Final answer:
The modification used to make CpG islands is methylation, an epigenetic process where a methyl group is added to the cytosine bases, often leading to gene silencing.
Step-by-step explanation:
The modification used to make CpG islands is methylation. CpG islands are regions in DNA with a high frequency of cytosine and guanine dinucleotide pairs (CG), often located in the promoter regions of genes. The process of methylation involves adding a methyl group to the cytosine of the CG pair, which can lead to gene silencing or other regulatory effects. This epigenetic modification affects how the DNA interacts with proteins, including histones, which in turn can affect gene expression. Highly methylated DNA regions with deacetylated histones usually result in tightly coiled and transcriptionally inactive DNA.