNA chemistry, structure, and function, similarly can play a role. The links below may be helpful to address some of these differences.

https://en.wikibooks.org/wiki/Structural_Biochemistry/Nucleic_Acid/Difference_between_DNA_and_RNA

https://en.wikipedia.org/wiki/Primer_(molecular_biology)

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Generally speaking, initiation of DNA replication in vivo at a defined origin of replication, requires an enzyme with DNA primase activity. DNA primase is a DNA-dependent RNA polymerase that generates short RNA primers that are used to prime DNA synthesis by the DNA polymerase holoenzyme at a so-called replication fork.

The classical polymerase chain reaction (PCR), on the other hand, is a way for humans to use an in vitro method to amplify relatively large amounts of double-stranded DNA from a specific fragment, using a DNA template target and two artificial, single-stranded DNA oligonucleotide primers (along with dNTP substrates, buffer, Mg++, a thermostable DNA polymerase, and a programmable thermocycler).

So the goals, or outcomes, of the two processes that you are trying to compare seem vastly different to me. Replicating a chromosome is presumably to keep an organism alive, and reproducing, and dividing, to produce progeny. Amplifying a desired DNA fragment is a tool to allow some subsequent process or assay to occur.

Your question does not have a clear sense (to me). There is no stated requirement for biochemical reactions created by human scientists to in any way be limited, or constrained by reactions that occur inside a living cell.

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