NCERT Notes for Class 8 Science Chapter 6: Reproduction in Animals

NCERT Notes for Class 8 Science Chapter - 6: Reproduction in Animals: Reproduction is essential for species to survive; it ensures their continued existence for many generations by passing on traits from one generation to the next.  Without it, individuals of similar genetic makeup would not continue beyond a single generation. Through these notes, the students will be able to revise essential topics like different modes of reproduction, sexual reproduction, Male reproductive organ, female reproductive organ, fertilization, test tube babies, external fertilization, development of an embryo, internal; fertilization, asexual reproduction, budding, binary fission and an introduction to the cloning process and the story of Dolly the sheep.

Reproduction

Reproduction is a vital process that is essential for the survival of all living organisms. In animals, reproduction involves the production of offspring that share genetic traits with their parents. The process of animal reproduction can be complex and varied, with different species exhibiting unique reproductive strategies. 

Modes of Reproduction

There are two ways that animals can reproduce, just like there are two ways that plants can reproduce: 

•  Sexual reproduction 
•  Asexual reproduction

Sexual Reproduction

• Animals have separate reproductive systems for males and females, just like plants.
• The male and female gametes combine to form the zygote, which then develops into a new individual in both plants and animals.
• Sexual reproduction is the process of reproduction that starts with the fusion of male and female gametes.

Male Reproductive Organs

• The male reproductive system consists of a penis, two sperm ducts, and two testes (plural, testis). 
• The male gametes known as sperms are produced in the testes. 
• The testes produce millions of sperm. 
• Despite their tiny size, sperms have a head, a central section, and a tail. 
• Each sperm is a single cell that contains all of the typical cell elements.

Female Reproductive Organs

• The uterus, a pair of ovaries, and oviducts (fallopian tubes) constitute the female reproductive system. 
• The female gametes known as ova (eggs) are produced in the ovary. 
• Each month, one of the ovaries of a human releases one developed egg into the oviduct.
• The uterus is where the embryonic development of the child occurs.
•  A single cell also makes up an egg, much like the sperm.

The egg could be very little, like in humans, or a lot bigger, like in hens. The biggest egg is an ostrich egg.

Fertilization

The reproductive process begins with the union of a sperm and an ovum. The egg may be fertilized when it interacts with one of the sperms, leading to fusion. It is known as fertilization when an egg and sperm combine in this manner.  The sperm and egg nuclei combine to create one nucleus during fertilization, which is found inside the zygote

Fertilization happens when a mother's egg cell and the father's sperm cell combine, resulting in the creation of a new individual who has inherited characteristics from both parents. This is evident by the similarities between some characters of brother and sister with their parents.

Internal Fertilisation

• Internal fertilization is the process of sperm and egg uniting within the female body, rather than externally.
• It's something many animals experience, such as humans, cows, dogs & hens.

Test Tube Babies

Some women have impaired oviducts, meaning they are unable to carry eggs. This impediment means these women may not be able to become pregnant due to the blocked oviducts. With the introduction of IVF, female eggs and male sperm can be externally combined to produce a fertilized egg.

A zygote is formed by the fusion of an egg & a sperm and is implanted in a woman's uterus after it has been nurtured for about a week. IVF babies are commonly referred to as Test Tube Babies.

External Fertilisation

During springtime and the rainy season, frogs and toads move to ponds & slow-flowing streams. The female then lays hundreds of eggs when these amphibians mate in water. Unlike chicken eggs, frog eggs are incredibly fragile & come without a shell. The eggs are encased in a jelly-like substance to keep them safe and stable. After being laid, the male fertilizes them by scattering sperm all over them.

The sperm's long tail helps it to move randomly in the water and eventually come into contact with the egg, leading to fertilization. External fertilization enables the fusion of a male and female gamete to take place outside the body of the female.  This process can be seen in abundance among aquatic organisms, including fish and starfish.

Though these animals generate vast amounts of sperm and eggs, not all of them get fertilized due to their exposure to wind, rain, and swift water. Additionally, some predators like fish also consume the eggs present in the pond. In order to ensure that at least some of the eggs and sperm are fertilized, enormous quantities of both must be produced.

