The Chloroplast Genome and Its Expression

Abstract

The studies of Schimper, Strasburger, and others in the 1880s demonstrated that chloroplasts in a sense have an existence of their own. Chloroplasts were found to proliferate by division of existing plastids and were passed on to daughter cells at the time of cell division. But more recent findings have indicated that the chloroplasts’ independence is quite limited. Plastids have their own DNA, which is quite distinct from nuclear DNA, and also have the ability to express the genetic information in their DNA. A question with much current interest, however, is the critical one of how much information chloroplast DNA actually contains. Although the total mass of DNA in a chloroplast is generally somewhat more than that in a bacterial cell, the genetic capacity of chloroplast DNA is much less than that of the bacterial chromosome. The reason for this paradox is that chloroplasts contain multiple copies of a relatively small molecule of DNA. As with mitochondria, the information in chloroplast DNA apparently is only that which is required, separate from the nucleus, to synthesize a few necessary functional proteins. However, even to accomplish the synthesis of these relatively few proteins, the chloroplast must contain its own ribosomes and complete machinery for protein synthesis. As work on the biosynthetic capabilities of the chloroplast proceeds, it is becoming quite clear that most of the plastid’s proteins and properties are determined by the nuclear genome and that most of its proteins are synthesized on cytoplasmic ribosomes.