Meiosis and Chromosomal Segregation 



27 



germination the haploid ascosporc divides tcrent strains of the same or different mating 

 mitotically and grows to produce the my- type will fuse to form a heterocaryon, hyphal 

 celium. Sometimes the hyphae of two dif- cells containing the nuclei of both strains. 



Mycelium 

 (mating type A) 



Mycelium 

 (mating type Bl 



scospore M 



rmination '"Y'fw 



Ascus with 

 ascospores (N) 



figure 2-9. Life cycle of Neurospora. See text for description. 



SUMMARY AND CONCLUSIONS 



Meiosis involves two successive, essentially mitotic divisions modified by the occurrence 

 of synapsis and of chiasmata formation during prophase I. and the nonoccurrence of 

 chromosomal replication during interphase I. Any particular chromosome in a genome 

 of a gamete has an equal chance of having a replica of a maternally- or a paternally- 

 derived centromere (because of the random manner in which different bivalents align 

 themselves on the spindle at metaphase I) and usually contains replicas of segments 

 originally derived from the other parent (as revealed by chiasmata). As a conse- 

 quence of meiosis the original pair of genomes becomes single in the gametes. Not 

 only do homologous chromosomal segments segregate, but chromosomal segments of 

 nonhomologs segregate independently. 



