do 



J. HKKHKHT TAYLOR 



ORIGINAL 



PARENT 



MOLECULE 



FIRST 



GENERATION 



DAUGHTER 



MOLECULES 



SECOND 



GENERATION 



DAUGHTER 



MOLECULES 



Fig. 4. Illustration of the segrogation of DXA chains according to the scheme 

 proposed by Watson and Crick. If the original labeled DNA contains two chains of 

 high density (X"), the daughter molecules after one replication in a medium 

 containing X'^ would have one chain with X" and one with X". After a second 

 rephcation in the X" medium one-half of the molecules would still have one chain 

 with X'^ and one with X", but the other half would have two chains with only X". 

 All molecules after one generation in the X" medium should have a density inter- 

 mediate between the original X" DX"A and the X" DX^A as shown in the fourth 

 frame of Fig. 3. (From Meselson and Stahl, 1958.) 



available that distribution in this same cell line was semi-conservative 

 at the chromosomal level (Taylor, 1960b). More recent studies have 

 shown that the particles of intermediate density from human cells can 

 also be separated into a hea\y and light subunit (Chun and Littlefield, 

 1961). 



A semi-conservative replication has also been demonstrated for phage 

 A (Meselson and Weigle, 1961), phage T7 (Meselson, 1960), and for 

 the alga Chlymadomonas (Sueoka, 1960). 



The most serious challenge to the simple interpretation of expcri- 



