Any DNA or mods that are not used in your genome slots when you reconstitute will be removed. Eventually we'll have a use for all of the extra pieces you have but for now they are just deleted! Any DNA in one of your slots will stay permanently until it's mutated, after which its lifetime is displayed by the number at the bottom right. Once that number reaches zero the mutation and the DNA it mutated from will be removed letting you apply a new piece of DNA.