Introduction to Reproduction
- Reproduction is the process of giving rise to an offspring.
- Reproduction is essential for the continuation of a species.
- There are two modes of reproduction:
- Sexual reproduction and
- Asexual reproduction
The type of reproduction in which only a single parent is involved is called asexual reproduction.
- There is no mixing of genetic information.
- Eg: buds in Hydra
The type of reproduction whcih involves two parents to give rise to an offspring is called as sexual reproduction.
- The males and females have different reproductive parts or organs.
- These organs produce the male and female gametes which fuse together to form the offspring.
- Genetic information from both parents is inherited.
- Eg: humans
It is a type of asexual reproduction in which a single-celled organism divides into two.
- Division of nucleus is followed by division of other cell organelles in the cytoplasm and finally the cytoplasm.
- Thus two cells are produced from one parent cell.
- Example: Amoeba.
- Amoeba reproduces by binary fission, which is dividing into two cells.
If one parent cell gives rise to many daughter cells, this type of fission is called as multiple fission.
- A bud starts as an outgrowth on the parent body.
- As this bud grows, it becomes a new individual organism.
- In some organisms, the buds remain attached to the parent’s body (example: yeast)
- In some organisms, it detaches from parent body and develops into a complete mature individual (hydra).
- This type of reproduction is called as budding.
- Reproduction essentially involves cell division.
- Without cell division life will not propagate.
- The two types of reproductive processes involve two different types of cell divisions.
Cell division is the process in which a parent cell divides into two or more daughter cells.
Cell division provides more cells:
- For growth
- Repair and damage control
- Gamete formation
There are two types of cell division:
Mitosis is a process where a single cell divides into two identical daughter cells.
- DNA replication occurs once followed by a single division.
- The number of chromosomes is maintained in this type of division.
- This type of division happens in asexual reproduction.
- It is also used for growth, repair and regeneration of tissues.
This is a cell division in which a single cell divides into four haploid cells.
- DNA replication occurs once followed by two divisions.
- The numbers of chromosomes are halved in meiosis.
- Meiosis takes place in sex organs to produce haploid gametes.
- Thus, the gametes have only half the number of chromosomes in them.
Sexual Reproduction in Humans
Male reproductive system
- The male reproductive system consists of a pair of testes, a pair of sperm ducts and a penis.
- The testes are located outside the male body.
- Sperms are produced in testes.
- Sperms are the male gametes.
- Testosterone is the primary male sex hormone and is produced by the testes.
- Testosterone is responsible for the development of sex organs, production of sperms and also the development of secondary sexual characteristics.
- Sperms are the male gametes produced by testes in humans.
- Millions of sperms are produced by the testes in a day.
- Each sperm is a single, microscopic cell having a head, a middle piece and a tail as its parts.
- Head contains a nucleus with a single set of chromosomes.
- The middle piece contains a large number of mitochondria that provide energy during active movement.
- Sperm is a motile gamete which moves through the female reproductive tract with the help of the tail.
Female reproductive system
- The female reproductive system consists of a pair of ovaries, a pair of fallopian tubes (oviducts) and a single uterus.
- Ova are produced by ovaries.
- Fallopian tubes (oviducts) carry the ova from ovaries to the uterus.
- Uterus holds the growing embryo/foetus and thus helps in holding the pregnancy.
- Estrogen or oestrogen is the primary female sex hormone secreted by ovaries.
- It is responsible for the development of female reproductive organs, secondary sexual characteristics, development of female gamete i.e.ovum and maintenance of menstrual cycle.
- The ovary produces the female gametes known as ovum or ova (plural).
- In human beings, a single mature egg or ovum is released into the oviduct/fallopian tube by the respective side ovary every month.
- The ovum is a single cell and is much larger than the sperm.
- It is non-motile and is carried to the uterus by the oviduct.
Fertilization is the process of fusion of the male gamete with the female gamete.
- Both the gametes containing haploid (single) set of chromosomes fuse and thus maintain the diploid (double) number of chromosomes.
- The fertilized ovum is now called a zygote.
- In human fertilization takes place in the fallopian tube and then the zygote is carried to the uterus.
- Inside the uterus, it undergoes repeated divisions and grows and becomes an embryo.
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