Cloning in bacteria using cloning to generate quantities of a specific. Resource that you can return to repeatedly. Low quantity of re will partially digest dna. Probe for any gene of interest good library a cdna libraries are created from mrna. Mrna must be reverse transcribed to cdna (complementary) Dttt primer allows for total mrna to be reverse transcribed cdna libraries cdna inserted in plasmids. Like total dna total cdna inserted into plasmids cdna lacks introns. Record of the complete mrna complement of a cell in cdna form. Can compare expression of patterns between cell types. Dna is the same in every cell, but mrna will differ by cell type. Essentially replication of a portion of a dna molecule. Producing dna by cloning is time intensive (multiple steps, bacteria) Polymerase chain reaction is much quicker (in vitro) Take small amount of dna (specifically amplify dna of interest) These 3 steps take place at a different temperature.