Chloroplast DNA in Expanding Spinach Leaves

The proportion of chloroplast DNA in total DNA from spinachleaves has been measured using the second order reassociationkinetics of a ³H-labelled chloroplast DNA probe in total DNAextracts. There was no significant difference between the proportionof chloroplast DNA in the basal and distal halves of 2 cm leavesand in the distal halves of 5, 8, and 10 cm leaves. The meanof all the observations was 21.1 ± 0.7%. There was littlechange in the average total DNA content of cells from any ofthe leaves but cells from larger leaves contained 130–170chloroplasts while cells from the basal half of 2 cm leavescontained about 20 chloroplasts which were smaller than thosefrom the larger leaves. Consequently the average number of copiesof the plastome per chloroplast in large leaves was about 30(5 x 10^–15 g DNA) and in the smaller chloroplasts in thebase of 2 cm leaves was 200 (32 x 10^–15 g DNA). Stainingwith the DNA fluorochrome 4, 6-diamidino-2 phenyl indole (DAPI)showed 10–15 plastid nucleoid areas in chloroplasts oflarger leaves, suggesting there are 2–3 copies of theplastome per plastid nucleoid.