Zygotes

A zygote (from Greek ζυγωτός zygōtos "joined" or "yoked", from ζυγοῦν zygoun "to join" or "to yoke"), or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. It is the earliest developmental stage of the embryo.

A zygote is always synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development. Zygotes are usually produced by a fertilization event between two haploid cells—an ovum from a female and a sperm cell from a male—which combine to form the single diploid cell. Such zygotes contain DNA derived from both the mother and the father, and this provides all the genetic information necessary to form a new individual. The term zygote is also used more loosely to refer to the group of cells formed by the first few cell divisions, although this is properly referred to as a morula.

In mammalian reproduction, after fertilization has taken place the zygote travels down the fallopian tube, while dividing to form more cells without the zygote actually increasing in size. This cell division is mitotic, and is known as cleavage. All mammals go through the zygote stage of life. Mammalian zygotes eventually develop into a blastocyst, after which they are more generally termed an embryo, and then a fetus.

A human zygote exists for about four days, and becomes a blastocyst on the fifth day. (From wikipedia)

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