Gene amplification in mammary gland at differentiation

Mouse mammary gland DNA was analyzed by isopycnic density gradient centrifugation in CsCl. The buoyant density of the pre-existing and newly made DNA of the mammary glands of virgin, pregnant and weaned mice showed the characteristic major and minor-(satellite) components of mouse DNA. However, most of the radioactive DNA of early lactating gland equilibriated as slightly heavier (0.004 g/cm 3) than the major band of mouse DNA and this heavily labeled peak in 6 day lactating gland was associated with a shoulder on the optical density curve. The results indicate a periodic amplification of the mammary cell genome at differentiation. Similarity of the buoyant density of the heavy DNA peak with that of the G+C rich mouse DNA also suggests involvement of the ribosomal genes.