Final answer:
Vertebrate cells methylate the cytosine nucleotide when it is adjacent to guanine in the CG sequence to reinforce cell identity. This occurs mainly in the promoter regions of genes within CpG islands and can lead to gene silencing.
Step-by-step explanation:
In vertebrate cells, DNA methylation primarily occurs on cytosine nucleotides that are next to guanine in the sequence CG, which is known as a CpG site. DNA methylation is a process by which a methyl group is added to the cytosine nucleotide. This modification often happens in regions called CpG islands, which are areas with a high frequency of CG pairs found in the promoter regions of genes and can lead to the silencing of those genes when methylated.
Therefore, the answer to the question 'To reinforce cell identity, vertebrate cells can methylate which nucleotide?' is A. cytosine that falls next to guanine in the sequence CG.