PCR can only copy small fragments of DNA, this is an obvious downside as although it is quick to produce thousands of copies of one fragment of DNA, for longer fragments i.e the whole gene or more than one it has to be done in stages and the replication is not always 100% accurate. Around 20% of the DNA fragments copied have incorrect base sequences, so not all DNA made via PCR is suitable for use in Gene Therapy.
hope this helps :)
hope this helps :)