Zygote

Zygote
Details
Days	0
Precursor	Gametes
Gives rise to	Morula
Identifiers
Gray's	p.45
Code	TE E2.0.1.2.0.0.9
Anatomical terminology

A zygote (from Greek ζυγωτός zygōtos "joined" or "yoked", from ζυγοῦν zygoun "to join" or "to yoke"),^[1] is 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.

In multicellular organisms, 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.

Oscar Hertwig and Richard Hertwig made some of the first discoveries on the zygote formation.

Animals

An animal zygote exists as a single cell before dividing into two blastomeres^[2] approximately 30 hours after fertilization (in humans). This pair of cells is also sometimes called the zygote, or simply the two-cell stage.

It travels down the oviduct, while continuing to divide^[3] mitotically, without actually increasing in size (cleavage).^[4] After these subsequent cell divisions, the mass of cells is called the compacting embryo. After four divisions the embryo consists of 16 cells, and it is known as the morula.^[5] By the fifth day of development, the blastocyst emerges as a result of further divisions and transformations (blastulation).^[6]

In other species

A Chlamydomonas zygote that contains chloroplast DNA (cpDNA) from both parents, such cells generally are rare since normally cpDNA is inherited uniparental from the mt+ mating type parent.These rare biparental zygotes allowed mapping of chloroplast genes by recombination.

See also

• Proembryo

References