MoBio DNA Methylation and CpG Island Chapter 7

Origin of CpG (CG) islands

The CG island is a short stretch of DNA in which the frequency of the CG sequence is higher than other regions. It is also called the CpG island, where "p" simply indicates that "C" and "G" are connected by a phosphodiester bond.

CpG islands are often located around the promoters of housekeeping genes (which are essential for general cell functions) or other genes frequently expressed in a cell. At these locations, the CG sequence is not methylated. By contrast, the CG sequences in inactive genes are usually methylated to suppress their expression. The methylated cytosine may be converted to thymine by accidental deamination. Unlike the cytosine to uracil mutation which is efficiently repaired, the cytosine to thymine mutation can be corrected only by the mismatch repair which is very inefficient. Hence, over evolutionary time scales, the methylated CG sequence will be converted to the TG sequence. This explains the deficiency of the CG sequence in inactive genes.

Inheritance of DNA methylation pattern

Any type of cells have their own methylation pattern so that a unique set of proteins may be expressed to perform functions specific for this cell type. Thus, during cell division, the methylation pattern should also pass over to the daughter cell. This is achieved by the enzyme, DNA methyltransferase, which can methylate only the CG sequence paired with methylated CG.

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Figure 7-F-2. Inheritance of the DNA methylation pattern. The DNA methyltransferase can methylate only the CG sequence paired with methylated CG. The CG sequence not paired with methylated CG will not be methylated. Hence, the original pattern can be maintained after DNA replication.