SUMMARY / RELATED TOPICS

Maternal to zygotic transition

Maternal to zygotic transition is the stage in embryonic development during which development comes under the exclusive control of the zygotic genome rather than the maternal genome. The egg contains stored maternal genetic material mRNA which controls embryo development until the onset of MZT. After MZT the diploid embryo takes over genetic control; this requires both zygotic genome degradation of maternal products. This process is important because it is the first time that the new embryonic genome is utilized and the paternal and maternal genomes are used in combination; the zygotic genome now drives embryo development. MZT is thought to be synonymous with midblastula transition, but these processes are, in fact, distinct. However, the MBT coincides with ZGA in many metazoans, thus may share some common regulatory features. For example, both processes are proposed to be regulated by the nucleocytoplasmic ratio. MBT refers to changes in the cell cycle and cell motility that occur just prior to gastrulation.

In the early cleavage stages of embryogenesis, rapid divisions occur synchronously and there are no "gap" stages in the cell cycle. During these stages, there is little to no transcription of mRNA from the zygotic genome, but zygotic transcription is not required for MBT to occur. Cellular functions during early cleavage are carried out by maternal products – proteins and mRNAs contributed to the egg during oogenesis. To begin transcription of zygotic genes, the embryo must first overcome the silencing, established; the cause of this silencing could be due to several factors: chromatin modifications leading to repression, lack of adequate transcription machinery, or lack of time in which significant transcription can occur due to the shortened cell cycles. Evidence for the first method was provided by Newport and Kirschner's experiments showing that nucleocytoplasmic ratio plays a role in activating zygotic transcription, they suggest that a defined amount of repressor is packaged into the egg, that the exponential amplification of DNA at each cell cycle results in titration of the repressor at the appropriate time.

Indeed, in Xenopus embryos in which excess DNA is introduced, transcription begins earlier. More evidence has been shown that transcription of a subset of genes in Drosophila is delayed by one cell cycle in haploid embryos; the second mechanism of repression has been addressed experimentally. Prioleau et al. show that by introducing TATA binding protein into Xenopus oocytes, the block in transcription can be overcome. The hypothesis that shortened cell cycles can cause repression of transcription is supported by the observation that mitosis causes transcription to cease.. The accepted mechanism for the initiation of embryonic gene regulatory networks in mammals is that there are multiple waves of MZT. In mouse, the first of these occurs in the zygote, where expression of a few pioneering transcription factors increases the expression of target genes downstream; this induction of genes leads to a second major MZT event To eliminate the contribution of maternal gene products to development, maternally-supplied mRNAs must be degraded in the embryo.

Studies in Drosophila have shown that sequences in the 3' UTR of maternal transcripts mediate their degradation These sequences are recognized by regulatory proteins that cause destabilization or degradation of the transcripts. Recent studies in both zebrafish and Xenopus have found evidence of a role for microRNAs in degradation of maternal transcripts. In zebrafish, the microRNA miR-430 is expressed at the onset of zygotic transcription and targets several hundred mRNAs for deadenylation and degradation. Many of these targets are genes. In Xenopus, the miR-430 ortholog miR-427 has been shown to target maternal mRNAs for deadenylation. MiR-427 targets include cell cycle regulators such as Cyclin A1 and Cyclin B2

Var─Śna

Varėna is a city in Dzūkija, Lithuania. The town was founded in 1862 near the Warsaw – Saint Petersburg Railway, 4 km south of Sena Varėna. At that time it was a small settlement, but following steady development it became the center of the district. In the interbellum period, following World War I, the town was annexed by Poland, renamed Orany, it was located near the Polish-Lithuanian border, in the Wilno-Troki County of the Wilno Voivodeship. In 1939, following the German-Soviet Invasion of Poland, Varėna was returned to Lithuania. On September 9, 1942 all the Jews of the town of Varėna were collected in the local synagogue. On that day though the Germans had tried to prevent him from doing so, the priest Jonas Gylys entered the synagogue and encouraged the Jews to be brave in their last hours rather than convert to Christianity. On the following day all of the Jews were taken from the synagogue to Eserekiai - a grove of trees near the village of Druckūnai, 1.5 kilometres from the town, on the side of the road leading to the village.

Two large pits had been dug there 25 metres apart, one for the women. Lithuanian nationalists shot them there; the murderers looted the possessions of their victims. According to the report of Karl Jaeger, commander of Einsatzkommando 3A, 831 Jews from Varėna – 541 men, 149 women, 141 children – were killed on that day. In 1946 around 2000 Poles were repatriated to Poland. Following industrialization in the 1970s, the town grew rapidly. There are 9,240 residents in Varėna; the Varėna district is the largest and most forested region in Lithuania. Lithuanian painter and composer Mikalojus Konstantinas Čiurlionis was born in Sena Varėna in 1875. Varėna is twinned with: Mikołajki, Poland The murder of the Jews of Varėna during World War II, at Yad Vashem website