DNA methylation in isolated nuclei during development of chick embryos

5-Methylcytosine content of chick embryo DNA was almost constant, independent of the age of embryos. Labelling experiments with isolated nuclei revealed that 5-methylcytosine was only the methylated product and the rate of DNA methylation in nuclei decreased with the age of embryos. The enzyme activity of nuclear extract paralleled the decrease. DNA from the 4 and 10-day embryos were methylated at the same rate by solubilized DNA methylase, indicating that the degree of DNA methylation does not vary with the age of embryos.