Synthesis of DNA during meiosis.

Chromosome pairing and crossing-over between homologous chromosomes in meiotic cells are two distinctive events which must in some way involve the DNA moiety of the chromosome. Ever since the uinambiguous demonstration by Taylor and McMaster' that DNA synthesis occurs during premeiotic iilterphase, explailations of pairing and crossing-over in terms of DNA synthesis during meiotie prophase have been more or less laid aside. At least onie case of thymidine incorporation during meiotic prophase, has been reported.2 However, this report has beenl treated casually, either because of the very few silver grains in the radioautographs or because of the possibility that the synthesis was no more than the extension of an otherwise conventional S-period. (S-period refers to period of DNA synthesis.) This communication demonlstrates the actuality of DNA syinthesis during meiotic prophase and its separation in time from the premeiotic S-period. The materials used in these studies were the microsporocytes of liliaceous plants. The advantages which such materials offer for a correlated study of biochemical and cytological events have been discussed in several earlier publications.3