Mitochondrial disorders

Not all DNA is contained within the cell nucleus. Mitochondria
have their own DNA consisting of a double-stranded circular
molecule. This mitochondrial DNA consists of 16 569 base pairs
that constitute 37 genes. There is some difference in the
genetic code between the nuclear and mitochondrial genomes,
and mitochondrial DNA is almost exclusively coding, with the
genes containing no intervening sequences (introns). A diploid
cell contains two copies of the nuclear genome, but there may
be thousands of copies of the mitochondrial genome, as each
mitochondrion contains up to 10 copies of its circular DNA and
each cell contains hundreds of mitochondria. The
mitochondrial genome encodes 22 types of transfer and two
ribosomal RNA molecules that are involved in mitochondrial
protein synthesis, as well as 13 of the polypeptides involved in
the respiratory chain complex. The remaining respiratory
chain polypeptides are encoded by nuclear genes. Diseases
affecting mitochondrial function may therefore be controlled
by nuclear gene mutation and follow mendelian inheritance, or
may result from mutations within the mitochondrial DNA.