CHROMOSOME DUPLICATION AND GENETIC RECOMBINATION 



185 



occur. Cytochemical studies for the most part indicate that DNA repli- 

 cation is completed before chromosome pairing takes place; if so, recom- 

 bination would necessarily be a consequence of breakage and exchange 

 (chiasmatype theory), and not of events occurring during replication 

 (copy-choice). A recent proposal, that recombination results from local- 

 ized transient pairing during replication which is undetectable cytologi- 

 cally, followed by conventional pachytene pairing later, avoids the timing 

 difficulty. This possibility will be discussed below (p. 202). 



In healthy, rapidly multiplying bacterial cultures, DNA is synthesized 

 continuously (or almost so) as demonstrated by the uniform rate of up- 

 take and incorporation of radioactive thymidine into DNA by all the 



80 



70 



60 



S 50 



o 



!40 



0) 



a. 



< 



30 - 



20 

 10 



4C -^ 



2C 



= \C 



10 15 20 25 30 35 

 Days of Development 



40 



45 



50 



A = Premeiosis 



B = Meiotic prophase 



C = Meiotic divisions I and II 



D = Microspore interphase 



E = Mitosis 



F = Pollen interphase 



■ = Times of uptake of P^^ jp^o DNA 



FIGURE 7.2. Relative DNA content of nuclei during meiosis and mitosis. In the 

 anther of the lily, Lilium longiflorum, the DNA content per nucleus was determined 



photometrically by Feulgen staining. Anthers exposed to P" 



'O4 incorporated 



radioactivity into their DNA only at the specific times when chromosome duplication 

 occurred, as shown by grain counts of autoradiographs correlated with photometric 

 determinations of DNA content and with stage of development (after Taylor and 

 McMaster, 1954, Chromosomo, 6:489). 



