Question

Gene transfer between the chloroplasts/mitochondria and the nuclear genome has been unidirectional from the organelle to the nucleus.

True False

Answer 1

* Gene transfer from organalle like chloroplast and mitochondria to the nuclear genome is unidirectional, so the answer is "True".

* Upto this date no study reveals any evidence of the reversal to happen and this has a potential reason too.

* chloroplast is very similar to cyanobacteria and mitochondria is similar to proteobacteria. So there is a hypothesis that once these free living prokaryotes were engulfed into eukaryotic cells through phagocytosis, may be during a pathogenic attack from these bacteria.  

* But instead of destroying these bacteriae, the eukaryotic cells developed a symbiotic relationship with these organisms. As we know the importance of chloroplast as the photosynthetic unit and mitochondria as the power house of a cell, they helped the eukaryotes to develop and in return the eukaryotic cells protected them.

* Symbiotic relationship is a mutually benefitting close physical relationship between two different organisms.

* The symbiosis between these organelle and eukaryotic cells is known as "Endosymbiosis".

* The process of development of endosymbiosis is "Symbiogenesis".

* Thus the genome of organelle has only semi autonomous properties, they are under the ultimate control of the eukaryotic cell nucleus, otherwise they can harm a eukaryotic cell by expressing some harmful genes in them.

Detrimental genes in these organelle can express and produce harmful cell destroying proteins which can cause apoptosis, so they are kept under close surveillance of cellular repairing mechanisms.

* The nuclear genome has no need to go to the organelle to get expressed, doing so, may cause strengthening of the organelle genome ( ie:- the invader genome) , which may result in invader getting an upperhand in the cell.

* But the organelle are found to be trying to incorporate small parts of its genome into the nucleus for better expression of that genes. This process is considered as an evolutionary mechanism for the betterment of the symbiotic relationship. In the nucleus, if that received genome fragment is found to be defective, it will be muted and will not be expressed.

* Thus the genome flow is unidirectional, from organelle to nucleus and not vice versa.