Genetic structure and function / P.F. Smith Keary.

  • Smith-Keary, P. F.
Date:
1975
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetic structure and function / P.F. Smith Keary. Source: Wellcome Collection.

    122/392 (page 101)
    THE GENETICS OF FUNGI 103 DNA is transcribed translated and replicated just as is chromosomal DNA. These DNA molecules vary from 5 jum (animals) up to 30 ¡im (Neuwspora or Saccharomyces) and it is estimated that each organelle may contain up to twenty molecules of DNA. Only a very limited number of genes could be accommodated on such a small molecule of DNA and there is very good evidence that the structural genes for at least some of the mitochondrial proteins are located on the chromosomes. One of the best-known examples of extra-chromosomal inheritance is the petite character of yeast, first studied by Boris Ephrussi and his colleagues (1950). In cultures of S. cerevisiae about 0.5-1 per cent of the cells give rise to colonies that are much smaller than normal; these petites lack certain respiratory enzymes and two cytochromes and so cannot grow aerobically. Although the petite character is quite stable Ephrussi found that it was immediately lost on crossing and that all four ascospores grew into normal sized colonies; in the same spontaneous segregation 1 in 100—200 cells grows into a petite colony owing to the segregation of mitochondria containing only mutant dna X the petite is crossed with normal yeast of a mating type meiosis each diploid cell \% Aa and has received some wild-type mitochondria from the normal parent ascospores germination 1 I I each ascospore has received at least one wild-type mitochondrion from the diploid parent cell and so grows into a normal sized colony. There is normal 2:2 segregation for the chromosomally located mating-type genes. Figure 6.6 Inheritance of the 'petite' character in Saccharomyces cerevisiae.