Bio-genesis of Mitochondria

Bio-genesis of Mitochondria

New mitochondria arises by growth and division of existing mitochondria or pro mitochondria Microscopic evidences support this idea. Mitochondria in the process of dividing have been observed under phase contrast microscope and membrane partition dividing mitochondria in half can be detected by electron microscope in green algae. Micromonas which contain single mitochondria but the microscopic observations do not rule out the possibilities that mitochondria also arise by other mechanism. In 1960’s David Luck designed experiment on Neurospora, that focused whether such alternative mechanism exist. 1st Neurospora colonies were incubated in 3H-choline (a phospholipid precursor), so it could be incorporated within mitochondrial membranes. Cultures were then transferred to non-radioactive medium and allow to grow for varying periods of time and finally examined by microscopic auto radiography and silver grains were counted to determine the distribution of radioactivity one of the 3 patters would be emerged:

1. If new mitochondria are formed by growth & division of existing mitochondria then each doubling in the number of mitochondria, after the 3H choline is removed it should be accompanied by 50 percent reduction in the no of auto radiographic grains/ mitochondria.

2. If de-novo synthesis occurs then new mitochondria formed after abnormal of 3H-choline should be non-radioactive while the radioactivity in the pre-existing mitochondria would remain the same.

3. If mitochondria develops from other membrane then the pre-existing radioactive mitochondria should retain their radioactivity and new mitochondria should at 1st be radioactivity because the membrane from which it develop would be radioactive. But as the time parse, the absence of 3H-choline from the medium would decrease in the amount of radioactive in the cellular membranes from which mitochondria develops and there form the average amount of radioactive/labeled mitochondria would decrease Data obtained in the studies followed 1st pattern suggesting that new mitochondria aries from growth and division of existing mitochondria.

In some cases mitochondria develop in the cells that appears to have no pre-existing mitochondria e.g. yeast which can survive either aerobic or aerobically. When switched to anaerobic conditions, the cells meet all their energy requirement through glycolysis and so mitochondria gradually disappears. When some cells are exposed to oxygen, mitochondria re-appears. Previously such conditions was thought to be de-novo formation. But electron microscopic examination revealed the presence of small double-membrane enclosed vesicular about 1mm in diameter that lacks costae and cytochrome but contains mt DNA. When anaerobically grown Yeast are exposed to-Oxygen the inner membrane forms crested that synthesize new cytochrome and other respiratory chain components. Because of their ability to differentiate into mitochondria the small double-membrane vesicles are called pro mitochondria.