Azygote (/ˈzaɪˌɡoʊt/; from Ancient Greek ζυγωτός (zygōtós)'joined, yoked', from ζυγοῦν (zygoun)'to join, to yoke')[1] is aeukaryoticcell formed by afertilization event between twogametes. The zygote'sgenome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. The sexual fusion of haploid cells is calledkaryogamy, the result of which is the formation of adiploid cell called the zygote or zygospore.
The zygote is the earliest developmental stage. In humans and most otheranisogamous organisms, a zygote is formed when anegg cell andsperm cell come together to create a new unique organism.
The formation of atotipotent zygote with the potential to produce a whole organism depends onepigenetic reprogramming.DNA demethylation of the paternalgenome in the zygote appears to be an important part of epigenetic reprogramming.[2] 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 ofbase excision repair and possibly other DNA-repair–based mechanisms.[2]
Inhuman fertilization, a released ovum (a haploid secondaryoocyte with replicate chromosome copies) and a haploidsperm cell (male gamete) combine to form a singlediploid cell called the zygote. Once the single sperm fuses with the oocyte, the latter completes the division of the secondmeiosis forming a haploid daughter with only 23 chromosomes, almost all of the cytoplasm, and the malepronucleus. 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 temporarily4n diploid. After approximately 30 hours from the time of fertilization, a fusion of the pronuclei and immediate mitotic division produce two2n diploid daughter cells calledblastomeres.[3]Between the stages of fertilization andimplantation, the developingembryo is sometimes termed as apreimplantation-conceptus. This stage has also been referred to as thepre-embryo in legal discourses including relevance to the use of embryonic stem cells.[4] In the US the National Institutes of Health has determined that the traditional classification of pre-implantation embryo is still correct.[5]
After fertilization, the conceptus travels down thefallopian tube towards theuterus while continuing todivide[6] without actually increasing in size, in a process calledcleavage.[7] After four divisions, the conceptus consists of 16 blastomeres, and it is known as themorula.[8] Through the processes of compaction, cell division, and blastulation, the conceptus takes the form of theblastocyst by the fifth day of development, just as it approaches the site of implantation.[9] When the blastocyst hatches from thezona pellucida, it can implant in the endometrial lining of the uterus and begin thegastrulation stage of embryonic development.
The human zygote has been genetically edited in experiments designed to cure inherited diseases.[10]
In plants, the zygote may bepolyploid if fertilization occurs between meiotically unreduced gametes.
Inland plants, the zygote is formed within a chamber called thearchegonium. 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.[citation needed]
The zygote can divide asexually bymitosis to produce identical offspring.[citation needed]
AChlamydomonas zygote containschloroplast DNA (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.[citation needed]
