Printed in Great Britain VARIABLE AMOUNTS OF DNA RELATED TO THE SIZE OF CHLOROPLASTS IV. THREE-DIMENSIONAL ARRANGEMENT OF DNA IN FULLY DIFFERENTIATED CHLOROPLASTS OF BETA

Glutaraldehyde-fixed strips of fully differentiated leaves of Beta vulgaris L. were treated with proteases. With this method it is possible to display DNA-regions in chloroplasts selec-tively and to reconstruct them from serial sections. As in young plastids, the DNA in mature plastids is also distributed within several regions which are separated rather clearly by thylakoids. The number of such regions depends upon the size of the organelle and thus upon its developmental state. The sliape of the regions is, in contrast to less-differentiated plastids, mostly elongated. The individual regions seem to contain unequal quantities of DNA-fibrils. A comparison of our ultrastructural results and biochemical data (DNA-amounts per plastid depending on the size of the organelle; kinetic complexity of chloroplast-DNA of Beta) as well as a comparison of the chloroplast with similar prokaryotic systems (as in bacteria, blue-green algae, and mitochondria) leads to the suggestion that each DNA-containing region can be regarded as a single nucleoid. In addition, each nucleoid already contains several (on average, 4-8) genetic units. Thus the chloroplast of Beta seems genetically polyvalent in at least 2 respects: (i) it is of polyenergidic organization, and (ii) the individual nucleoids can be polyploid to varying degrees