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The continuity of life on earth and the duration of the existence of any kind of living organisms, from plants and animals to human beings, are supported by the process of reproduction. The paper aims to analyze it because this ability of life forms to multiply and produce a new generation of the same species is one of their main features. Reproduction is vital for all of them because every organism exists as a result of it. For a species, its meaning consists in replacing those of its representatives who perish. It ensures the continuity of existence. Besides, under suitable conditions, reproduction enables to increase the total number of species. The paper focuses on understanding its essentials and how it works in animals and human beings. It also puts emphasis on the human reproductive system and the division of sex cells. Using the literature review of various scholarly sources, it is possible to admit that the essence of reproduction is the formation of germ cells, confluence of male and female reproductive cells, fertilization, and the development of a new daughter organism from the fertilized egg cell. Data analysis findings indicate that due to fertilization, a new organism appears with a more diverse set of chromosomes and hence with more diverse hereditary traits, as a result of which it may turn out to be more adapted to the environment.

Keywords: sexual, asexual reproduction, fertilization, division of sex cells

 

 

 

 

 

 

 

Reproduction

Reproduction is a process whereby the genetic material is transferred from the parent generation to the next one. It does not only ensure the reproduction of traits of this species but of specific parents. Each new organism must go through a series of growth and development stages before it has the ability to reproduce (Benagiano, Carrara, & Filippi, 2010). Since some organisms die before reaching the reproductive stage (or sexual maturity) as a result of being destroyed by predators, diseases, and various kinds of random events, the species can be preserved only under the condition that each generation will produce more descendants than parent individuals that have participated in reproduction.

Animal Reproduction

Reproduction can be both sexual and asexual. In the second case, a new specimen is formed by means of spores or special cells without the participation of gametes (Benagiano et al., 2010). Bacterial cell division into two daughter cells can be an example of asexual reproduction (Kraaijiveld & Bast, 2012). Only one parent is involved in the latter, which divides or buds, resulting in the formation of two or more new organisms, similar in their hereditary characteristics to the parent (Tabata, Ichiki, Tanaka, & Kageyama, 2016). So, mainly unicellular animals, for example, amoebas, breed. Hydra reproduces by budding, where only a small part of the body separated from the parent organism develops into an independent specimen.

The most perfect form is sexual reproduction. The simplest animals, such as paramecia, first conjugate forming a pair, and then cells are separated, whereby each is divided into two new ones. There are animals, where one organism has both male and female reproductive parts, which give rise to a new specimen through merging. Some animals can develop from unfertilized eggs (Tabata et al., 2016). For example, in honey bees, the queen bee and working individuals appear from fertilized eggs, and male individuals called drones are reproduced from unfertilized ones.

In many vertebrates, reproduction occurs after the fertilization of the egg by the spermatozoid (male germ cell) in the female’s body. As a result, a zygote is formed, which is divided several times, turning into an embryo, and subsequently into an adult organism (Benagiano et al., 2010). So, it is important to understand the nature of animal reproduction because it has so much in common with the one of a human being.

How Human Reproduction Works

Human reproduction is a physiological function, which is necessary for the preservation of man as a biological species. It begins with conception (fertilization), from the moment of the penetration of the male reproductive cell (sperm cell) into the female one (egg) (Epstein, 2018). The fusion of the nuclei of these two cells is the beginning of the formation of a new individual. The human fetus develops in the womb of a woman during pregnancy, which lasts around 265-270 days (Epstein, 2018). At the end of this period, the uterus begins to contract spontaneously and rhythmically, becoming stronger and more frequent with time. It leads to the amniotic sac (fetal bladder) being torn. Finally, a mature fetus is expelled through the vagina, which means that a child is born (Martin, 2013). Soon the placenta also departs. Starting with the contractions of the uterus and ending with the expulsion of the fetus and afterbirth, the whole process is called childbirth.

