Sexual Reproduction in Animals

Summary

Reproduction
It is the biological process by which existing organisms give rise to their offspring. Reproduction is carried out by different living organisms to perpetuate their species. Reproduction can be of two types – Sexual mode and Asexual mode.



Reproduction in animals
Sexual mode of reproduction: It involves the union of two opposite sex cells by the process of fertilisation to give rise to zygote, the single cell. This type of reproduction involves two organisms, the male and the female. Reproductive organs in humans produce gametes - eggs (ova) and sperms. Egg (ovum) is the female gamete produced by female reproductive organ. Sperms are male gametes produced by male reproductive organ. A zygote is the future individual formed by the fusion of an egg and a sperm.

Reproductive organs
Male reproductive organs : These include a pair of testes, a pair of epididymis, two sperm ducts, a pair of seminal vesicles, urethra and a penis. Male reproductive glands include prostrate gland and Cowper’s gland.

a) Testis is an endocrine gland forming gonad in males. Testes are placed in a bag like structure called as scrotum which descends from the abdominal region. Each testis is made up of coiled tubules which produce male gametes, sperms. Testis also secretes a hormone called as testosterone which maintains entire reproductive activities that occur in a male.

b) Sperm duct is also known as vas deferens. They are two in number, each one arising from testis placed on either side. They transport sperms into penis. They also collect fluids secreted by different glands. These secretions are rich in proteins to enrich the sperms. Sperms along with these secretions form thick substance called as semen.

c) Penis is a muscular organ which transfers semen into female reproductive tract. Penis receives both urinary tube and sperm duct and serve as a common transporting organ for urine and semen. It opens out through a small tube called as urethra.


 

Female reproductive organs : These consist of a pair of ovaries, two oviducts, also called fallopian tubes, uterus, vagina and vulva.

a) Ovary is a endocrine gland which serves as a gonad in females. They are two in number placed on either side of the abdomen. Ovaries are stimulated during onset of puberty and release one ovum in one menstrual cycle. These also secrete female hormones estrogen and progesterone  which regulate and maintain female reproductive cycle.
 
b) Oviducts or fallopian tubes are thin and long tubes conveying ovum to outside. Fimbriae located at the end of the fallopian tube grab the mature ovum released by ovary. They allow ovum to travel in the tube. Ovum waits for the sperm to fertilise it. Whether the ovum is fertilised or not, oviducts convey it to uterus.

c) Uterus is a hollow muscular organ which has the capacity to bear the child. It is otherwise called as womb. After fertilisation, female reproductive hormones bring in many changes to the uterus, so as to bear the growing embryo.
In the absence of fertilisation, the thickened wall of ovary ruptures and tears itself into pieces. These torn out pieces along with blood vessels and degenerated ovum is released out as menstrual flow.

d) Vagina lies at the end of uterus and allows uterus to contact the external environment. It is lubricated with continuous discharge from the secretory cells lining the reproductive tract.
 
Sex cells
Sex cells are also called as gametes. Gametes are the reproductive cells produced by sex organs, the gonads of the body. Gametes are of opposite sex, male gamete and a female gamete.
       •  Male gonads testes produce sperms, the male gametes. Sperms have a head, a middle piece and a tail. Sperms contain cell membrane, cytoplasm and nucleus.
       •  Female gonads ovaries produce female gametes called ova or eggs. 

  



Ovulation

It refers to release of ovum from the ovary. Egg is released on 14th day of the female menstrual cycle.

Copulation
It is the act in which a male reproductive organ enters the female reproductive tract to discharge the male gametes.

Fertilisation
It is the fusion of male and female gametes to give rise to a single cell – zygote. Fertilisation can be external fertilisation or internal fertilisation.
 


       •  Internal fertilisation takes place in the body of a female who delivers the baby on completion of the gestation period.
       •  External fertilisation takes place in frog. During rainy season as female frog lays eggs entangled in jelly like substance on the surface of water. Immediately, male frog deposits sperms over the eggs. Sperms move with the help of their tail and randomly fertilise the eggs.
       •  In-vitro fertilisation can be explained in case of test tube babies. Freshly ovulated eggs are collected from women who are incapable of bearing babies. These eggs are maintained under sterilized conditions and are fertilised with the semen collected from the husband of the woman. As the zygote is formed, it develops for seven days under controlled lab conditions. At this stage, embryo is transferred into the uterus of surrogate mother. Foetus develops in the mother’s womb and is delivered at the end of nine months.



Zygote
It is the super cell marks the beginning of a new individual.The zygote divides repeatedly to form a ball of cells making an embryo.



Embryo
It refers to the early stage of development in an animal. It gets embedded in the wall of uterus, part of the female reproductive system.The development of the embryo takes place in the mother’s uterine wall. All parts of the body start developing in an embryo which finally transforms itself into foetus.

Parturition
It is the process of delivering the baby by the mother at the end of gestation period.

Transmission of characters
Human beings carry 23 pairs of chromosomes in each cell. Of these 23pairs, one pair is the sex chromosomal pair. Males possess XY chromosomes and females possess XX chromosomes.Gene is a structural and functional unit of heredity and variations. Gene is a DNA segment on the chromosome. Genes control the expression of characteristics.
       •  The process of passing traits from parent to offspring is called heredity.
       •  Sperms are the male gametes possessing a head, body and a tail. They either comprise X chromosome or Y chromosome.
       •  The eggs are the female gametes produced by the ovary. They all comprise of X chromosome.
       •  Sperms and eggs are haploid with one set of chromosomes. They fuse to form a diploid zygote in the process of fertilization. As sperms are from father and egg is from mother, the child receives genetic material from both the parents. The characters of parents are transmitted to the child.

Differences between sexual reproduction and asexual reproduction

SEXUAL REPRODUCTION ASEXUAL REPRODUCTION
It is a mode of reproduction which involves two parents in giving rise to offspring. It is a mode of reproduction which involves only one parent giving rise to offspring.
Gonads, endocrine glands bring about the process of reproduction. Reproductive structures are not involved. Vegetative parts of individuls are used for the process of reproduction.
Sex cells develop by the process of meiosis. Cells taking part do not undergo meiosis.
Gametes of opposite sex fuse to form a single cell, the zygote. Gametes are not formed. Single parent cell gives rise to daughter cell.
Zygote undergoes different stages of development to give rise to new individual. Single cell develops fast to give rise to new individual.
Time period involved in the production of offspring is more. Time period involved in the production of new individual is less.
Offspring formed do not resemble the parents exactly. They exhibit variability. Offspring formed are exact individuals of the parents.

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