In this article we will discuss about the meaning of mitochondria.
Mitochondria are cytoplasmic organelles containing their own DNA and some enzymes that catalyse the process of oxidative phosphorylation. Mitochondria are not autonomous bodies because they require their own genes and nuclear genes to function normally.
The role of mitochondria in cytoplasmic inheritance was first understood when Ephrussi and his colleagues (1955) studied petite mutants (petite meaning small) in yeast (Saccharomyces cerevisiae). When a cell divides, approximately equal numbers of mitochondria pass into daughter cells. They can originate from pre-existing mitochondria, and can probably divide transversely.
Several mutant strains of yeast are referred to as petite mutants because they form smaller colonies on agar as compared to normal yeast. The growth rate of petites is slow due to absence of respiratory enzymes, cytochromes a, b and c, and due to deficiency in some dehydrogenases present in normal mitochondria.
The petite strains are of three types. In the first type called segregational petites when an individual is crossed with a normal strain, a 1:1 ratio of normal: petite results after segregation. This suggests Mendelian inheritance and the petite strain has originated due to a mutation in nuclear genes.
The second and third types of petite strains arise when normal yeast cells are treated with the acridine dye euflavin or with ethidium bromide dyes which are known to intercalate in double stranded DNA.
The petites so formed do not segregate regularly when crossed with normal yeast, and fall under two types—neutral and suppressive. When a neutral petite is crossed with a normal type, all the progeny in the next and successive generations are normal and the petite trait never reappears.
If a haploid cell of the neutral petite strain is mated to a haploid vegetative cell of a normal strain, a diploid zygote is formed. The diploid cells produced by this zygote reproduce asexually, sometimes may even divide meiotically to form spores, but the petite trait is never visible in the progeny. The genetic explanation of the trait is that there is a cytoplasmic factor (p+) which is present in the normal strains of yeast but absent in petites (p–).
The suppressive petites when crossed to normal cells of yeast show the petite trait in the progeny but in non-mendelian ratios. Suppressive petite mutants are found to have mutant DNA in their mitochondria. The mutant mitochondria replicate and transmit the mutant phenotype to the progeny cells. Mitochondrial mutants are also known in Neurospora, Paramecium and Trypanosoma.