Final answer:
Cytosine methylation in mammalian cells occurs in CpG dinucleotides and is often found in areas where CpG sites are clustered together, forming CpG islands. The sequence that would most likely NOT have its cytosine methylated is GCTT.
Step-by-step explanation:
Cytosine methylation is a common epigenetic modification in DNA, and it is typically catalyzed by mammalian methyltransferases. Methylation of cytosine occurs in CpG dinucleotides (where a cytosine is followed by a guanine), but it does not occur in all CpG sites. In mammalian cells, cytosine methylation is often found in areas where CpG sites are clustered together, forming CpG islands. These CpG islands are typically located near gene promoter regions.
Based on this information, the sequence that would most likely NOT have its cytosine methylated is option B, GCTT. This is because GCTT does not contain a CpG dinucleotide (C followed by a G). The other sequences (GCGG and TCGT) contain CpG dinucleotides and are more likely to have their cytosine methylated.