Complex sentence: DNA methylation in vertebrates typically occurs at CpG sites (cytosine-phosphate-guanine sites; that is, where a cytosine is directly followed by a guanine in the DNA sequence); this methylation results in the conversion of the cytosine to 5-methylcytosine.
Split into simpler sentences: DNA methylation in vertebrates typically occurs at CpG sites (cytosine-phosphate-guanine sites, that is, where a cytosine is directly followed by a guanine in the DNA sequence). This methylation results in the conversion of the cytosine to 5-methylcytosine.