Madanus did a good job explaning , but I ' ll give it a go as well . In the transgene lab lab 2 , you are looking for a large piece of novel DNA . So , all you need are primers specific to this piece of DNA . If you get a product , it ' s there . If you don ' t , it isn ' t . Pretty simple . In the 1 st lab , they could have been doing the restriction digest just to give you experience with digests . However , there is a specific test where you cut a PCR product . That test is when you ' re looking for a single base pair change , so there is only one nucleotide difference in the possible outcomes . PCR can ' t detect a single bp change by the absenceexistance or size of the product , so you need to add the restriction enzyme . However , it ' s a bit tricky , as you need to A know the two possible single nuclotide polymorphisms SNPs you are looking for AND B one of those two possibilities needs to either create or destroy a restriction site . If both of those criteria are met , then this is a quick way to test for which SNP your sample has .