Chromosome duplication first of all requires the replication of the DNA. It is now clear that the replication of most if not all of the chromosomal DNA follows the scheme based on precise base pairing as originally proposed by Watson and Crick [50]. The pattern of replication was first clearly indicated by experiments showing the semiconservative distribution of chromosomal DNA labeled with H3-thymidine (Taylor et al. [48]) and later verified by density gradient centrifugation studies with Escherichia coli DNA labeled with the heavy isotope of nitrogen (Meselson and Stahl [24]). Similar results were obtained with phage T 7 and λ [23, 25] and with the alga, Chlamydomonas (Sueoka [38]). Subsequently, semiconservative distribution of DNA in mammalian cells was demonstrated by the use of bromouracil as a density label (Djordjevic and Szybalski [7]; Simon [36]; Chun and Littlefield [6]). Recently semiconservative replication of DNA has been demonstrated in higher plant cells with N15 as a density label (Filner [8]) and also with bromouracil as a density label (Haut and Taylor [13]).