In developmental biology

Developmental biology

Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...

, midblastula or midblastula transition (MBT) is a stage during embryonic development in which zygotic

Zygote

A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...

 gene transcription is activated. There are three major characteristics of pre-MBT embryos. Firstly, all of the embryonic cells undergo cell division

Cell division

Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...

 at the same time. Secondly, zygotic chromatin

Chromatin

Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...

 is condensed, hypo-acetylated and H3 methylated

Histone methylation

Histone methylation is the modification of certain amino acids in a histone protein by the addition of one, two, or three methyl groups. In the cell nucleus, DNA is wound around histones...

, indicating that most of the genes are in a repressed heterochromatic

Heterochromatin

Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin...

 state. Finally, embryos are observed to translate

Translation (genetics)

In molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...

 only maternally inherited mRNA, i.e. that mRNA which is present in the oocyte

Oocyte

An oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...

 when it is fertilised. The mRNA is localised in different parts of the oocyte, so that as the embryo divides it is segregated

Mendelian inheritance

Mendelian inheritance is a scientific description of how hereditary characteristics are passed from parent organisms to their offspring; it underlies much of genetics...

 into different cells. This segregation is thought to underlie much of the differentiation of cells that occurs after MBT. Once MBT has taken place, the embryo begins to transcribe its own DNA, cells

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 become motile and cell division becomes asynchronous. Since the cells are now transcribing their own DNA, this stage is where differential expression of paternal genes is first observed.

Timing

The timing of MBT varies between different organisms. Zebrafish MBT occurs at cycle 10, whilst it occurs at cycle 13 in both Xenopus and Drosophila. Cells are thought to time the MBT by measuring the nucleocytoplasmic ratio, which is effectively the ratio between the volume of cytosol and the amount of DNA. Evidence for this hypothesis comes from the observation that the timing of MBT can be sped up by adding extra DNA, or by halving the amount of cytoplasm. The exact method by which the cell achieves this control is unknown, but it is thought to involve a cytosolic protein.