New living organisms cannot be produced without the process of reproduction. During sexual reproduction, two persons produce offspring, which has genetic characteristics of both parents. The main function of the reproductive system is to create male and female gametes and to ensure the growth and development of new organisms (Parker, 2009). Both female and male reproductive organs consist of external and internal structures. The main of them are gonads (testicles and ovaries), which are responsible for the production of germ cells (sperm and ova) and hormones. Other reproductive organs belong to secondary reproductive structures, which help in the growth and maturation of gametes, as well as the development of offspring.

The reproductive system of a woman is formed by external (labia and clitoris) and internal genital organs (ovaries, fallopian tubes, uterus, and vagina), and mammary glands, which are paired organs where a secretion is produced for infant feeding) (Parker, 2009). The main sex organs in women are two ovaries located at the funnel ends of fallopian tubes. They contain immature eggs, which are formed in the body of a woman even before her birth. Their maturation in woman’s ovaries occurs from the end of puberty to the end of the reproductive period (Parker, 2009). Every woman ovulation occurs every month, whereby one of the eggs reaches full maturity and leaves the ovary. After its release, it enters the fallopian tube, moving to the uterus. If the egg is not fertilized, menstruation occurs. In addition to eggs in ovaries, there are secretory cells that release sex hormones (estradiol and progesterone).

Fallopian tubes are paired organs that link ovaries to the uterine cavity. The total length of the fallopian tube is about 12 cm (Parker, 2009). By grabbing a mature egg from the ovary, it provides its nourishment and movement to the uterus. The latter is a hollow, unpaired muscular organ where the embryo and the fetus develop from a zygote during pregnancy. It has the body, to which the fallopian tubes are attached, and the cervix, which is the narrow end of this organ. The uterus passes into the vagina, through which sperm enters the female body.

The reproductive function in males is reduced to the production of the sufficient number of sperm with normal motility being capable of fertilizing mature eggs. Male genital organs include testicles (testes) with their ducts, penis, and the auxiliary organ called the prostate gland (Parker, 2009). The purpose of the male reproductive system is to produce and transport sperm (male reproductive cells) and semen, dump it into the female reproductive tract while having sex, and to produce male sex hormones to keep the male reproductive system working (Parker, 2009). So, female and male reproductive systems play an important role in producing new generations of human beings on earth.

Division of Sex Cells

Sexual reproduction exists almost in all living organisms. It is associated with the formation of highly specialized germ cells, gametes, which are formed from diploid cells by means of a special type of cell division, meiosis. As a result of it, the initial number of chromosomes in cells is halved (a diploid one becomes haploid) (Almeling, 2011). Male gametes ripen in male genital glands, testes, and this process is called spermatogenesis. Female germ cells are developed in ovaries in the process of oogenesis. Meiosis occurs as a result of two consecutive divisions of the parent diploid cell. Each of them includes four phases (Almeling, 2011). As a result of a series of serial events, the replicated genetic material in the parent cell is apportioned to four daughter cells (Almeling, 2011). The latter are haploid because they consist of half the number of chromosomes from the former. Altogether sex cells of a human contain a set of 23 chromosomes. Meiosis is similar to the process of the division of non-sex cells, known as mitosis. The latter produces two daughter cells that are genetically identical and contain the same number of chromosomes as the parent cell. They are diploid because they include two sets of chromosomes. Human diploid cells include 23 pairs or 46 chromosomes (Almeling, 2011). When germ cells coalesce during fertilization, haploid cells become diploid. Sex cells division causes genetic variation, and it is a very important element of sexual reproduction.

Conclusion

Nature has endowed living organisms with different possibilities of extending the genus. Asexual reproduction, such as division or budding, is widespread among lower forms of life. However, among higher organisms, sexual reproduction prevails. Many biologists believe that the main advantage of the reproduction process is that the offspring are more diverse, creating material for natural selection and helping to accelerate evolution. The fusion of the male and female reproductive cells, which leads to fertilization and the development of a new daughter organism from the fertilized egg, creates a new organism with a more diverse set of chromosomes. It means with more diverse inherited traits, as a result of which it may be more adapted to the environment. So reproduction plays a vital role in the maintenance of life on earth.