Before the embryo undergoes the midblastula transition it is in a state of fast and constant replication of cells. The cell cycle is very short. The cells in the zygote are also replicating synchronously, always undergoing cell division at the same time. The zygote is not producing its own mRNA but rather it is using mRNAs that were produced in the mother and loaded into the oocyte in order to produce proteins necessary for zygotic growth. The zygotic DNA is not being used because it is repressed through a variety of mechanisms such as methylation. This repressed DNA is sometimes referred to as heterochromatin and is tightly packed together inside the cell because it is not being used for transcription.
Characteristics of the MBT
Before the zygote undergoes the midblastula transition it is in a state of fast and constant replication of cells.
At this stage, the zygote starts producing its own mRNAs that are made from its own DNA, and no longer uses the maternal mRNA. This can also be called the maternal to zygotic transition. The maternal mRNAs are then degraded. Since the cells are now transcribing their own DNA, this stage is where expression of paternal genes is first observed.
When the zygote begins to produce its own mRNA, the cell cycle begins to slow down and the G1 and G2 phases are added to the cell cycle. The addition of these phases allows the cell to have more time to proofread the new genetic material it is making to ensure there are no mutations. The asynchronous nature of the cell divisions is an important change that occurs during/after the MBT.
The timing of MBT varies between different organisms. Zebrafish MBT occurs at cycle 10, but it occurs at cycle 13 in both Xenopus and Drosophila. Cells are thought to time the MBT by measuring the nucleocytoplasmic ratio, which is the ratio between the volume of the nucleus, which contains DNA, to the volume of cytosol. Evidence for this hypothesis comes from experiments showing that the timing of MBT can be sped up by adding extra DNA to make the nucleus larger, or by halving the amount of cytoplasm. The exact methods by which the cell achieves this control is unknown, but it is thought to involve proteins in the cytosol. In Drosophila, the zinc-finger transcription factor Zelda is bound to regulatory regions of genes expressed by the zygote, and in zebrafish, the homeodomain protein Pou5f3 has an analogous role. Without the function of these proteins MBT gene expressionsynchrony is disrupted, but particular mechanisms of coordinating the timing of gene expression are still unknown but being studied.
