(18)
14. Beskrive organiseringen af mitokondrielt DNA og angive i hvilke
organeller/organismer DNA er linært eller cirkulært.
Devlin, s.59-60, fig.2.37
Stryer, s.125 - fig.5.18
Genomic DNA can be linear or circular.
Linear genome is found in eukaryotes, while circular mainly in prokaryotes.
Circular DNA results from the formation of phosphodiester bonds between the 3`- and 5`- termini of linear polynucleotides.
Many viruses have circular DNA at some point in their life cycle.
Most (if not all) DNA in bacteria is circular. This includes the bacterial chromosomes as well as smaller extra-chromosomal DNA called plasmids.
Mitochondria have also circular DNA.
mtDNA
Stryer, s.493
Devlin, s.587-8,
fig. 13.61
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 and
22 tRNA species, which are required for protein synthesis in mitochondria.
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
Mitochondria are though to be descendants of aerobic prokaryotes that invaded and set up a symbiotic relationship with eukaryotic cells. That is why they have some prokaryotic characteristics, so antibiotics can slightly effect them too.
tilbage til molekylær biologi