Transcription and degradation of heavy (pre 28S)RNA and 4S RNA during embryogenesis in Limnaea.

Short (15 min) and long (1 hour or more) pulses with P32 have been compared at different developmental stages of Limnaea. In short pulses, as opposed to long-pulses, the relative percentage of 4S RNA is at a more or less constant level at all stages of development. Heavy RNA, i. e. heavier than 28S, is traceable with both short and long pulses in the uncleaved and early morula stages. Transcription of heavy RNA seems to decrease significantly immediately before mitosis. No other RNA fraction shows such a marked rhythm associated with mitosis.