2 Rounds of PCR

[Henrik Wengholt]

Artifact after 2 rounds of PCR

When the product obtained after one round (30 to 40 cycles) of polymerase 
chain reaction (PCR) is subjected to a second round of PCR with the same
primers the result is often a smear rather than the expected product. The
smear as well as nonspecific bands are seen with most primer sets after the 
second round of PCR even without added template (this is probably not due to
contamination). What is the cause of the phenomenon and how can it be
avoided ?
P.s. In my case it is not possible to use internal (to the first primer set)
primers for the second round of PCR. 

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Henrik Wengholt                      email:      hww at hrl.dk
Hagedorn Research Laboratory            ph:      +45-31-68-08-60