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The methylation state of an individual CpG island:
is fixed; such genes are permanently turned off.is fixed, but this has no effect on whether genes are expressed.can change over time in response to environmental cues, but this has no effect on gene expression.can change over time in response to environmental cues, allowing genes to be turned on or off as needed.is random sometimes the cytosines are methylated and sometimes they're not, but the state is independent of the environment or cell type.

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Answer:

can change over time in response to environmental cues, allowing genes to be turned on or off as needed

Explanation:

DNA methylation is a process of gene expression regulation. It involves the methylation (addition of methyl groups) of the DNA which usually repress gene transcription.

CpG islands are DNA sequences rich in C and G nucleotides, in the form of repeating dinucleotides. They are often found within the promotor of the gene. CpG islands (more preciesly citosine in CpG) can be methylated by DNA methyltransferases. That methylation can change te gene expression and this type of regulation is called epigenetics.

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