In this article we will discuss about the presence of DNA in mitochondria and chloroplasts.
The presence of organelle-specific DNA in mitochondria and chloroplasts was identified with certainty over three decades ago. The occurrence of a specific chloroplast DNA was first demonstrated by Chun et al. in 1963, and of mitochondrial DNA a year later by Luck and Reich (1964).
It is now fairly well established that organelle DNA is distinct from nuclear DNA in several respects. That it can carry out synthesis of RNA and protein on its own synthesizing ‘machinery’ and replicate itself. There are mitochondrial and chloroplast ribosomes. The genomes of these organelles contain genes for the rRNA species present in both large and small subunits of their respective ribosomes.
Mitochondrial DNA (mt DNA):
Mitochondria contain an open or closed circular DNA molecule. DNA from different species shows a wide range in average base composition ranging between less than 20% to more than 50% of G + C content. It has been found that the amount of DNA present in mitochondria is sufficient to code for proteins.
Grossman(1971) have determined that a minimum base composition of 35% G + C is needed to code for an average protein. Thus regions of mt DNA containing very small amounts of G and C nucleotides (in cases where the average base composition is less than 30%) may not be able to code for a protein.
Recent evidence indicates that there is one copy of the gene for each of the rRNA species associated with the large and small subunits of mitochondrial ribosomes. The positions of these two genes have been mapped in some lower eukaryotes.
In the mold Neurospora crassa the two genes for rRNA are adjacent to each other, whereas in yeast (Saccharomyces cerevisiae) they are separated. With the use of radioactive isotopes it has been shown that in N. crassa and S. cerevisiae some of the mitochondrial inner membrane proteins are synthesised by the mitochondrial protein synthesizing apparatus.
The existence of mutations have been recorded in the mitochondrial genome. A study of the oligomycin resistance mutations have led to the identification of 3 distinct linkage groups in mtDNA. Recently Tzagaloff (1975) have been able to identify a number of mutants with defects in cytochrome oxidase and coenzyme Q-cytochrome c reductase.
mtDNA has also been studied in a limited number of higher plants. A closed circular DNA with molecular weight ranging between 60 – 80 x 106 Daltons has been seen in spinach, lettuce and pea. However, higher plant mtDNA has not shown the presence of rRNA and tRNA genes. No protein synthesis is recorded for plant mtDNA.
Chloroplast DNA (ctDNA):
Circular DNA molecules have been observed in ctDNA of both higher plants and algae. It contains genes for both large and small rRNA species present in chloroplast ribosomes. Hybridisation studies have indicated one gene per chromosome for each rRNA in tobacco, whereas in bean, lettuce, pea, spinach and maize there are two genes per chromosome. Similar studies have demonstrated the presence of genes for about 25 tRNA species each of size about 25,000 Daltons.
Radioactive isotope studies in vivo have clearly shown that protein synthesis occurs in chloroplasts. One such protein is the large subunit of ribulose diphosphate carboxylase (termed fraction I protein) which is a major constituent of chloroplast as well as other membrane proteins.
The large subunit of fraction I protein and some other membrane proteins are also synthesised by isolated chloroplasts. The small subunit of fraction I protein is coded for by nuclear DNA.