(94)
11. Beskrive opbygningen af mitokonriel DNA. Definere afvigelserne i den
genetiske kode samt hvordan de translateres af færre tRNA'er
Devlin, s.251-254
Stryer, s.493
Devlin, s.587-8
0.3% of human DNA is found in mitochondria.
Mitochondria are semiautonomous organelles that live in an endosymbiotic relation with the host cell. Mitochondria generate most of the ATP required by aerobic cells. Ca. 100 mitochondria are found in a metabolitic active cell.
These organelles contain their own genome, a circular double-stranded DNA (mtDNA). The mitochondrial DNA comprises 16,569 bp and encodes:
13
respiratory chain proteins
2
mitochondrion-specific rRNAs, a small and a large one
22
tRNA species, which are required for protein synthesis in mitochondria.
Even though there are only 22 in comparison to the 64 tRNAs in the cytoplasm,
they are still enough to translate the 20 amino acids. This basically means
that the code is degenerated in 2 codons, which both code for the same amino
acid.
All the genes lie on the external strand of the circular DNA and are very closely packed, that sometimes one sequence is used by two adjacent genes.
Mitochondria have the ability to transcribe and their own DNA, but they are not self-replicating organelles. Over 90% of all mitochondrial proteins are encoded in nuclear DNA and imported in the cytosol. There are ca. 100 proteins all in all.
Mitochondria’s genetic code is slightly different:
UGA codes for tryptophan instead of being a stop codon
AUA codes for methionine and not isoleucine, as it does in the cytosolic DNA
AAG and AGA are stop codons, instead of coding for arginine
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.
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 mitochondria that the 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 working on bacteria and other prokaryotes can have slight influence on our mitochondria.
The wobble hypothesis is very important in mitochondria, whose genetic code is different from the one of the rest of the cell. Mitochondria have only 22tRNAs, 61 codons and they need to code for 20 amino acids. The low number of tRNAs and high number of codons means that the wobble hypothesis is highly applied and therefore extremly important in mitochondria
tilbage til molekylær biologi