Sexual Reproduction
Sexual reproduction is the process in which a zygote is formed after the fusion of two gametes of opposite sexes. As a result of development in this zygote, a new organism is formed.
In sexual reproduction, male and female gametes are produced in two different organisms.
- Unisexual
- Bisexual or Hermaphrodite
1. Unisexual
'When male gametes are formed in male organism and female gametes are formed in female organism, then such organisms are called unisexual organisms.'
2. Bisexual or Hermaphrodite
'When male and female gametes are formed in an organism, then this type of organism is called bisexual organism or hermaphrodite organism.'
Sexual reproduction occurs in different ways in all plants and animals because the external morphology, internal morphology and physiology of all these organisms and plants are different. But they all perform sexual reproduction according to the same system. Growth of the organism to a certain stage is necessary for sexual reproduction, that is, maturity of the body of the organism is necessary for sexual reproduction. Which we usually call Juvenile Phase in animals and Vegetative Phase in plants.
Some animals show secondary sexual characteristics during reproductive stage for reproduction. In non-primates, mammals such as cow, dog, deer, cheetah, buffalo, sheep, etc., the cyclical change that occurs at the time of reproduction is called Oestrus Cycle. While in primates, such as monkeys, humans, etc., it is called Menstrual Cycle or M.C.
Mendelian Disorder Occur in Human BeingsStages of Sexual Reproduction
After the organism has grown to a certain stage, there are several stages of activities for sexual reproduction, the details of which are as follows –
- Pre – Fertilization
- Gametogenesis
- Isogamous Type
- Anisogamous Type Or Heterogametic
- Oogamous Type
- Gamete Transfer
- Fertilization
- External Fertilization
- Internal Fertilization
- Post – Fertilization
- Zygote
- Embryogenesis
- Oviparous
- Viviparous
1 . Pre – Fertilization
In this phase, the events before the fusion of gametes come, under which the process of gametogenesis and gamete transfer comes.
- Gametogenesis
- Isogamous Type
- Anisogamous Type Or Heterogametic
- Oogamous Type
- Gamete Transfer
a . Gametogenesis
In the process of gametogenesis, male gametes are formed from male gametes and female gametes are formed from female gametes. This action takes place by meiosis, but in algae and fungi gametes are formed without meiosis.
Gametes are haploid. If the parents are haploid, they undergo mitosis. If the parents are diploid, they undergo meiosis.
On the basis of behavior, composition and nature of gametes, gametes can be of three types –
- Isogamous Type
- Anisogamous Type Or Heterogametic
- Oogamous Type
( i ) Isogamous Type
When gametes are similar in external shape and size, but functionally different, then such gametes are called homozygous. They are represented by +ve and -ve symbols. Like – in Ulothrix, Chlamydomonas etc.
( ii ) Anisogamous Type Or Heterogametic
When the gametes are similar in external appearance, but different in size and function, then such gametes are called heterozygous. One of these gametes is large and inactive, while the other gamete is small and active. As a result of the fusion of these two gametes, a zygote is formed.
( iii ) Oogamous Type
In this the male gamete is small and active and contains flagella. Female gametes are large, inactive and flagellate. Male gamete is called Antherozoid and female gamete is called Ovum. Oospore is formed by their conjugation.
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b . Gamete Transfer
Gamete transfer is an important process that occurs between gametogenesis and fertilization. During gamete transfer, male and female gametes produced in gametogenesis come in contact with each other for the process of fertilization. In this process, female gametes are often non-motile and basifixed, whereas male gametes are motile or motile.
In some plants like algae, bryophyta, etc., medium is required for the male gamete to reach the female gamete. Many male gametes are destroyed in this, due to which male gametes are produced in excessive numbers.
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The most important step in sexual reproduction is fertilization, in which the fusion of male and female gametes takes place, resulting in the formation of a diploid zygote.
Parthenogenesis
Sometimes female gametes start forming new organisms without fertilization. This is called parthenogenesis. Like – in bees, some reptiles and some birds.
In angiosperms, the process of fruit formation without fertilization is called parthenocarpy.
Types Of Fertilization
Mainly Two types of fertilization occur, That is -
- External Fertilization
- Internal Fertilization
a . External Fertilization
When the integration of gametes takes place in an external medium outside the body of the organism, then this type of integration or conjugation is called external fertilization. Like – in algae, in fishes, in amphibians. In this process, a large number of zygotes are formed, from which innumerable organisms are formed. But the chances of survival of these creatures are very less, because higher class creatures use them as their food.
b . Internal Fertilization
When the fusion of gametes takes place inside the body of the organism, then this type of fusion or conjugation is called internal fertilization. Like – in terrestrial animals (reptiles and birds), mammals and higher class plants (bryophyta, pteridophyta, angiosperms and gymnosperms).
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3 . Post – Fertilization
In the process of sexual reproduction, all those events or activities which happen after the process of fertilization are called post-fertilization process. Under this mainly comes the process of Zygote formation and Embryogenesis.
- Zygote
- Embryogenesis
- Oviparous
- Viviparous
- Oviparous
- Viviparous
a . Zygote
As a result of the process of fertilization, the gametes of both the opposite sexes fuse together to form a zygote. In the organisms in which the process of fertilization has taken place outside the organism, the zygote is also formed outside the body in the same medium, while in the organisms in which the zygote is formed inside the organism, the formation of the zygote is also inside the organism body. only happens. The growth and development of the zygote depends on the environment in which the zygote grows. Under adverse conditions, the zygote remains at rest before germination, and when conditions are favorable, embryogenesis takes place in it.
b . Embryogenesis
The process of development of embryo from zygote is called Embryogenesis. Embryogenesis involves mitosis and cell differentiation. Cells increase in number by the process of division, while different groups of cells are formed by differentiation, from which tissues, organs and organ systems are formed. In this way a complete organism is created.
Animals are divided into two parts on the basis of embryonic development –
- Oviparous
- Viviparous
( i ) Oviparous
Oviparous animals lay fertilized or unfertilized eggs. These creatures lay their eggs in a safe place. A hard layer of calcium is found on these eggs to protect them from the external environment. The embryo formation of the organism takes place inside these eggs and after the embryo formation, after a certain time interval, the progeny organisms come out of the eggs. Like – in the creatures of reptiles and birds.
( ii ) Viviparous
In viviparous animals, the embryogenesis of the zygote takes place within the body of the female. After a certain time the child is born through the act of delivery.
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