Question

An animal is undergoing a cleavage pattern that is bilateral and mesoblastic and, after closely examining its cleavage pattern, synchronous division is lost after the third cellular division. What might be causing this loss of synchronous division??

Answer 1

Fertilization leads to the formation of a single-celled zygote which has a high cytoplasm to nucleoplasm ratio. This leads to rapid division of this zygote without cell entering G1 and G2 phases. As a result, each successive daughter cell has a lower cytoplasmic to the nucleoplasmic ratio. However, this happens only until a particular ratio is achieved. This drive towards achieving a stable cytoplasmic to nucleoplasmic ratio is what scientists believe drives the rapid earlier divisions. Hence these divisions are synchronous. The zygotic genes are kept silent for the period and only the maternal mRNA is transcribed. However, once the desired ratio is achieved, the zygotic transcription begins. Thus, the control shifts from maternal mRNA to the zygotic DNA and all the cells begin expression at varied time scales. This is what is referred to as the midblastula transition. The cell divisions that occur during this period are the first asynchronous divisions. This gives polarity to the developing embryo which allows the layout of anterior-posterior, dorso-ventral and eventually left-right axes.