Abstract

DNA is generally considered to be confined to the nucleus. Small amounts of DNA sometimes found in other cell fractions are usually attributed to contamination by nuclear material ( Allfrey 1959). The presence of significant amounts of extranuclear DNA occurring in the cytoplasm of the amphibian oöcyte ( Stich 1962), the kinetosomes ( Clark and Wallace 1960), and in the chloroplasts of plant cells ( Chun et al. 1963) is well documented but is regarded rather as an exception than as a rule, because in these cases DNA is associated with cellular components absent from higher cells. This paper presents evidence that preparations of mitochondria from baker's yeast, purified by flotation in density gradients, contain a significant quantity of DNA. The amount found is far in excess of that accounted for by the polydeoxyribonucleotide component ( Appleby and Morton, 1960) of mitochondrial cytochrome b ₂. Since the mitochondrion represents an organelle present in most cells, including those of mammals, the occurrence of DNA in yeast mitochondria suggests the possibility, that extranuclear DNA is much more common that hitherto suspected Mitochondrial function in the yeast cell is in part controlled by extrachromosomal genetic factors ( Ephrussi and Hottinguer 1951), whose exact chemical nature is unknown. The presence of DNA in yeast mitochondria is thus of special interest.