The organisms need to multiply to maintain their population in the environment thus balancing the biodiversity and the ecosystem of the Earth. Reproduction is regarded as one of the criteria based on which the classification of the organisms is done in the present day classification system. Every organism has evolved its way of reproducing that is maximally suited to them and their body forms. A simpler organism reproduces by the means of asexual reproduction as it is fast, has minimum genetic variations, and saves energy for the organism. However, higher complex organisms have billions of cells in their body and therefore it is impossible to create an entirely new individual of the same species just by binary fission, budding, regeneration, and vegetative propagation. Thus, the mode of sexual reproduction evolved with the evolution of multicellular complex organisms. It is slower compared to the asexual mode of reproduction and involves a lot of genetic variation among the individuals of the same species; it is the best strategy to reproduce without creating any discrepancy in the biodiversity.
Sexual mode of reproduction occurs in fungi, plants, and animals. The gametes are haploid cells that divide by meiosis in which the daughter cells receive one set of chromosomes from the parents. The ova or the egg is the female gamete and the sperms are the male gametes. The process of formation of these gametes is an entirely different topic of discussion that is out of the scope of this article. Suppose an organism contains two sets of chromosomes like humans, then after the first meiosis, the daughter cells will receive only one set of chromosomes. They then further divide in a mitotic manner referred to as meiosis II and thus four daughter cells are formed, one parent each containing half the number of chromosomes compared to the parent cell.
Fertilization is the process of fusion or the mixing of the two gametes from the different sexes to form a single cell zygote. During the process of meiosis, the female gamete called the ovum is formed from the primary oocyte and the male gamete called the sperm is formed from spermatogonium in human adults.
After the formation of the two gametes, they need to come together in the near vicinity to result in fertilization. The ova in humans is immotile and is released into the fallopian tube of the female reproductive tract. The sperms are motile and need to be transferred to the female reproductive tract to proceed successfully for fertilization. This is achieved by the process of coitus. Once the sperms are released into the female genitalia, they travel to the fallopian tube where they encounter the egg in the ampullary-isthmic junction. The sperms fuse with the egg and the chromosomes of the two gametes also fuse. Once the fusion occurs, the zygote is formed. The process of fertilization is very complex at the molecular level. The zygote is the first cell that further undergoes division to develop into a whole new individual. The study of the development of the zygote is called developmental biology and is a newly growing research field of biology.
Zygote Formation and Development:
The stage that involves the division of the zygote into many cells that eventually get specialized into different tissues and organ systems is called the post-fertilization phase of reproduction. The single-celled zygote contains all the genetic information required to divide and form new tissues. Differential gene expression occurs in which different genes are expressed in different regions of the embryo and other genes that are not required by those tissues are turned off. The zygote contains the nutrition required for cell development and after a few days gets implanted into the inner uterine lining of the female reproductive system.
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