The mechanisms by which chloroplasts and bacteria divide are also similar. Both utilize FtsZ proteins, which are self-assembling GTPases related to tubulins (see Chapter 16). Bacterial FtsZ is a soluble protein that assembles into a dynamic ring of membrane-attached protofilaments beneath the plasma membrane in the middle of the dividing cell. The FtsZ ring acts as a scaffold for recruitment of other cell-division proteins and generates a contractile force that results in membrane constriction and eventually in cell division. Presumably, chloroplasts divide in very much the same way. Although both employ membrane-interacting GTPases, the mechanisms by which mitochondria and chloroplasts divide are fundamentally different. The machinery for chloroplast division acts from the inside, as in bacteria, while the dynamin-like GTPases divide mitochondria from the outside (see Figure 14–63). The chloroplasts have remained closer to their bacterial origins than have mitochondria, since the eukaryotic mechanisms of membrane constriction and vesicle formation have been adapted for mitochondrial fission.
The RNA editing and RNA processing that is prevalent in chloroplasts owes everything to their eukaryotic hosts. This RNA processing includes the generation of transcript 5ʹ and 3ʹ termini and the cleavage of polycistronic transcripts. In addition, an RNA editing process converts specific C residues to U and can change the amino acid specified by the edited codon. These and other RNA-based 23S 16S 23S 16S liverwort chloroplast DNA 121,024 bp
KEY:
Figure 14–67 The organization of the liverwort chloroplast genome. the chloroplast genome organization is similar in all higher plants, although the size varies from species to species—depending on how much of the DNa surrounding the genes encoding the chloroplast’s 16S and 23S ribosomal rNas is present in two copies.
processes are catalyzed by protein families that are not found in prokaryotes. One can ask why the expression of so few chloroplast genes needs to be so complex. One explanation is that the expression of chloroplast and nuclear genes must be closely coordinated. More generally, the bacterial concept of the operon as a co-regulated set of genes in a single transcription unit has been largely abandoned in chloroplasts. Polycistronic transcripts are cleaved into smaller fragments, which then require splicing or RNA editing to become functional.
Organelle Genes are Maternally Inherited in animals and plants
In Saccharomyces cerevisiae (baker’s yeast), when two haploid cells mate, they are equal in size and contribute equal amounts of mitochondrial DNA to the diploid zygote. Mitochondrial inheritance in yeasts is therefore biparental: both parents contribute equally to the mitochondrial gene pool of the progeny. However, during the course of the subsequent asexual, vegetative growth, the mitochondria become distributed more or less randomly to daughter cells. After a few generations, the mitochondria of any given cell contain only the DNA from one or the other parent cell, because only a small sample of the mitochondrial DNA passes from the mother cell to the bud of the daughter cell. This process is known as mitotic segregation, and it gives rise to a distinct form of inheritance that is called non-Mendelian, or cytoplasmic inheritance, in contrast to the Mendelian inheritance of nuclear genes.