Cytoplasmic Inheritance

• The DNA in human mitochondria is circular and not condensed with histones
• Each organelle may include multiple copies of its DNA
• The human mitochondria has 16,569 basepairs
• All proteins coded are subunits of complexes involved in oxidative phosphorylation required for ATP production inside the mitochondria
• All genes found in the mitochondria are translated inside the mitochondria
• heteroplasmy: both normal and defective copies of the organelle DNA may be present in the cells of an individual
• Maternal inheritance in man:

  • all reflect the loss of ATP availability
  • effects are seen where energy is needed most
  • all show variability in organs due to mitochondrial heteroplasmy
• maternal inheritance examples:

  • Lebers hereditary optic neuropathy (LHON)
  • Kearns-Sayer syndrome (KSS)
  • Myoclonic Epilepsy with Ragged Red Fibers (MERF)
• Lebers hereditary optic neuropathy:

  • causes optic nerve degeneration
  • results from a G to A transition at position 11,778 causing an arg to his substitution in NAD dehydrogenase subunit 4
• Kearns-Sayer syndrome:

  • vision loss before age 20, hearing loss, organ failure
  • results from various deletions, including at least one tRNA
  • a larger proportion of defective mitochondria is typical on aging
• Mycoclonic Epilepsy with Ragged Red Fibers:

  • muscle twitching epilepsy, ataxia, ragged red fibers in muscle biology
  • usually an A to G transition in a mt-tRNA for lysine
• In maternal inheritance, the mitochondria do not go through the sperm, so if the father has the gene, he will not pass it on
• Since mitochondrial defects are passed only through the egg, all progeny of an affected mother will inherit the trait
• Affected individuals are almost always "heteroplasmons", meaning they have a mix of functional and nonfunctional mitochondria
• The proportion of mitochondria with the defect that enter the egg can vary, meaning the level of problem can vary among the progeny
• When cells divide by mitosis, sorting of mitochondria into daughter cells is random
• In pines and other gymnosperm plants, chloroplasts come from the egg, but mitochondria come through the pollen
• Yeast of opposite mating types fuse to form a diploid before mating so there is a mixing of cytopalsm
• yeast can grow by fermentation without functional mitochondria, but in doing so they form relatively tiny "petite" colonies. This reflects the fact that glycolysis provides only 2 ATP per glucose rather than 30 from oxidative phosphorylation using mitochondria
• In corn, you can cross a male inbred sterile with male inbred fertile to get a hybrid. 50% will be fertile and the other 50% will be sterile
• Crossing corn resulting in southern corn leaf blight
• infectious heredity: caused by a virus or microbe that can pass through the egg but not the sperm
• examples of infectious heredity:

  • mouse mammory tumors can result from a virus that could be passed from the mother to her pups via milk
  • CO2 sensitive drosophila are the result of a virus that can be egg-transmitted
  • "killer" (a particle in paramecia) can be passed through the cytoplasm from a heterozygous or homozygous dominant individual. If the recipient is homozygous recessive, the "particle" unwraps and releases viruses that kill the host
• Infectious heredity mimics cytoplasmic inheritance
• maternal effect: occurs when a substance deposited in the egg affects development of the progeny, no matter their own genotype
• Human mtDNA is 16,569 base pairs in a closed circle
• Plant and fungal mtDNAs are usually larger
• Typical ctDNA is more than 121,000 base pairs in a circle
• Organelles generally pass from one generation to the next only via the egg, not sperm or pollen
• All progeny inherit the organelle DNA sequence of their mother
• DNA in organelles codes for:

  • rRNA for translation inside organelle
  • tRNA for translation inside organelle
  • mRNA for some subunits of proteins that are made in and function inside the organelle
• Proteins coded by mRNA include ct/mt ribosomal proteins and an RNA polymerase, cytochrome oxidases, ATPases, and electron transport complex component in mtDNA, and phytosystem, cytochromes, and electron transport complex proteins in ctDNA
• Other protein subunits for many of the same complexes are coded for by nuclear genes, transcribed in the nucleus, translated in the cytoplasm, and transported into the organelle with the aid of "signal sequences" built into the beginning of the chain of amino acids.
• Inheritance of defects in the organelle genes is strictly maternal
• The best test for maternal inheritance is to make reciprocal crosses. Progeny of an affected female will all show the trait, but if an affected male is crossed to a normal female, all progeny will be normal.
• If a white sector of a plant is used as a maternal parent, all progeny will be white, while those from a sectored female may be white, sectored or green
• Mitochondrial DNA defects in man include defects in vision and muscles; areas where high energy is needed
• Lebers hereditary optic neuropathy:

  • affected female by normal male = all affected
  • normal female by affected male = all normal
• Defects must be relatively mild or the lack of functional energy metabolism would be lethal
• When mitochondria are defective, the plant can show cytoplasmic male sterility