developmental biology Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of regeneration, asexual reproduction, metamorphosis, and the growth and differentiation of st ...

, midblastula or midblastula transition (MBT) occurs during the

blastula Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (fro ...

stage of

embryonic development An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

in non-mammals. During this stage, the embryo is referred to as a blastula. The series of changes to the blastula that characterize the midblastula transition include activation of zygotic

gene transcription Transcription is the process of copying a segment of DNA into RNA. The segments of DNA transcribed into RNA molecules that can encode proteins are said to produce messenger RNA (mRNA). Other segments of DNA are copied into RNA molecules called ...

, slowing of the

cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and sub ...

, increased asynchrony in cell division, and an increase in cell motility.

Blastula Before MBT

Before the embryo undergoes the midblastula transition it is in a state of fast and constant replication of cells. The

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 In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein. mRNA is created during the ...

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 ( genetic material) is not being used because it is repressed through a variety of mechanisms such as

methylation In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These ...

. This repressed DNA is sometimes referred to as

heterochromatin Heterochromatin is a tightly packed form of DNA or '' condensed DNA'', which comes in multiple varieties. These varieties lie on a continue between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a rol ...

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.

Activation of Zygotic
Gene Transcription Transcription is the process of copying a segment of DNA into RNA. The segments of DNA transcribed into RNA molecules that can encode proteins are said to produce messenger RNA (mRNA). Other segments of DNA are copied into RNA molecules called ...

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.

Cell Cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and sub ...
Changes

When the zygote begins to produce its own mRNA, the cell cycle begins to slow down and the G1 and

G2 phase G2 phase, Gap 2 phase, or Growth 2 phase, is the third subphase of interphase in the cell cycle directly preceding mitosis. It follows the successful completion of S phase, during which the cell’s DNA is replicated. G2 phase ends with th ...

s 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.

Cell Motility Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...

Timing of 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

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s in the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

. 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 (a paralog of mammalian POU5F1 (OCT4) has an analogous role.{{cite journal, last1=Harrison, first1=MM, last2=Li, first2=XY, last3=Kaplan, first3=T, last4=Botchan, first4=MR, last5=Eisen, first5=MB, date=Oct 2011, title=Zelda binding in the early Drosophila melanogaster embryo marks regions subsequently activated at the maternal-to-zygotic transition, journal=PLOS Genet, volume=7, issue=10, page=e1002266, doi=10.1371/journal.pgen.1002266, pmid=22028662, pmc=3197655 Without the function of these proteins MBT gene expression synchrony is disrupted, but particular mechanisms of coordinating the timing of gene expression are still unknown but being studied.

References

Animal developmental biology