Mitochondria have their own DNA molecules, usually multiple copies in one mitochondrion, in circular form, located in several nucleoid regions, with no histone association (naked). Mitochondrial genome size varies, plants have mitochondrial average 150 -200 kb, but human mitochondria genome is only 16 kb. It encodes enzymes required for oxidative phosphorylation and mitochondrial electron transfer. A cell can have different types of mitochondria (heteroplasmy) or same type of mitochondria (homoplasmy).
Mitochondrial DNA replication
Mitochodnrial DNA has H (heavy) strand which is GC-rich and light (L) strand which is AT-rich. A region of 1121 bp containing the replication origin is called D-loop, which is also the transcriptional start point for both H and L strand. Mitochondrial gene transcrips are polycistrons, RNA are processed after transcription. Translation is more similar to prokaryotes, with some unusual codon usage (non-universal).
Like mitochondria, chloroplast also contain circular double stranded DNA molecules. They too do not associate with histones. Chloroplast genome contain two inverted repeats called IRA and IRB, therefore many genes encoded by chloroplast genome have two copies. Both transcription and translation procedure is similar to prokaryotes.
This is a feature unique to mitochondria and chloroplasts. RNA editing means post-transcriptional modification of the primary sequence of RNA through nucleotide deletion, insertion, or base modification.
Origin of mitochondria and chloroplast –endosymbiont hypothesis
Mitochondria and chloroplast were free-living prokaryotes. These organisms invaded primitive eukaryotes and established symbiotic relationship. The evidence aree listed below: MtDNA and cpDNA are cicular, similar to bacteria, they have similar ribosomes with bacteria; they are similar in size; and they all have similar double membranes.
Both mitochondria and chloroplast genes are located in cytoplasm, therefore they are extranuclear genetic materials. They have distinct inheritance pattern as compared to Mendelian inheritance. Usually they show maternal effect: the offsprings inherit the trait from their mother.
Mutations in mitochondria
Mutation types include point mutations, deletion and duplication, or quantitatively loss of entire molecules. Mutation in mtDNA is a cause of aging and many other disorders.