- Extranuclear inheritance
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Extranuclear inheritance is the transmission of genes that occur outside the nucleus. It is found in most eukaryotes and is commonly known to occur in cytoplasmic organelles such as mitochondria and chloroplasts or from cellular parasites like viruses or bacteria (1,2,3,7).
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Extranuclear Inheritance of Organelles
Mitochondria are organelles which function to produce energy as a result of cellular respiration. Chloroplasts are organelles which function to produce sugars via photosynthesis in plants and algae. The genes located in mitochondria and chloroplasts are very important for proper cellular function, yet the genomes replicate independently of the DNA located in the nucleus, which is typically arranged in chromosomes that only replicate one time preceding cellular division. The extranuclear genomes of mitochondria and chloroplasts however replicate independently of cell division. They replicate in response to a cells increasing energy needs which adjust during that cells lifespan. Since they replicate independently, genomic recombination of these genomes is rarely found in offspring contrary to nuclear genomes, in which recombination is common. Mitochondrial disease are received from the mother, sperm does not contribute for it.
Extranuclear Inheritance of Parasites
Extranuclear transmission of viral genomes and symbiotic bacteria is also possible. An example of viral genome transmission is perinatal transmission. This occurs from mother to fetus during the perinatal period, which begins before birth and ends about 1 month after birth. During this time viral material may be passed from mother to child in the bloodstream or breastmilk. This is of particular concern with mothers carrying HIV or Hepatitis C viruses (2,3). Examples of cytoplasmic symbiotic bacteria have also been found to be inherited in organisms such as insects and protists (7).
Types of Extranuclear Inheritance
Three general types of extranuclear inheritance exist. These are vegetative segregation, uniparental inheritance and biparental inheritance.
• Vegetative segregation results from random replication and partitioning of cytoplasmic organelles. It occurs with chloroplasts and mitochondria during mitotic cell divisions and results in daughter cells that contain a random sample of the parent cell’s organelles. An example of vegetative segregation is with mitochondria of asexually replicating yeast cells (8).
• Uniparental inheritance occurs in extranuclear genes when only one parent contributes organellar DNA to the offspring. A classic example of uniparental gene transmission is the maternal inheritance of human mitochondria. The mother’s mitochondria are transmitted to the offspring at fertilization via the egg. The father’s mitochondrial genes are not transmitted to the offspring via the sperm. Very rare cases which require further investigation have been reported of paternal mitochondrial inheritance in humans, in which the father’s mitochondrial genome is found in offspring (4). Chloroplast genes can also inherit uniparentally during sexual reproduction. They are historically thought to inherit maternally, but paternal inheritance in many species is increasingly being identified. The mechanisms of uniparental inheritance from species to species differ greatly and are quite complicated. For instance, chloroplasts have been found to exhibit maternal, paternal and biparental modes even within the same species (5,6).
• Biparental inheritance occurs in extranuclear genes when both parents contribute organellar DNA to the offspring. It may be less common than uniparental extranuclear inheritance, and usually occurs in a permissible species only a fraction of the time. An example of biparental mitochondrial inheritance is in the yeast, Saccharomyces cerevisiae. When two haploid cells of opposite mating type fuse they can both contribute mitochondria to the resulting diploid offspring (1,8).
References
(1) Birky et al.,1994.Relaxed and Stringent Genomes: Why Cytoplasmic Genes Don't Obey Mendel's Laws. J. Hered. 85, 355-366. [1]
(2) Sangeeta et al., 2007. Hepatitis C in Pregnancy. American Journal of Perinatology 24, 251-256. [2]
(3) Duff 1996. HIV infection in women. Primary Care Update for OB/GYNS 3, 45-49. [3]
(4) Schwartz et al., 2003. New patterns of inheritance in mitochondrial disease. Biochemical and Biophysical Research Communications 310, 247-251.[4]
(5)Birky et al.,1995. Uniparental inheritance of mitochondrial and chloroplast genes: mechanisms and evolution. PNAS 92, 11331-11338. [5]
(6) Hansen et al., 2006. Paternal, maternal, and biparental inheritance of the chloroplast genome in Passiflora (Passifloraceae): implications for phylogenic studies. Botany 94, 42-46. [6]
(7) Sapp 2004. The dynamics of symbiosis: an historical overview. Can. J. Bot. 82, 1046–1056. [7]
(8) Birky et al.,1978. Vegetative segregation of mitochondria in yeast: Estimating parameters using a random model. MGG 158, 251-261. [8]
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