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Chapter 16: Mitochondrial DNA and Extranuclear Inheritance Chapter Summary The cytoplasm of eukaryotic cells contains complex organelles, the most prevalent of which are mitochondria, found in both plant and animal cells, and chloroplasts, found in plant cells. Mitochondria are responsible for respiratory metabolism. Chloroplasts are responsible for photosynthesis. Both mitochondria and chloroplasts contain DNA molecules that encode a variety of proteins. The genes contained in mitochondrial and chloroplast DNA are not present in chromosomal DNA. Hence, traits determined by these genes are not inherited according to Mendelian principles, but show extranuclear (cytoplasmic) inheritance instead. Extranuclear inheritance is also observed for traits that are determined by bacteria and viruses transmitted in the cytoplasm from one generation to the next. In eukaryotic microorganisms, cytoplasmic mixing usually takes place in zygote formation, and both parents contribute cytoplasmic particles to the zygote. In multicellular animals, however, the egg contains a large amount of cytoplasm and the sperm very little, so cytoplasmic transmission results in maternal inheritance. Progeny resembling the mother is characteristic of both maternal inheritance and maternal effects. Maternal inheritance results from the transmission of extranuclear factors by the mother. Maternal effects occur when the nuclear genotype of the mother determines the phenotype of the offspring—for example, through the molecular organization of the egg or by nurturing ability. The direction of shell coiling in snails results from a maternal effect on the orientation of the spindle in the first mitotic division in the zygote. Variegation in the four o’clock plant is caused by mutations in chloroplast DNA that eliminate chlorophyll. Variegated plants contain patches of green and white derived from wildtype cells and from cells with mutant chloroplasts, respectively. Inheritance of the variegated phenotype depends on the chloroplast content of the egg because pollen does not contain chloroplasts. The inheritance of resistance to certain antibiotics in Chlamydomonas is also cytoplasmic and depends on mutations in the chloroplast. Mitochondrial defects account for the cytoplasmic inheritance of some petite mutations in yeast. Petites pro- duce very small colonies because the defective mitochondria are unable to carry out aerobic metabolism. Petite cells can use only anaerobic metabolism, which is so inefficient that small colonies are produced. 310
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311 There are two major types of petites. Segregational petites result from a mutation in a nuclear gene and show strictly Mendelian inheritance. Neutral petites show non-Mendelian inheritance (4 : 0 segregation in ascospores) and result from large deletions in mitochondrial DNA. Male sterility in maize also results from defective mitochondria that contain certain types of deletions or rearrangements in the DNA. Male sterility
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