A zygote is a single cell which has formed as the result of the merging of an egg cell and a sperm cell. In other words, it's a fertilized egg, with the potential to develop into a living organism. The formation of a zygote represents the moment at which conception occurs, making it an important landmark in the process of fetal development from an ethical, legal, and moral standpoint as well as a scientific one.
Conception
One egg and one sperm cell are necessary in normal conception to begin the process of making a baby. An adult human generally has 46 chromosomes (packets of genetic material) in the vast majority of his cells, 23 from the mother and 23 from the father. A woman's egg contains half the normal amount, and a man's sperm contains the other half. The egg and the sperm meet in the fallopian tubes through which the egg moves from the ovary towards the uterus. Here, these half-sets of chromosomes come together to form a full set of genetic material.
Origin of Zygote
The term “zygote” means “yoked together” in Greek, reflecting the fact that the cell contains genetic material from both parents, brought together for the first time to create a cell with a full set of genes. A zygote is the first step in the creation of a completely new human, with a typically unique set of genes. If all goes well, the zygote will undergo cleavage, dividing and reproducing itself to create a small cluster of cells known as a blastocyst. The development of the blastocyst takes around five days from the moment of conception in humans.
Multiple Zygotes
In the case of twins, the twins may be monozygotic, or dizygotic twins. Monozygotic twins are produced from the same zygote, which means that each twin has identical genetic information. Dizogotic or fraternal twins are the result of the formation of two separate zygotes. This can occur when a woman releases two eggs at ovulation, instead of just one, and both eggs become fertilized. In this case, the twins will share some genetic material, but they will not be identical.
Development of the Embryo
Once the blastocyst implants in the uterine wall, it becomes an embryo, and at eight weeks, the developing baby is reclassified as a fetus. Development occurs very rapidly during the embryonic period, as the cells rapidly divide and multiply to lay out the structures which will eventually become fully mature, ranging from the spinal cord to the organs. By six weeks, very basic brain activity equivalent to that observed in comatose patients has been documented, and the embryo develops the ability to move shortly after this.
Ethical Implications
While the terminology which surrounds developing organisms at various states of development is highly scientific and clearly defined, beliefs about when life begins are a bit more complex. Some people believe that the formation of a zygote represents the beginning of life, and that the single cell already has a soul, which makes it equivalent to a living being outside the uterus. Others do not share this belief, or they may value later stages of development more highly. These contradictory beliefs sometimes cause political and social friction as people debate sensitive issues such as abortion and stem cell research.