Sexual Reproduction and Events/Process of Sexual Raproduction

 

 Sexual Reproduction:-

     It involves the production of male gametes and female gametes either by same parent or different parent.

Bisexual:- when the two types of different gametes are produced by the same individual , it is regarded as bisexual.

Unisexual:- When the male gametes are produced  from male parent and female gamete produced from female parents the parents are regarded as unisexual or heterosexual .

                          During the process of sexual reproduction , the male and female gametes fuse to form the zygote, which develop into a new organism.All the organisms shows the diversity or some phases while grow to a certain maturity in their life before  they start reproducing sexually. The phases are:-

juvenile  phase:-   The period of growth between their birth up to their  reproductive maturity,

Vegetative Phase:- In plants, the period of growth between seed germination upto initiation of flowering.

Reproductive Phase:- The later part,when organism start reproducing sexually, is called reproductive  phase.

Events in sexual reproduction

In Spite of their morphological, anatomical and physiological differences, the fundamental events in sexual reproduction are same. These events can be categorized as follows:-

i) Pre-fertilization events  ii)Fertilization  and  (iii) Post fertilization

i) Pre-fertilization events

The formation of two types of gametes - male and female gametes , inside the gametangia, is called gametogenesis. The reproductive units in sexual reproduction is gametes.The gametes are generally of two types- male gamete and female gamete. The gametes of all the organisms are usually haploid cells,i.e possess single cell or n number of chromosomes .Different types of gametes are as follow :-

a) Isogametes or Homogametes:-the two gametes are morphologically similar.i.e, they appear similar in size, shape, and activity. example - in some algae. It is not possible to distinguish them into male and female gametes.

b) Heterogametes :- In these type of organisms, the male and female gametes are morphologically distinct.      

c) Microgametes or Spermatozoa:- The male gametes are called microgametes or spermatozoa. These are minute and motile so that they swim to the female for fertilization

d)  Macrogametes or Ova :- The female gametes are termed as macrogametes or ova.These are usually large, non-motile and often have a store of food to nourish the developing embryo. 

Sexuality in organisms

    The male and female gametes are in most of the animals produced by male and female gametes respectively. Such animals are termed as unisexual or dioecious.

In some animals such as liver fluke, earthworms, leech, both kind of gametes are produced by single individual. Such animals are called bisexual, monoecious or hermaphrodite.

Cell Division during gamete formation

          

The gametes are generally formed by meiotic divisions. Therefore they are haploid or n number of chromosomes. In sexual reproduction,  the male and female gametes fuse to form zygote  by the process of fertilization. the two haploid gametes form the diploid or 2n number of chromosomes. The zygote give rise to offspring by the mitotic divisions . Thus, offspring is also a diploid like its parents which formed haploid gametes by meiosis for its creation.Meiosis and fertilization are the two important events in sexual reproduction that keep the number of chromosomes constant from generation to generation.

Gamete Transfer:-

    Once the male and female gametes are formed inside the gametangia, they are allowed to come in contact with each other to facilitate their fusion (syngamy/fertilization).

    In plants transfer of pollen grains from another to the stigma , is called pollination. It is of two types -self and cross pollination.

In self pollination transfer of pollen grains to stigma occurs within the same flower e.g peas whereas in the cross pollination transfer of pollen grains are transferred from the anthers of male flower to the stigma of female flower through the air, water,insects or animals .

 

In case of animals, the male gametes are transferred to the female gametes by three distinct pattern depending on whether fertilization and embryonic development occur within or outside the maternal  body:-

External fertilization and external development:-

This type of pattern is found in many aquatic animals, such as Obelia,Nereis, Labeo and Frog.Parents release sperms and eggs into the surrounding water, where fertilization occurs and zygote develop into offspring.For this pattern to succeed, male and female shed their gametes at the same time and place and in large numbers.The parents may or may not make a physical contact for releasing the gametes.

Internal Fertilization and external development:-

Sperms are passed from the male to the female with an intromittent organ.The female reproductive tract provides a confined, protected place where sperms and eggs can easily meet without any danger.The zygote passing down to the female reproductive tract to the exterior can acquire secretion, membrane or shell for protection of the developing embryo.

Internal Fertilization and internal development:-

Internal development provides additional advantage to the embryo.The mother's body provides exactly the right chemical conditions, warmth and nourishment also.

As the mother carries the embryo wherever she goes,it is not vulnerable to predators who attack externally developing eggs.

ii) Syngamy and fertilization

It is the fusion of the two entire gametes to form a zygote. The whole description is given in my earlier post.

iii) Post fertilization events

All the events that occur during sexual reproduction after the fusion of gametes are called post -fertilization events.

a) The zygote:-

Fusion of two haploid (n)gametes results in the formation of single diploid (2n) zygote. The zygote is the first cell of diploid organism including human beings. However there are certain organism where zygote undergoes meiosis. Whether the zygote divides by meiosis to form haploid spore or divides by meiosis to develop diploid by depends upon the type of life cycle occurs in the organism.

b) Embryogenesis:-

The process of development of embryo from the zygote is the known as embryogenesis. Development of offspring from reproductive units , such as buds or fragments or in asexual reproduction is called blastogenesis.Both embryogenesis and blastogenesis have the same target to develop a new individual by the process office and cell division and differentiation.

In animals, the development of zygote takes place either inside the body of female parent or outside. On this basis the animals are grouped into two categories:--  oviparous and viviparous.

Oviparous:-

These animals lay fertilized or unfertilized eggs.e.g , reptiles,and birds lay fertilized eggs in a safe place in the environment.The eggs are covered by hard calcareous shell . After passing a variable incubation period, the young ones hatch out from the eggs.

Viviparous 

 These animals give birth to young ones. The zygote develops into young ones inside the body of the female.