Final answer:
RecBCD changes its activity upon encountering a recognition site, which is a specific DNA sequence that signals the enzyme to initiate DNA repair processes.
Step-by-step explanation:
When RecBCD encounters a recognition site (5'GCTGGTGG3'), its activity changes. The correct answer is a. Recognition. RecBCD is an enzyme complex in bacteria that has multiple activities, including helicase and nuclease activity. Its function is closely related to the repair of double-stranded breaks in DNA. A recognition site, often a specific DNA sequence, alters the activity of RecBCD as it binds to DNA and initiates its repair processes. After binding to a recognition site, the RecBCD enzyme unwinds the DNA and degrades it until it encounters a chi (crossover hotspot instigator) sequence. This signals RecBCD to cut the DNA, leading to the formation of a 3'-ended single-stranded DNA overhang. This overhang is essential for the process of homologous recombination, an important DNA repair mechanism.