Answer: annealing

AnswerCrossword Clue
annealing(of DNA) Undergo this process
annealingRecombine (DNA) in the double-stranded form following separation by heat
annealingHeat (metal or glass) and allow it to cool slowly, in order to remove internal stresses and toughen it
annealingANNEAL, to toughen (to make tough (strong and resilient))