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 afertilization 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. A zygote is formed by the fusion of sperm and ovum in oviduct. In an animal during the first few cellular divisions, a zygote is properly referred to as a morula, or mass of cells. A zygote undergoes cellular division for four days, and on the fifth day it forms a blastocyst.
In animal development, 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 ismitotic, 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 afetus.
A human zygote exists as a single cell before undergoing cleavage, forming blastomeres, and becomes a blastocyst on the fifth day.
IN OTHER SPECIES
A biparental zygote is a Chlamydomonas (a kind of algae) zygote that contains chloroplast DNA (cpDNA) from both parents.