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

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Abstract
Incorporation of 3H nucleosides into DNA and RNA was examined in partially synchronized cells of R. pipiens embryos as the neurula and tailbud stages. 3H-thymidine and deoxyguanosine are incorporated into the DNA in 2 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 degree of incorporation of labeled deoxyguanosine to labelled thymidine into DNA suggests that earlier replicating DNA at both stages may be GC-rich compared to later replicating DNA. Incorporation of 3H-uridine into RNA druing the S phase differs between the neurula and tailbud stages. Pulse and continous 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.