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17. Forklare hvorfor proteinsynthesen foregår lidt anderledes i mitochondrier
Devlin, s.251-254

Mitochondria are cell organelles that contain their own circular DNA molecule. They encode for 13 respiratory chain proteins and RNA molecules which are mitochondrion specific: 22 tRNA, which are enough to translate all amino acids and 2 rRNAs, a small and a large one.

Their translocation is independent of the rest of the cell, and they have besides their own tRNAs and rRNA - an RNA polymerase, aminoacyl tRNA synthetase and ribosomes. All these are unique for the mitochondrion, but are all synthesized in the cytosol and have to be imported. That is why mitochondria are not self-replicating organelles; they depend on the nuclear DNA. Around 90% of all proteins required by mitochondria are synthesized in the cytosol.

Few differences between the mitochondrion and cytosolic translation:   

• The number of tRNAs is smaller in mitochondria, only 22

• The genetic code is slightly different. It is mainly because of the fact that mitochondrias genetic code is not universal. Fx. UGA, the termination sequence in eukaryote cells codes for tryptophan in mitochondria. The stop codons in mitochondria are AGG and AGA

• Mitochondrial ribosomes are smaller, the rRNAs shorter

• The initiation tRNA is fMet-tRNAi, like in prokaryotes, but the formyl-modification process is different

• The aminoacyl tRNA synthetase is different

Mitochondria have a high degree of prokaryotic characteristics, since they are though to be descendants from aerobic prokaryotes that invaded and set up a symbiotic relationship with a eukaryotic cell. That is why antibiotics that have effect on bacteria and other prokaryotes can have slight influence on our mitochondria (but still 1000 times less influence on mitochondria then on bacteria).

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