Abstract
Early embryonic cleavages are characterized by short and highly synchronous cell cycles made of alternating S- and M-phases with virtually absent gap phases. In this contracted cell cycle, the duration of DNA synthesis can be extraordinarily short. Depending on the organism, the whole genome of an embryo is replicated at a speed that is between 20 to 60 times faster than that of a somatic cell. Because transcription in the early embryo is repressed, DNA synthesis relies on a large stockpile of maternally supplied proteins stored in the egg representing most, if not all, cellular genes. In addition, in early embryonic cell cycles, both replication and DNA damage checkpoints are inefficient. In this article, we will review current knowledge on how DNA synthesis is regulated in early embryos and discuss possible consequences of replicating chromosomes with little or no quality control.
Highlights
The early embryonic cell cycles of most metazoans are usually contracted compared to those of somatic cells [1]
In the majority of animals, embryonic cell divisions are very rapid and highly synchronous including a replication phase (S-phase) and a division phase (M-phase), with short or absent intermediate G1- and G2- phases [3]. These amazingly fast embryonic cell cycles, typical of animals with external development, can be explained as an adaptation to ensure the subsistence of laid eggs in the hostile external environment and the need to proceed to the hatching stage as quickly as possible
Reactivation of of cellular from synthesis and translation, several cellular processes are inactive during the early stages from DNA synthesis and translation, several cellular processes are inactive during the early stages of of development
Summary
The early embryonic cell cycles of most metazoans are usually contracted compared to those of somatic cells [1]. In the majority of animals, embryonic cell divisions are very rapid and highly synchronous (with some exceptions [2]) including a replication phase (S-phase) and a division phase (M-phase), with short or absent intermediate G1- and G2- (gap) phases [3]. If one may think that replicating the Xenopus genome in such a short time is a fast process, it is even more astonishing to find out that the Drosophila genome is replicated in less than 4 min [5] These observations raise the following questions: What makes DNA synthesis so fast in these embryos?.
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