TIMING OF INCORPORATION OF TRITIATED NUCLEOSIDES INTO DNA AND RNA OF EMBRYONIC CELLS OF RANA PIPIENS

The incorporation of tritiated nucleosides into DNA and RNA has been examined in partially synchronized cells of Rana pipiens embryos at the neurula and tailbud stages. Tritiated thymidine and deoxyguanosine are incorporated into the DNA in two maxima, or waves, during the S phase at both stages. More DNA replicates in the early maximum at the neurula stage than at the tailbud stage. A comparison of the degree of incorporation of labelled deoxyguanosine to labelled thymidine into DNA suggests that earlier replicating DNA at both stages may be GC-rich compared to later replicating DNA. The incorporation of tritiated uridine into RNA during the S phase also differs between the neurula and tailbud stages. Pulse and continuous label experiments indicate that at the neurula stage the highest rate of RNA synthesis occurs late in the S phase whereas at the tailbud stage the higher rate of RNA synthesis has shifted to an interval earlier in the S phase.