Zygote

Lore & Background

While working with sea urchins, Hertwig saw and was able to prove that only one sperm entered the ovum and sometime after the two pronuclei fused into one nucleus. From these observations Hertwig was able to conclude that an essential part of fertilization was the fusion of the two pronuclei to form the nucleus.

Reader's Guide

The zygote is the earliest developmental stage in multicellular organisms. In humans and most other anisogamous organisms, a zygote is formed when an egg cell and sperm cell come together to create a new unique organism. The formation of a totipotent 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 human fertilization, a released ovum and a haploid sperm cell combine to form a single diploid cell called the zygote. 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 blastomeres. The human zygote has been genetically edited in experiments designed to cure inherited diseases.

Did You Know?

• The term 'zygote' comes from Ancient Greek ζυγωτός (zygōtós) meaning 'joined, yoked'.
• In humans, the zygote's chromosome number temporarily becomes 4n diploid before pronuclear fusion.
• In land plants, the zygote is formed within a chamber called the archegonium.
• In fungi, the zygote may enter meiosis or mitosis depending on the life cycle of the species.