Development of Embryo

• During fertilization, a zygote is created and it then starts the process of development into an embryo. 
• The zygote divides multiple times, creating a cluster of cells. Subsequently, these cells group together to form various body tissues and organs.
• An embryo is the initial stage of a growing organism in the uterus. 
• After being implanted into the uterine wall, it begins to develop further into its complete form with all its body parts such as hands, legs, head, eyes, and ears.
• The fetal stage is when the baby is still in the womb and its body parts are identifiable. 
• After the fetus has finished developing, it's time for the mother to give birth to the newborn.

Viviparous and Oviparous Animals

Viviparous animals are those that reproduce by giving birth to offspring. The infants are born by their mothers. A few examples of viviparous creatures are dogs, cats, cows, etc. Oviparous animals are defined as those that lay eggs. This is because the mother will lay them in an external environment instead of keeping them inside her own body.

A hen’s egg is white in color and spheroid in shape. Lizard eggs are the size of fingernails and white in color. Crow eggs are bluish-green in color. These are all examples of oviparous animals that lay eggs.

Also Read: Difference between Oviparous and Viviparous Animals

Young ones to Adult

From the moment they are born, babies grow and develop until they reach adulthood. However, their youthful appearance may not remain the same as it was when they were young. For example, adults possess a different look than younger frogs and silkworms.

Frogs experience three different phases in their life cycle - egg, tadpole (larva), and adult. Tadpoles look nothing like adults, but with time, they turn into frogs capable of jumping and swimming. Metamorphosis is an important process when it comes to the transformation of larvae into adults. This transformation involves a lot of physical changes that do not take place in humans as they age, hence why it does not occur in them.

Asexual Reproduction

Only one parent is involved in asexual reproduction. It occurs in microbes like amoebas and small creatures like hydras. Regenerative cells are used by Hydra to give rise to new individuals during its budding phase. This happens due to repeated cell division at one single point which causes the formation of buds as an outgrowth.

Budding is a process wherein small individual buds gradually detach from their parent's body and grow into new, young organisms. This unfolding of events occurs when the buds have fully developed.

Budding in Hydra

• The Hydra reproduces asexually through the process of budding. Its body is covered with protrusions called Buds, which gradually blossom into their own individual Hydra. 
• This process is very similar to that of yeast reproduction.
• Different Hydras can have varying numbers of buds.

Binary Fission in Amoeba

• Reproduction in amoeba happens through a process called Binary Fission, wherein it splits into two individuals. 
• The only cell present in an amoeba divides itself and its nucleus divides into two, forming two distinct nuclei.
• The process of asexual reproduction in amoeba involves cell division, where one nucleus is shared between two daughter cells.
•  As a result, the parent amoeba gives rise to two offspring without mating with another partner.

Story of Dolly

• A team of researchers led by Ian Wilmut at the Roslin Institute in Scotland, achieved a scientific milestone when they cloned a sheep named Dolly. C
• Cloning is the process of producing an exact replica or copy of cells, other living components, or organisms.
• On July 5th, 1996, a historic event occurred as Dolly became the world's first cloned animal. 
• A cell was taken from a female Finn Dorsett sheep's mammary gland and an egg was extracted from a Scottish Blackface ewe for this cloning process.
• The nucleus of the egg was removed and replaced with the nucleus of a mammary gland cell from a Finn Dorsett sheep using a Scottish Blackface ewe as the host. 
• The ewe then successfully implanted and carried this egg to completion.
• After a successful development, Dolly was born and it was revealed that her genetic material had been sourced from Finn Dorsett sheep.
• Despite being delivered by a Scottish Blackface ewe, she was an exact clone of the donor sheep.
• Contrary to what one may think, Dolly did not exhibit the traits of a Scottish blackface ewe since the nucleus from its egg was removed. 
• Dolly was actually born healthy and she went on to have several offspring through traditional mating.
• Sadly, Dolly died from a lung-related ailment on 14th February 2003. 
• Subsequent clone attempts have proven to be unsuccessful, with most passing away either pre- or post-birth. This has sparked contemplation about the ethics of cloning in general.