class 8 Reproduction in Animals Short notes science chapter 9

Fission

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.

Cell Division

• 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
• Regeneration
• Repair and damage control
• Gamete formation

There are two types of cell division:

1. Mitosis
2. Meiosis

Mitosis

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
  

Meiosis

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.

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

• 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

• 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.

Oestrogen

• 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.

Ovum

• 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
  

Fertilisation

Fertilisation 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.

 

 Asexual Reproduction: The type of reproduction in which only a single parent is involved, is called asexual reproduction.

Binary Fission: In binary fission, a single-celled individual reproduces by dividing itself into two. Example: Amoeba.

Budding: In this type of reproduction, a lateral bud arises from the body’ of the parent organism, it matures and gets detached from the body to behave as a new organism.

Eggs: Eggs (or Ova) are female gametes.

Embryo: Zygote, during its development, divides repeatedly to form a ball of cells. The cells then form groups to form tissues and ultimately organs of the body. This structure is called embryo

Fertilization: The fusion of ovum and the sperm is called fertilization.

Internal Fertilisation: Fertilisation that takes place inside the female body is called internal fertilisation. This is observed in human beings and other animals such as cows and dogs.

External Fertilisation: Fertilisation that takes place outside the female body is called external fertilisation. This is common in aquatic animals such as frogs, fish, starfish, etc.

Foetus: It is the stage of embryo in which all the body parts are identifiable in its developmental stage.

Viviparous: Animals such as human beings, cows and dogs which give birth to the young ones are called Viviparous animals.

Oviparous: Animals such as hen, frog and butterfly which lay eggs are called oviparous animals.

Tadpoles: In the life process of a frog, we find three distinct stages, that is egg → tadpole → adult. These tadpoles get transformed into adults which are capable of jumping and swimming, and are finally transformed into frog.

Metamorphosis: The drastic change which transforms a larva into an adult in case of frog is called metamorphosis. •

Sexual Reproduction: The process of reproduction, which results from the fusion of male and female gametes is called sexual reproduction.

Sperms: The male gametes.

Ova: The female gametes.

Zygote: The nuclei of sperm(n) and egg(n) are fused during fertilization, to form a single nucleus. Egg after fertilization is called Zygote.

Cloning: Cloning is the creation of an organism that is an exact genetic copy of another. This means that every single bit of DNA is the same between the two organisms.

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