|Gives rise to||Morula|
A zygote (from Greek ζυγωτός zygōtos "joined" or "yoked", from ζυγοῦν zygoun "to join" or "to yoke"), is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information necessary to form a new individual. In multicellular organisms, the zygote is the earliest developmental stage. In single-celled organisms, the zygote can divide asexually by mitosis to produce identical offspring.
In fungi, the sexual fusion of haploid cells is called karyogamy. The result of karyogamy is a diploid cell called a zygote or zygospore. This cell may then enter meiosis or mitosis depending on the life cycle of the species.
In plants, the zygote may be polyploid if fertilization occurs between meiotically unreduced gametes.
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.
In human fertilization, a release ovum (a haploid secondary oocyte with replicate chromosome copies) and a haploidsperm cell (male gamete)—combine to form a single 2n diploid cell called the zygote. Once the single sperm enters the oocyte, it completes the division of the second meiosis forming a haploid daughter with only 23 chromosomes, almost all of the cytoplasm, and the sprem in its own pronucleus. The other product of meiosis II is the second polar body with only chromosomes but no ability to replicate or survive. In the fertilized dughter, DNA is then replicated in the two separate pronuclei derived from the sperm and ovum, making the zygote's chromosome number temporarily 4n diploid. After approximately 30 hours from the time of fertilization, fusion of the pronuclei and immediate mitotic division produce two 2n diploid daughter cells called blastomeres.
Between the stages of fertilization and implantation, the developing human is called the preimplantation conceptus or the proembryo. It is not correct to call the conceptus an embryo, because it will later differentiate into both intraembryonic and extraembryonic tissues, and can even split to produce multiple embryos (identical twins).
After fertilization, the conceptus travels down the oviduct towards the uterus while continuing to divide mitotically without actually increasing in size, in a process called cleavage. After four divisions, the conceptus consists of 16 blastomeres, and it is known as the morula. Through the processes of compaction, cell division, and blastulation, the conceptus takes the form of the blastocyst by the fifth day of development, just as it approaches the site of implantation. When the blastocyst hatches from the zona pellucida, it can implant in the endometrial lining of the uterus and begin the embryonic stage of development.
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.
In the amoeba, reproduction occurs by cell division of the parent cell: first the nucleus of the parent divides into two and then the cell membrane also cleaves, becoming two "daughter" Amoebae.
- "English etymology of zygote". myetymology.com.
- Blastomere Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica Inc., 2012. Web. 06 Feb. 2012.
- Larsen's Human Embryology. 4th Ed. Page 4.
- O’Reilly, Deirdre. "Fetal development". MedlinePlus Medical Encyclopedia (2007-10-19). Retrieved 2009-02-15.
- Klossner, N. Jayne and Hatfield, Nancy. Introductory Maternity & Pediatric Nursing, p. 107 (Lippincott Williams & Wilkins, 2006).
- Neas, John F. "Human Development" Archived July 22, 2011, at the Wayback Machine.. Embryology Atlas
- Blackburn, Susan. Maternal, Fetal, & Neonatal Physiology, p. 80 (Elsevier Health Sciences 2007).
- Human zygote edited genetically
Oocyte + Sperm
|Stages of human development
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