A zygote (from Greek ζυγωτός zygōtos "joined" or "yoked", from ζυγοῦν zygoun "to join" or "to yoke"), 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. In single-celled organisms, the zygote divides to produce offspring, usually through Mitosis, the process of cell division.
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 (female gamete) and a sperm cell (male gamete)—which combine to form the single diploid cell. Such zygotes contain DNA derived from both parents, and this provides all the genetic information necessary to form a new individual. In land plants, the zygote is formed within a chamber called the archegonium. In seedless plants, the archegonium is usually flask-shaped, with a long hollow neck through which the sperm cell enters. As the zygote divides and grows, it does so inside the archegonium. With onset of the first cellular divisions, an animal zygote transforms into a morula, or a mass of cells.
In mammalian reproduction, after fertilization has taken place the zygote travels down the fallopian tube, while dividing to form more cellsO’Reilly, Deirdre. " Fetal development". MedlinePlus Medical Encyclopedia (2007-10-19). Retrieved 2009-02-15. without the zygote actually increasing in size. This cell division is mitotic, and is known as cleavage.Klossner, N. Jayne and Hatfield, Nancy. cleavage blastocyst&as_brr=3&ei=mJaYSbj2CZjEM-Hc9KQJ Introductory Maternity & Pediatric Nursing, p. 107 (Lippincott Williams & Wilkins, 2006). 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 as a single cell before undergoing cleavage, forming , Blastomere Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica Inc., 2012. Web. 06 Feb. 2012. and becomes a blastocyst on the fifth day.Blackburn, Susan. Maternal, Fetal, & Neonatal Physiology, p. 80 (Elsevier Health Sciences 2007).