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Cell division

.reproduce,

.and again segregate randomly in cell division.

.and again segregate randomly in cell division.

Homo-plasmic cell

Heteroplasmic cells

Homoplasmic cells

Homo-plasmic cell

Heteroplasmic cells

Homoplasmic cells

Conclusion: Most cells are heteroplasmic, but, just by chance, some cells may receive only one type of organelle (e.g., they may receive all normal or all mutant).

20.5 Organelles in a heteroplasmic cell divide randomly into the progeny cells. This diagram illustrates replicative segregation in mitosis; the same process also takes place in meiosis.

When replicative segregation occurs in somatic cells, it may create phenotypic variation within a single organism; different cells of the organism may possess different proportions of mutant and wild-type sequences, resulting in different degrees of phenotypic expression among tissues. When replicative segregation occurs in the germ cells of a hetero-plasmic cytoplasmic donor, the offspring may show quite different phenotypes.

The disease known as myoclonic epilepsy and ragged-red fiber disease syndrome (MERRF) is caused by a mutation in an mtDNA gene. A 20-year-old person who carried this mutation in 85% of his mtDNAs displayed a normal pheno-type, whereas a cousin who had the mutation in 96% of his mtDNAs was severely affected. In diseases caused by mutations in mtDNA, the severity of the disease is frequently related to the proportion of mutant mtDNA sequences inherited at birth.

A number of traits encoded by organellar DNA have been studied. One of the first to be examined in detail was the phe-notype produced by petite mutations in yeast (I Figure 20.6). In the late 1940s, Boris Ephrussi and his colleagues noticed that, when grown on solid medium, some colonies of yeast were much smaller than normal. Examination of these petite colonies revealed that growth rates of the cells within the colonies were greatly reduced. The results of biochemical studies demonstrated that petite mutants were unable to carry out aerobic respiration; they obtained all of their energy from anaerobic respiration (glycolysis), which is much less efficient than aerobic respiration and results in the smaller colony size.

Some petite mutations are inherited from both parents and are defects in nuclear DNA. However, most petite mutations are inherited from only a single parent; such mutants possess large deletions in mtDNA or, in some cases, are missing mtDNA entirely. Because much of their mtDNA encodes enzymes that catalyze aerobic respiration, the petite mutants are unable to carry out aerobic respiration and therefore cannot produce normal quantities of ATP, which inhibits their growth.

20.6 The petite mutants have large deletions in their mtDNA and are unable to carry out oxidative phosphorylation. Electron micrograph of mitochondria in (a) a normal yeast cell and (b) a petite mutant. (Part a, David M. Phillips/Visuals Unlimited; part b, .)

Another known mtDNA mutation occurs in Neu-rospora. Isolated by Mary Mitchell in 1952, poky mutants grow slowly, display cytoplasmic inheritance, and have abnormal amounts of cytochromes. Cytochromes are protein components of the electron-transport chain of the mitochondria and play an integral role in the production of ATP. Most organisms have three primary types of cytochromes: cytochrome a, cytochrome b, and cytochrome c. Poky mutants have cytochrome c but no cytochrome a or b. Like petite mutants, poky mutants are defective in ATP synthesis and therefore grow more slowly than normal wild-type cells.

In recent years, a number of genetic diseases that result from mutations in mtDNA have been identified in humans. Leber hereditary optic neuropathy (LHON), which typically leads to sudden loss of vision in middle age, results from mutations in the mtDNA genes that encode electron-transport proteins. Another disease caused by mitochon-drial mutations is neurogenic muscle weakness, ataxia, and tetinitis pigmentosa (NARP), which is characterized by seizures, dementia, and developmental delay. Other mitochondrial diseases include Kearns-Sayre syndrome (KSS) and chronic external opthalmoplegia (CEOP), both of which result in paralysis of the eye muscles, droopy eyelids, and, in severe cases, vision loss, deafness, and dementia. All of these diseases exhibit cytoplasmic inheritance and variable expression (see Chapter 5).

A trait in plants that is produced by mutations in mito-chondrial genes is cytoplasmic male sterility, a mutant phe-notype found in more than 140 different plant species and inherited only from the maternal parent. These mutations inhibit pollen development but do not affect female fertility.

A number of cpDNA mutants also have been discovered. One of the first to be recognized was leaf variegation in the Mirabilis jalapaa, which was studied by Carl Correns in 1909 (see p. 000 in Chapter 5). In the green alga Chlamy-domonas, streptomycin-resistant mutations occur in cpDNA, and a number of mutants exhibiting altered pigmentation and growth in higher plants have been traced to defects in cpDNA.

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