Formation and Development of Zygotes
Gamete refers to a haploid sex cell that is a sperm in males and egg (oocyte) in females. Zygote is the diploid cell that results from the fertilization between an egg and a sperm.
In mammals, the sperm (male gamete) fertilizes the egg (ovum, the female gamete) and the fertilized egg is called a zygote. The ovum, and therefore the zygote, is much larger than a normal cell. The zygote contains one set of chromosomes from each gamete; so it has all the genetic information encoded in it for development. However, the genes are not immediately activated to produce proteins. First, the zygote undergoes mitotic cell division several times. This is called cleavage, a process during which the zygote subdivides into many smaller cells. The over all size of the zygote itself does not change. Mammalian zygotes eventually develop into a blastocyst, after which they are more generally termed an embryo, and then a fetus.
This video takes the example of human reproduction to illustrate the process of fertilization and the development cycle of a zygote into a fetus:
Gametes are haploid in nature. A haploid cell has only one set of chromosomes; hence it only has half of the genetic material necessary to form a complete organism. Zygote is formed when gametes fuse and is hence diploid in nature. A diploid cell has paired chromosomes, and hence has the complete genetic material necessary to form a complete organism.
Gametes are formed by the process of meiosis and hence carry only one type of each chromosome.23 autosomes and either an X chromosome or a Y chromosome (23, X/Y). While oocyte can have only an X chromosome the sperm can carry either an X or a Y chromosome. Zygotes have two copies of all chromosomes. Sex chromosomes present can be either XX or XY (46, XX/XY). While an XX carrying zygote will form a female, an XY carrying zygote will give rise to a male.
Sperms are formed in the testis, a part of male reproductive system. Oocytes are formed in the ovaries, a part of female reproductive system. Zygote is formed when the sperm and the egg fuse in the fallopian tube, a part of female reproductive system.
Oocyte is arrested at metaphase of the second meiotic division before fertilization. Mature sperms formed via meiosis do not undergo cell divisions. Zygote formed by the process of fertilization undergoes rapid mitotic divisions to form blastocyst and subsequently the fetus.
The sperm is a tadpole-like structure that has a head, a mid section and a tail. They have very less cytoplasmic content. Oocyte is the largest human cell and is just visible to the naked eye. The cell consists of a large amount of cytoplasm and is spherical in shape.
The ovum and zygote are not externally active and move passively moved by the fluid-flow in the oviduct .The sperm cells are externally active and mobile, using their tails to swim against the stream of fluid in the oviduct.
The gametes on fusion during fertilization give rise to the zygote. The zygote by undergoing mitosis forms the fetus which gives rise to the organism.