Final answer:
Methylation of CpG islands leads to transcriptional repression by causing DNA to coil tightly, reducing gene expression.
Step-by-step explanation:
The modification of cytosine phosphate guanine (CpG) islands associated with transcriptional repression is methylation. DNA methylation involves adding a methyl group to the cytosine base within CpG islands, commonly found in promoter regions of genes. This epigenetic change impacts how DNA interacts with proteins, including histones, leading to tight coiling of the DNA and reduced access for transcription machinery, thereby silencing gene expression.