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K Pillai, Dept of Zoology, Hindu College, Moradabad email@example.com Cytoplasmic Inheritance Cytoplasmic Inheritance : Cytoplasmic Inheritance Also known as Extrachromosomal inheritance or Extranuclear inheritance Different cytoplasmic organelles like mitochondria, chloroplasts, plasmids and endosymbionts participate in inheritance. Two main types are: maternal inheritance and infectious inheritance. Maternal inheritance : Maternal inheritance Mitochondria and chloroplasts are essential components of the cytoplasm of eukaryotic cells. These contain the genetic material DNA. These cytoplasmic organelles are usually inherited with the egg cytoplasm from the maternal parent. So it is termed ‘maternal inheritance’. Infectious inheritance : Infectious inheritance The cytoplasm of animal and plant cells is complex. In addition to the normal cellular components, numerous parasites can infect the cytoplasm and replicate there. These parasites include bacteria and viruses. This is termed infectious inheritance. Cytoplasmic inheritance Examples : Cytoplasmic inheritance Examples 1. Shell coiling in snail Limnaea peregra 2. Plastid inheritance in Mirabilis jalapa 3. Cytoplasmic male sterility in maize 4. Kappa particles in Paramecium aurelia 5. Sigma virus in Drosophila melanogaster 6. Milk factor in mice 7. Transmissible spongiform encephalopathy 1. Shell coiling in Snail : 1. Shell coiling in Snail In snails (gastropods) the shell is spirally coiled. Snails exhibit two types of coiling of their shell: (i) shell coiled to right is dextral (ii) shell coiled to left is sinistral. The gene for dextral coiling is dominant D, sinistral coiling is recessive d. Boycott and Driver (1923) showed that the character of coiling is determined by the gene of the mother and not by the individual's own gene. Sinistral Dextral 2. Plastid inheritance : 2. Plastid inheritance Carl Correns (1909) studied the inheritance of leaf variegation called 'albomaculatus' in the 'four o' clock plant' Mirabilis jalapa. In this, normal green tissue of leaf is irregularly spotted with patches of paler green or white. Flowers on wholly green branches produce seeds that grow into normal plants. Flowers on variegated branches yield offsprings of three kinds- green, white and variegated in variable proportions. Flowers from branches wholly white give progeny without chlorophyll. But in every case the source of pollen has no influence on the offspring. So cytoplasm of the egg influence the type of leaf in Mirabilis. Slide 9: Plastid inheritance in Mirabilis jalapa 3. Cytoplasmic male sterility : 3. Cytoplasmic male sterility In several crops like maize (Zea mays) and many other plants, cytoplasmic control of male sterility is known. If the female parent is male sterile (having plasma gene for male sterility) the F1 progeny would always be male sterile. Cytoplasmic male sterility is passed down maternally. 4. Kappa Particles in Paramecium : 4. Kappa Particles in Paramecium Tracy Sonneborn, an American geneticist made many discoveries in the field of extranuclear inheritance. Sonneborn (1943) described the inheritance of some cytoplasmic particles known as kappa and their relation to nuclear gene in Paramecium aurelia. There are two strains of Paramecium one killer and the other sensitive. The killer secretes a toxic substance called paramecin which kills the sensitive. The paramecia of killer strain contain large number of particles in their cytoplasm. Slide 13: Studies by Preer (1974) indicated that kappa particles are endosymbiont gram negative bacterium Caedobacter taeniospiralis living in the cytoplasm of Paramecium. Kappa particles (100-200 per cell) are usually 1-5mm in length bounded by a double membrane and are Feulgen positive. Kappa particles contain DNA and protein and require a nuclear gene (K, “little k” strains are sensitive) for maintenance 5. Sigma Virus in Drosophila : 5. Sigma Virus in Drosophila Philip L'Heritier and Teissier reported an example of cytoplasmic inheritance in Drosophila. These flies are sensitive to carbon dioxide and can be immobilized by exposing them to CO2. Some strains of Drosophila are much more sensitive to CO2 than the normal. The CO2 sensitivity of Drosophila is caused by a small virus like particle called sigma. Some physical characters of sigma are known. They are much smaller than kappa, approximately 0.07 mm in diameter. They are bullet-shaped very similar to vesicular stomatis virus (VSV). The nucleic acid content of sigma is not established, but it is assumed that sigma contain RNA. 6. Milk Factor in Mice : 6. Milk Factor in Mice Bittner reported a particular type of mammary cancer in mice susceptibility of which was maternally transmitted. Milk factor resembles in many respects with a virus. It has been discovered to be transmissible also by saliva and semen. 7. Transmissible spongiform encephalopathy : 7. Transmissible spongiform encephalopathy Prions are small, disease-causing agents that consist entirely of proteins. Prions have no nucleic acid genome. Prions are thought to be normal proteins that can adopt different configurations. They are found in the brain of all animal species. Prions cause fatal brain degeneration diseases known as transmissible spongiform encephalopathy (TSE). Slide 17: Prions are inherited maternally in the cytoplasm. They can also be infectious in cases where animals consume meat from infected animals. Mad cow disease reported in Britain (1995) and killed thousands of cattle, was an example of TSE. Stanley Prusiner (United states) won Nobel Prize of 1997of Physiology or Medicine for the studies on prions and mad cow disease. 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