Whilst most DNA is found in the cell nucleus, a small amount of double-stranded DNA is present in the mitochondria. It encodes protein components of the respiratory chain and some special types of RNA

Mitochondrial inheritance has the following characteristics:

• inheritance is only via the maternal line as the sperm contributes no cytoplasm to the zygote
• none of the children of an affected male will inherit the disease
• all of the children of an affected female will inherit the disease
• generally, encode rare neurological diseases
• poor genotype:phenotype correlation - within a tissue or cell there can be different mitochondrial populations - this is known as heteroplasmy

Mitochondrial inheritance has the following characteristics:

• No introns >> prone to get mutations x 10 than nuclear DNA
• No protective DNA repair enzymes >> accumulate throughout life; changes along aging
• inheritance is only via the maternal line as the sperm contributes no cytoplasm to the zygote
• all children of affected males will not inherit the disease
• all children of affected females will inherit it (maternal inheritance)
• generally encode rare neurological diseases
• poor genotype:phenotype correlation - within a tissue or cell there can be different mitochondrial populations - this is known as heteroplasmy)

Common clinical features of mitochondrial disorders

• neurological
  • seizures
  • developmental delay / learning difficulties
  • ataxia, spasticity, peripheral neuropathy
  • stroke-like episodes (esp. MELAS)
  • sensorineural hearing loss
  • ophthalmoplegia and ptosis
• muscular
  • proximal myopathy, exercise intolerance
  • lactic acidosis (due to impaired oxidative phosphorylation)