378 



CHAPTER 29 



soma! genes recombine in postmeiotic 

 di\ isions. 



These studios ' suggest the existence of an 

 extensive extrachromosomal gene system in 

 Chlamydomonas. Some of these genes may 

 be located in the DNA of the chloroplast; 

 this DNA is reported to have a base ratio dis- 

 tinctly different from Chlamydomonas nu- 

 clear DNA. 



Mitochondria 



Mitochondria (Fig. 29-6) are organelles 

 consisting of a smooth continuous outer 

 membrane and an inner membrane which 

 folds inward to form double layers or 

 cristae. 10 The outer membrane probably 

 controls permeability; the inner membrane, 

 its cristae, and elementary particles contain 

 most of the insoluble respiratory enzymes 

 whose function provides the main source of 

 energy for the cell. Mitochondria which ap- 

 pear to be dividing transversely have been 

 observed; it is very likely that most, if not 

 all, mitochondria arise from the division of 

 pre-existing mitochondria." DNA is a nor- 

 mal component of mitochondria. 12 This 

 DNA (Figure 29-6) has a unique buoyant 

 density and is probably double-stranded.' ; 



Certain strains of yeast produce tiny colo- 

 nies on agar. When such organisms are 

 crossed with normal-sized individuals, we 

 obtain a two normal to two tiny ratio in the 

 meiotic products after segregation. Such 

 tiny strains due to mutant nuclear genes are 

 called segregational petites. When normal 

 yeast cells are treated with an acridine dye 

 (eutlavin). numerous petite colonies arise. 11 

 When these strains, called vegetative petites, 

 are crossed with normal yeast, the petite 



,J See R. Sager (1965). 



10 See D. F. Parsons (1963). and D. E. Green 



(1964). 



» See D. J. L. Luck (1963). 



J- See M. Chevremont (1963). and G. Schatz, 



E. Haslbrunner, and H. Tuppy (1964). 



,:! See D. J. L. Luck and E. Reich (1964). 



14 See B. Ephrussi (1953). 



phenotype does not segregate regularly. The 

 ease with which vegetative petites are pro- 

 duced by acridine dyes and their subsequent 

 failure to segregate properly provide good 

 evidence that they are caused by extrachro- 

 mosomal mutants. The characteristic slow 

 growth of petites is attributable to the ab- 

 sence of respiratory enzymes known to 

 reside in the mitochondria. Although no 

 change in mitochondrial morphology has 

 been detected in petites. it is clear that the 

 presence of certain mitochondrial enzymes 

 is controlled by chromosomal as well as by 

 extrachromosomal genes. It has not yet 

 been proved, however, that these extrachro- 

 mosomal genes are located in the mitochon- 

 drial DNA. 



In Neurospora a slow-growing strain, 

 poky, when crossed with a wild strain, fails 

 to show segregation and is unlinked to any 

 chromosome. 17. The poky phenotype is ap- 

 parently due to a mutant of an extrachromo- 

 somal gene. In poky individuals certain en- 

 zymes normally present in the mitochondria 

 are altered and so is mitochondrial morphol- 

 ogy. Fusion of hyphae from wild type and 

 poky strains produces a heterocytosome — 

 a mixture of the two kinds of cytoplasm. 

 Such fused hyphae are wild-type at first but 

 later become poky, with the nuclear geno- 

 type having no effect upon the outcome. Is 

 this outcome due to selection favoring an 

 apparently detrimental extrachromosomal 

 gene? We do not know. As in the case of 

 petites, the location of the extrachromosomal 

 genes involved in poky is unknown. 



Centrosomes, Kinetosomes, and Kinetoplasts 



The centrosome is an organelle often found 

 at each pole of a spindle, particularly in ani- 

 mal cells. A granular structure — the cen- 

 triole — is sometimes seen within it; similar 

 granules can sometimes be seen within the 

 centromere (Figure 29-7). The granules 



'■"•See M. B. Mitchell and H. K. Mitchell (1952). 



