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A zygote (, ) is a
eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
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 of a new individual organism. In multicellular organisms, the zygote is the earliest developmental stage. In humans and most other
anisogamous Different forms of anisogamy: A) anisogamy of motile cells, B) egg_cell.html"_;"title="oogamy_(egg_cell">oogamy_(egg_cell_and_sperm_cell),_C)_anisogamy_of_non-motile_cells_(egg_cell_and_spermatia)..html" ;"title="egg_cell_and_sperm_cell.html" ;" ...
organisms, a zygote is formed when an egg cell and sperm cell come together to create a new unique organism. In single-celled organisms, the zygote can divide asexually by mitosis to produce identical offspring. German zoologists
Oscar Oscar, OSCAR, or The Oscar may refer to: People * Oscar (given name), an Irish- and English-language name also used in other languages; the article includes the names Oskar, Oskari, Oszkár, Óscar, and other forms. * Oscar (Irish mythology) ...
and
Richard Hertwig Richard Wilhelm Karl Theodor Ritter von Hertwig (23 September 1850 in Friedberg, Hesse – 3 October 1937 in Schlederloh, Bavaria), also Richard Hertwig or Richard von Hertwig, was a German zoologist and professor of 50 years, notable as the fir ...
made some of the first discoveries on animal zygote formation in the late 19th century.


Humans

In
human fertilization Human fertilization is the union of a human egg and sperm, occurring primarily in the ampulla of the fallopian tube. The result of this union leads to the production of a fertilized egg called a zygote, initiating embryonic development. Scient ...
, a released ovum (a haploid secondary oocyte with replicate chromosome copies) and a haploid sperm cell ( male gamete) combine to form a single
diploid cell Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
called the zygote. Once the single sperm fuses with the oocyte, the latter completes the division of the second meiosis forming a haploid daughter with only 23 chromosomes, almost all of the cytoplasm, and the male
pronucleus A pronucleus () is the nucleus of a sperm or egg cell during the process of fertilization. The sperm cell becomes a pronucleus after the sperm enters the ovum, but before the genetic material of the sperm and egg fuse. Contrary to the sperm cell, ...
. The other product of meiosis is the second polar body with only chromosomes but no ability to replicate or survive. In the fertilized daughter, 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, a fusion of the pronuclei and immediate mitotic division produce two 2n diploid daughter cells called
blastomere In biology, a blastomere is a type of cell produced by cell division (cleavage) of the zygote after fertilization; blastomeres are an essential part of blastula formation, and blastocyst formation in mammals. Human blastomere characteristics In ...
s. Between the stages of fertilization and implantation, the developing embryo is sometimes termed as a ''preimplantation-
conceptus A conceptus (from Latin: ''concipere'' to conceive) is an embryo and its appendages (adnexa), the associated membranes, placenta, and umbilical cord; the products of conception or, more broadly, "the product of conception at any point between fert ...
''. This stage has also been referred to as the pre-embryo in legal discourses including relevance to the use of embryonic stem cells. In the US the National Institutes of Health has determined that the traditional classification of pre-implantation embryo is still correct. After fertilization, the conceptus travels down the fallopian tube towards the
uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...
while continuing to divide 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 A morula (Latin, ''morus'': mulberry) is an early-stage embryo consisting of a solid ball of cells called blastomeres, contained in mammals, and other animals within the zona pellucida shell. The blastomeres are the daughter cells of the zygot ...
. 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 The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a specialized extracellular matrix that surrounds the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first ap ...
, it can implant in the endometrial lining of the uterus and begin the gastrulation stage of embryonic development. The human zygote has been genetically edited in experiments designed to cure inherited diseases.


Fungi

In fungi, the sexual fusion of haploid cells is called karyogamy. The result of karyogamy is the formation of a diploid cell called the zygote or zygospore. This cell may then enter meiosis or mitosis depending on the life cycle of the species.


Plants

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.


Reprogramming to totipotency

The formation of a
totipotent Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta. According to Latin terms, Pluripotentia means the ability for many thin ...
zygote with the potential to produce a whole organism depends on epigenetic reprogramming. DNA demethylation of the paternal genome in the zygote appears to be an important part of epigenetic reprogramming. In the paternal genome of the mouse, demethylation of DNA, particularly at sites of methylated cytosines, is likely a key process in establishing totipotency. Demethylation involves the processes of base excision repair and possibly other DNA- repair- based mechanisms.


In other species

A '' Chlamydomonas'' zygote contains
chloroplast DNA Chloroplast DNA (cpDNA) is the DNA located in chloroplasts, which are photosynthetic organelles located within the cells of some eukaryotic organisms. Chloroplasts, like other types of plastid, contain a genome separate from that in the cell n ...
(cpDNA) from both parents; such cells are generally rare, since normally cpDNA is inherited uniparentally from the mt+ mating type parent. These rare biparental zygotes allowed mapping of chloroplast genes by recombination.


In protozoa

In the
amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudop ...
, 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.


See also

* Breastfeeding and fertility * Fertilization * Proembryo


References

{{Authority control Developmental biology Human pregnancy