The Biology of Continuation by Sexual Reproduction

ABSTRACT :

The basic need of an animal mind is continuation. Continuation of an active life. Continuation of life requires reproduction. 'Reproduction' is a compound word that signifies 'Re' i.e. again and production. Therefore reproduction is an essential life-procedure that enables a species to perpetuate its members. The development of life in animals those are capable of sexual reproduction starts from formation of gametes (both male and female) at the beginning of their adult periods. Due to sexual reproduction parental characters are passed from one generation to the next through the union of two opposite sex gametes. When meiosis cell division occurs then sexual gametes are formed. The offspring grows different aspects of variations in their bodies by the recombination of its parental chromosomes.
In general, four types of biological reproduction are found among the living organisms, 1) Vegetative propagation, 2) Asexual, 3) Sexual and 4) Parthenogenesis.

SEXUAL REPRODUCTION AND ITS TYPES :

The biological reproduction where new organism is produced when the union of male and female gametes occurs is called sexual reproduction. In case of animals male gamete is called Sperm and female gamete is called Ovum or egg. New animal is produced from the complete development of a fertilized egg generally.
Sexual reproduction can be categorized into : 1) Syngamy and 2) Conjugation.
Syngamy can be classified into three categories : 1) Isogamy, 2) Anisogamy and 3) Oogamy. Let's go through them.

A. ISOGAMY : In this case male and female gametes are morphologically and physiologically identical, that is both are equal in their shape, size and actions. Example : Monocysts.

B. ANISOGAMY : In this case two gametes are dissimilar in shape and size and also in physiological features but union of male and female gametes occurs outside the female reproductive organ. Example : a plant Chlamydomonous.

C. OOGAMY : Like anisogamy here also two gametes are dissimilar in shape and size and also in physiological features but here female gamete remains confined within the female reproductive organs and male gametes enter within the female reproductive organ and then any one of the male gametes fertilizes the female gamete.

Conjugation is found mainly in Bacterial cells where a conjugation tube is formed between the two Bacteria and the donor bacteria gives its genetic element to the recipient bacteria.

PRE-NATAL PERIOD AND POST-NATAL PERIOD :

In the sexually developed animals the period starting from the fertilization of an ovum and which is continued upto the hatch or birth of the young is known as Pre-natal period. On the other hand, the Post-natal period is started from the birth of the new born young and is continued upto its death. A series of consecutive developmental stages are found in the body of a new born animal. The total development of an organism from the fertilization to the adult stage of age is called ontogenetic development.

OVIPARITY, OVOVIVIPARITY AND VIVIPARITY :

1) OVIPARITY : This is the process where eggs are developed outside the female parent and the female lays eggs outside in the water and in this case fertilization occurs outside the female body mainly inside the water. Example : Fish, Frog etc.

2) OVOVIVIPARITY : This is the process where fertilization occurs within the female body and also the embryo is developed inside the mother body but offsprings do not share the nutritive compounds with the mother. So, they grow inside their mother body without any nutritional help. Example : Shark, Snake etc.

3) VIVIPARITY : In this process fertilized egg is implanted in the uterine wall of females and is developed there until birth, and offsprings are benefited nutritionally with their mothers. In Human females, the events of fertilization occur inside the Fallopian tube (oviduct), after that the fertilized egg travels down into the uterus and implanted on the uterine wall.

GAMETOGENESIS :

The process of formation of male gametes and female gametes in the reproductive organs or gonads of males and females respectively is commonly called Gametogenesis.
It is of two categories : 1) Spermatogenesis and 2) Oogenesis,

1) Spermatogenesis : This is the process by which mature sperms (spermatozoa) are formed from spermatogonia in the testes. ('Testes' is plural but 'Testis' is singular).
Firstly, Sperm mother cell produces spermatogonium then it produces primary spermatocyte, after that secondary spermatocyte is formed from primary spermatocyte. Then, spermatids are produced from secondary spermatocytes and from spermatids matured sperms or spermatozoa are produced.
NOTE : The last stage of spermatogenesis where sperms are produced from spermatids is called Spermiogenesis.

2) Oogenesis : This is the process by which matured female gamete or ovum is produced in Ovary. When the ovum is expelled out from ovary (ovulation) then it is called secondary oocyte.
Firstly, Oogonium is produced from egg mother cells. Then primary oocytes and then secondary oocytes and First polar bodies are formed. After all, Ovum and secondary polar body are formed. Polar bodies are degenerated later.

FERTILIZATION :

The union/fusion of male gamete and female gamete is called Fertilization. Based on the site of occurrence it is of two categories :

1) External Fertilization : This type of fertilization of ovum occurs outside the female body part. Both male and female animals release their gametes outside the body of them (mainly in water), and then fertilization occurs. Example : Fishes and Amphibians.

2) Internal Fertilization : This type of fertilization of ovum occurs inside the female genital tract (Fallopian tube). Sperms are deposited into the female genital organ with the help of copulatory organ. Example : Higher animals like reptiles, birds and mammals.

After fertilization fertilized zygote is implanted in the uterine wall (implantation), and develops there. If implantation occurs any other part of the female reproductive system besides uterus then the pregnancy may be life threatened. This is called Ectopic pregnancy, where the embryo is surgically removed.

FORMATION OF TWINS :

1) Monozygotic twins : These twins are developed from a single fertilized ovum. The fertilized ovum (Zygote) divides into two parts and each part form a new embryo. So, these two embryos are genetically identical and have same sex. These twins are also called Identical twins.

2) Dizygotic twins : Most of the time ovary produces one ovum at a time. If two ova are released from an ovary then each ovum is fertilized by sperms. Then two zygotes are formed those are genetically not identical, and may be of same sex or opposite sex.

ONTOGENETIC DEVELOPMENT :

This is the chain of development procedure of a Zygote after fertilization. After fertilization Zygote is formed. Then formation of Morula, Blastula and Gastrula occurs. From Gastrula embryo is formed and it develops. Embryo after morphogenesis and organogenesis produces a new young animal. Cleavage i.e. the segmentation occurs by which zygote develops by a series of repeated mitotic cell divisions.

PARTHENOGENESIS :

This is a process of reproduction in which new young organism is produced without fertilization. New organism is produced directly from the female ovum.
NOTE : The formation of seedless fruit without the act of pollination and fertilization is called Parthenocarpy.
Parthenogenesis may be Natural or Artificial. If a male organism is produced from a mother ovum then parthenogenesis is called Arrhenotoky (example : Drone honey bees) and if a female organism is produced from a mother ovum then parthenogenesis is called Thelytoky (example : Gall flies).

FURTHER READINGS MAY BE DONE FROM :

1) Embryology by Balinsky,
2) Developmental Biology by Gibert,
3) Fertilization by Frank J. Longo (1987), University of Lowa, USA.,
4) Oxford Dictionary of Biology,
5) www.associatedcontent.com/article/607435/human_fertilization_the_polygamous.html