DNA Labeling by Isolated Wheat Chloroplasts

In wheat, growth is from a basal meristem: The cells of the young shoot form a developmental array from the tip (oldest) to the base (youngest) of the leaf. In cells from a specific part of the leaf (and so of a certain age), chloroplast DNA is found to be replicating. In seedlings up to 10 d old, chloroplast DNA replication is found in the region from 1 to 4 cm from the leaf base (1 ) If chloroplasts are isolated from this region of leaf and incubated in the presence of a radioactively labeled DNA precursor, radioactively labeled DNA can be produced in vitro.