Since blastular cells differ in size within the Xenopus embryo, how can nuclear/cytoplasmic ratios regulate transcription?
Because Drosophila genetics has uncovered a number of zygotic lethal mutations, a number of markers of early zygotic transcription have been discovered. These represent the gap and pair rule genes which will be discussed in greater detail in the next section of lectures. By definition, zygotic lethal genes represent anything that is lethal when homozygous. The earliest zygotic lethals such as the gap gene Krupple and the pair rule gene fushi tarazu (ftz) are expressed as early as cell cycle 10, when the cellular blastoderm is forming. Expression is of these genes is detected by in situ hybridization; by cycle 10 both genes appear to be expressed according to their normal pattern. Interestingly, expression can be detected as early as cell cycle 8; however at this time expression is limited to a few scattered nuclei in the periphery of the embryo. This result suggests that while the bulk of early transcription occurs at cycle 10, the regulation of zygotic transcription is leaky, and tran
