Final answer:
The longest stretch of the DNA sequence likely to adopt the Z-conformation is CGCGCGTGCGCGCGCG, a region rich in alternating cytosine and guanine bases.
Step-by-step explanation:
The longest part of the sequence that is likely to take up the Z-DNA conformation is the repeated CG sequence: CGCGCGTGCGCGCGCG.
When we consider nucleic acid structure prediction based on sequence, it's important to remember that different DNA sequences have propensities to form different secondary structures. Z-DNA formation is often associated with alternating purine and pyrimidine bases, particularly in regions where cytosine (C) follows guanine (G). In the provided sequence, the longest stretch that follows this pattern is the one mentioned above.
This stretch stands out because it contains alternating C and G bases, which are known to form more stable GC pairs, which promotes the formation of Z-DNA under certain conditions. Z-DNA is a left-handed helical form of DNA, which differs from the more common B-DNA form that is right-handed.