Describe the inheitance pattern that gives rise to "poky" yeast.

Expert Solution

Poky yeast is a mutant with defective mitochondrial function, they are also called petite mutants. Such mutants have sustained gross deletions or total loss of mtDNA and are, in consequence, incapable of assembling mitochondrial ribosomes and carrying out mitochondrial protein synthesis. Petite mutants have been of particular value in investigating the role of mtDNA, establishing which mitochondrial proteins are synthesized extramitochondrially and constructing gene maps of mtDNA.

The mitochondrial heredity has been exemplified by yeast (Saccharomyces cereviceae) and Neurospora crassa.

Strains of yeast S. cereviceae produce tiny colonies when grown on agar medium.In his experiments Ephrussi (1953) observed that one or two out of every one thousand colonies were only about one-third or one- half of the diameter of the remainder. The small colonies are termed as petite colonies.

Cells from the normal large colonies, when spread on culture medium, further produced a small proportion of petite colonies and this happened so time after time. The cells from the small colonies were true breeding and they produced only petites.

The petite phenotype can result either from mutation of nuclear genes or from mitochondrial genes. This type of petite mutation is called segregational petite or nuclear petite. Petite mutants resulted due to mutation in a nuclear gene follow Mendelian pattern of inheritance with segregation occurring in heterozygotes.

When the individuals of petite colony are crossed to the individuals from normal large sized colony, normal zygotes are formed which produce normal cells vegetatively. When meiosis takes place in diploid cells, haploid cells are recovered that will form petite and normal colonies in 1: 1 ratio as shown below:

Any colony formed evidently due to mutant nuclear genes, they are called segregational petites.

Following biochemical studies have established that the slow growth of petite colonies was due to the loss of aerobic respiratory enzymes particularly cytochrome a and b and enzyme cytochrome oxidase occurring in mitochondria of the cells and the utilization of the less efficient fermentation process by the cells.

gladiator answered 1 year ago

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