Evolutionary Origin of Mitochondria

The old theory of the ‘bacterial’ origin of mitochondria has been revived with recent discoveries of similarities in structure and physiology of bacteria and mitochondria. According to this symiobtic theory, primitive host cells, which carried out anaerobic respiration by glycolysis, were invaded by bacteria-like parasites which respired aerobically through the Kerbs cycle and by oxidative phosphorylation. These invading organisms later developed an endosymbiotic relationship with the host cells and became the mitochondria. This theory is based on the several striking similarities between bacteria and mitochondria.

1. Morphology. The general dimensions of bacteria and mitochondria are similar. Rod-shaped bacteria are similar in shape to many types of mitochondria.

2. Localization of the respiratory chain. The inner membrane of the mitochondrion is similar to the bacterial plasma membrane with respect to the respiratory chain. The projections of the plasma membrane (mesosomes) in some bacteria are similar to the cristae of mitochondria. The outer mitochondrial membrane is similar is similar to microsomal membranes in enzymatic constitution. The synthesis of the inner membrane is under the control of MDNA, while that of the outer membrane is under the control of nuclear DNA. These factgs have led to the view that the inner membrane of the mitochondrion has been evolved from the primitive bacterial membrane, which has subsequently become enclosed in an outer membrane derived from the host cells.

3. Chemical constitution. There is a similarity between the lipid composition of bacterial and mitochondrial membranes. The high selectively of the mitochondrial membrane indicates the presence of a permease system to that present in bacteria.

4. Ribosomes. The small size of mitochondrial ribosomes (55-60S) is comparable to that of bacterial ribosomes (70S) but not with that of non-bacterial ribosomes (80S).

5. Drug sensitivity. The drug chloramphenicol inhibits protein synthesis of mitochondria and bacteria but not that of higher cells. This inhibition is because chloramphenicol can bind with the mitochondrial and bacterial ribosomes, but not with the 80S non-bacterial ribosomes.

6.  DNA structure. The DNA of both mitochondira and bacteria is circular. Circular DNA is found only in the prokaryotes. Thus MDNA is more closely related to the DNA of microorganisms than to chromosomal DNA of higher organisms.

7. Protein synthesis machinery. The presence of DNA polymerase, RNA polymerase, tRNA activating enzymes, ribosomes and amino acid incorporating activity in mitochondria all indicate the presence of an autonomous proteins synthesis machinery similar to that of free-living organisms. The amount of MDNA is, however, insufficient to code all the required proteins of the mitochondrion. Control by nuclear DNA is also indicated.