Structure of Organelle Genomes
Maternal Inheritance Overheads
Structure of Organelle Genomes
One basic concept of biology suggests that eukaryotic organelles arose after a primitive eukaryotic cell engulfed (swallowed, invaginated) a prokaryotic cell. This is called the endosymbiotic theory. After this invagination event, the concept continues, duplicate functions already possessed by the eukaryotic cell were eliminated, and only those prokaryotic functions that were advantageous to the eukaryotic cell were maintained. In particular, energy transduction functions, such as ATP and NADPH production, were maintained.
The two organelles that arose were the mitochondria, found in all eukoryotic cells, and chloroplasts which are found in plants and algae. Some of the genetic information of the prokaryotic cell was transferred to the nucleus of the eukaryotic cell. Chloroplast and mitochondrial sequences have been found in the nucleus of plant cells. Furthermore, chloroplast sequences have been found in the mitochondrial of plant cells. This is clear evidence that genetic control of certain biochemical functions was relinquished (or taken from) the progenitor cells. The following diagram shows the flow of genetic information from organelles in a plant cell.
It is important to note that all the basic functions of the Central Dogma of Molecular Genetics are found in organelles. This functions include DNA replication, RNA transcription and protein translation. Thus, certain gene products will result from the expression of the organelle DNA. The number of protein products from mitochondrial transcription is limited. One set of genes that are expressed are the rRNAs and tRNAs that are required for translation. Five known gene products are produced from the mammalian mitochondrial genome. These include subunits I, II and III for cytochrome oxidase, the apoprotein for cytochrome b and subunit 6 of the mitochondrial ATPase. In addition to these gene products, six ORFs have been identified. (ORF represents Open Reading Frame.) These are sequences that have transcription start and stop signals that bracket sequences that could produce a protein product, but the actual function of the product is not known. Finally, as we saw above the expression of organelle genes has a unique effect on the inheritance of certain traits.
This transfer of genetic information during the evolution of eukaryotic cells has also required the development of cooperative gene expression systems between the organelle and the nucleus. Cytochrome oxidase is one of the mitochondrial enzymes involved in ATP generation. As stated above subunits I, II, and III are encoded in the mitochondria. The remainder of the subunits of this seven subunit protein are encoded in the nucleus. The best studied example of coupled nuclear DNA/chloroplast DNA gene expression is for the protein RUBISCO (ribulose bisphoshpate carboxylase oxygenase). This is the enzyme that begins the fixation of atmospheric CO2 into sugar molecules. This enzyme has 16 subunits, eight large subunits and 8 small subunits. The large subunits are encoded by the chloroplast DNA whereas, the small subunits are encoded by nuclear DNA. Thus, for those proteins that are encoded by genes in two different cellular locations, gene expression has to be coordinated between the two locations for proper protein functioning.
Chloroplast Genomes
The chloroplast genomes of plants exhibit a far greater conversation of structure than plant mitochondrial genomes. These genomes are circular and the size of higher plant chloroplast DNAs are either *150 kb (for example, spinach) or *120 (for example pea). The difference in size can be accounted for by a deletion from the larger genome to generate the smaller. The gene order among all higher plant chloroplast genomes is essentially conserved.
As you would predict, many of chloroplast DNA genes encode proteins that are involved in photosynthesis. In total, the genome appears to encode for a complete set of rRNA and tRNA genes and *45 protein products. There are some differences between species, but these differences primarily are between higher plants and algae, which also contain chloroplast DNA.
RFLP proof that chloroplast DNA can be maternally inherited
As has been stated previously, to demonstrate that a trait is maternally inherited specific crosses need to be made to generate the required offspring. A number of crosses where made between cultivated tomato (L. esculentum) and a number of wild species. As we discussed previously, chloroplast DNA is a circular molecule *150 kb in size. Digestion of this molecule produces relatively few fragments. Within the plant cell, chloroplast DNA represents a significant portion (*15%) of the DNA. This is a result of the large number of chloroplasts per cell (*50) and the large number of chloroplast DNA molecules per chloroplast (*150).
Chloroplast DNA was obtained from F1 plants of a number of crosses in which L. esculentum was the female in the cross. As can be seen below, in each case the F1 restriction fragment pattern was identical to L. esculentum (sample 8). This is conclusive evidence that chloroplast DNA is inherited in a maternal manner. [The figures shown below are from Palmer and Zamir (1982) Proceedings of the National Academy of Science USA 79:5006.]
Mitochondrial Genomes
All of the eukaryotic species that have been analyzed to date have DNA in the mitochondria. But the size of this mitochondrial genome varies.
Size of Mitochondrial G enomes
Species |Size (kb) -------------------------- Human | 16 Drosophila | 18 Yeast | 75 Turnip | 218 Corn | 570 Muskmelon | 2000 --------------------------
All of the mitochondrial genomes are circular. Even though there is a large size discrepancy between different species, especially between plants and animals, the number of genes that are expressed in each species is nearly the same, about 20. Thus the extra DNA that is found in the larger genomes does not appear to be required. Studies have shown that much of this DNA is in fact repeated sequences.
RFLP proof that mitochondrial DNA can be maternally inherited
Experiments designed to show that mitochondrial DNA was maternally inherited were analogous to those which demonstrated that chloroplast DNA was maternally inherited. Mouse is the experimental organism used to demonstrate this principle here. Reciprocal crosses were made between the species Mus domesticus, common mouse, and the wild species Mus spretus. F1 progeny where then backcrossed, with one of the two species serving as females. The restriction enzyme HincII is diagnostic for the mitochondrial DNA of the two species. M. domesticus mt DNA contains five restriction sites for the enzyme that generates four fragments whereas M. spretus mt DNA contains eight restriction sites which generate seven fragments. Only two of the fragments are the same size and presumably contain the same sequence information.
The figure below depicts the results of the backcross experiments. [The figure is from Gyllenstein et al. (1985) Journal of Heredity 76:321.] As you can see the female contributes the mitochondrial genome. These experiments have been repeated in other species, such as human, and the female has been shown to contribute the mitochondrial genome.
Copyright © 1997. Phillip McClean