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Organelle heredity
DNA contained in mitochondria or chloroplasts determines certain phenotypic characteristics of the offspring
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Infectious heredity
results form symbiotic or parasitic association with a microorganism
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Maternal effect
- nuclear gene products are stored in the egg and then transmitted through the ooplasm to the offspring
- gene products in the ovule are distributed to the cells of the developing embryo and influence its phenotype
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Analysis of mutant alleles in chloroplast and mitochondria are difficult because:
- the function of these organelles is dependent on gene products from the nucleus and the organelle DNA, making the origin of mutations affecting organelle function difficult
- organelles are contributed to each progeny cell and exhibit great variation (heteroplasmy) making detection of the mutation hence analysis more complicated than Mendelian inheritance
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Chlamydomonas
excellent model system for studying organelle heredity because it has a single large chloroplast that exhibits a uniparental inheritance pattern
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Ruth Sager (1954)
showed that streptomycin-resistant Chlamydomonas show differences in reciprocal crosses with susceptible strains, and the trait is passed through the female parent
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Mitchell and Mitchell (1952)
reveal that mitochondria likewise contain a diverse genetic system, and mutations are transmitted through the cytoplasm
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3 types of mutations
segregational, neutral, suppressive
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Segregational petites
mitochondrial mutation studies with yeast Saccharomyces cerevisae have revealed that a small percent of these mutations are nuclear in nature
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Neutral petites
cross with neutral petites show that mitochondria are inherited form both parental cells and are replicated in the offspring
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Suppressive mutation
may not be observed initially in offspring but becomes apparent after a time
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Endosymbiotic theory
- states that mitochondria and chloroplasts arose independently about 2 billion years ago form free-living bacteria
- Progenitors posses the abilities now attributed to these organelles such as aerobic respiration and photosynthesis
- theory proposes that these bacteria were engulfed by larger eukaryotic cells and a beneficial symbiotic relationship was developed
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Amount of DNA in today's mitochondrai
is less than 10%, very miniscule when compared to DNA of free-living bacteria from which it was derived
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Chloropast DNA length
ranges from 100-225 kb and genes carried on the DNA encode products involved in phtosynthesis and translation
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Comparison of Mitochondrial DNA (mtDNA) and DNA in chloroplasts (cDNA)
- mitochondria DNA is smaller because the introns are absent and gene repititions are rare
- most of protein encoding genes are located on single strand
- replication in mitochondria is dependent on genes encoded by the nuclear DNA
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Human mitochondrial DNA
- contains 16,569 base pairs
- codes for 13 over 70 proteins required for aerobic cellular respiration
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Why is mtDNA susceptible to mutations?
- doesn't have histones to protect from mutations
- mitochondria have high concentrations of reactive oxygen species (ROS) generated by cell respiration [ROS damages organelle contents such as proteins, lipids, and mtDNA)
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Heterplasmy
- condition in which adult cells have a variable mixture of normal and abnormal organelles
- if a zygote receives a large number of organelles through the egg; a mutation in one or a few will be diluted out by many mitochondria that lack the mutation and function normally
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For human disorder to be attributed to mtDNA:
- the inheritance must exhibit a maternal inheritance pattern
- the disorder must reflect a deficiency in the bioenergetic function of the organelle
- there must be a specific mutation in a mitochondrial gene
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Three disorders arising from mtDNA are:
- myoclonic epilepsy and ragged red fiber disease (MERRF)
- Leber's heredity optic neuropathy (LHON)
- Kearns-Sayre syndrome (KSS)
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Maternal effect
- implies that an offspring's phenotype is under the control of nuclear gene products present in the egg
- nuclear genes of the female gamete are transcribed and the genetic products accumulate in the egg cytoplasm
- these products are distributed among newly formed cells, influencing patterns/traits established early in development
- Maternal effect genes, the genotype of hte female parent and not that of the embryo determines the phenotype of the offspring by overriding the genotype of the progeny
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Examples of maternal effect
- Ephestia (larva)
- small Limnaea peregra (shell coiling)
- Drosophila
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