Modes Of Reproduction

Reproduction is a fundamental biological process that ensures the continuation of a species across generations. Without reproduction, species would eventually disappear. Just like essential life processes such as digestion, circulation, and respiration are crucial for individual survival, reproduction is crucial for the survival of the species.

Animals reproduce in different ways to produce new individuals. Broadly, there are two main modes of reproduction in animals:

1. Sexual Reproduction
2. Asexual Reproduction

Different animals produce young ones that may look different from the adult initially, such as chicks from hens, caterpillars from butterflies, tadpoles from frogs, or may resemble the adults more closely, like kittens from cats and puppies from dogs.

Sexual Reproduction

Sexual reproduction involves the participation of two individuals, typically a male and a female. Animals that reproduce sexually possess distinct male and female reproductive organs. These organs produce specialised reproductive cells called gametes.

Sexual reproduction begins with the fusion of a male gamete and a female gamete. The cell formed by this fusion is called a zygote. The zygote is the very first cell of a new individual and develops into a complete organism.

Male Reproductive Organs

In humans and many other animals, the male reproductive system includes:

• A pair of testes (singular: testis)
• Two sperm ducts
• A penis

The testes are responsible for producing the male gametes, which are called sperms. Millions of sperms are produced by the testes. Each sperm is a single cell, albeit very small, and typically consists of a head, a middle piece, and a tail. The tail helps the sperm to move towards the female gamete.

Female Reproductive Organs

In humans and many other animals, the female reproductive system includes:

• A pair of ovaries
• Oviducts (also known as fallopian tubes)
• The uterus

The ovaries produce the female gametes, which are called ova (singular: ovum) or eggs. In human females, usually one mature egg is released by one of the ovaries into the oviduct each month. Like the sperm, the egg is also a single cell, although generally much larger than a sperm.

The uterus is the part of the female reproductive system where the development of the baby takes place after fertilisation.

Fertilisation

The process of sexual reproduction begins with fertilisation, which is the fusion of a male gamete (sperm) and a female gamete (ovum). When sperms encounter an egg, one sperm may fuse with the egg cell. This fusion involves the joining of the nuclei of the sperm and the egg to form a single new nucleus within the fertilised egg.

The cell resulting from fertilisation is called a zygote. The zygote is the first cell of the new individual, inheriting genetic material (characteristics) from both the mother and the father. This is why offspring often show traits from both parents.

Fertilisation can occur either inside or outside the female body:

• Internal Fertilisation: This type of fertilisation takes place inside the body of the female. It is common in many animals, including mammals (like humans, cows, dogs), birds (like hens), and reptiles.
• External Fertilisation: In some animals, fertilisation occurs outside the female's body, typically in water. This is observed in many aquatic animals such as frogs, fish, and starfish. During external fertilisation, the female releases eggs into the water, and the male releases sperms over them. Fertilisation then occurs in the water.

Aquatic animals like frogs and fish often lay hundreds or thousands of eggs and release millions of sperms. This large number is necessary because the eggs and sperms in water are exposed to risks from water currents, wind, rain, and predators. Producing vast numbers increases the chances that at least a few eggs will be fertilised and develop into new individuals.

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Development Of Embryo

Following fertilisation, the resulting zygote begins to develop. The single-celled zygote undergoes rapid and repeated cell division, forming a ball of cells. These cells then start to differentiate, organising into groups that will develop into the various tissues and organs of the body. This developing structure is called an embryo.

In animals with internal fertilisation (like humans), the embryo travels to the uterus and becomes embedded in the uterine wall. This embedding process is crucial for the embryo to receive nutrients and oxygen necessary for its continued growth and development. The embryo continues to develop within the uterus, gradually forming identifiable body parts such as hands, legs, head, eyes, and ears.

The stage of development where all the major body parts can be identified is called a foetus. The foetus continues to grow and mature until its development is complete, at which point the mother gives birth to the baby.

In animals with external fertilisation (like frogs), the development of the embryo takes place outside the female's body, within the protective egg coverings floating in water. The embryos grow and develop inside these eggs until they hatch into young forms (like tadpoles), which then grow and transform into adults.

Viviparous And Oviparous Animals

Animals can be categorised into two groups based on whether they give birth to live young ones or lay eggs:

• Viviparous Animals: These are animals that give birth directly to young ones that are already developed. The embryo develops fully inside the mother's body. Examples include humans, cows, dogs, and cats.
• Oviparous Animals: These are animals that lay eggs. The embryo develops inside the egg, and a young one hatches from the egg after incubation. Examples include birds (like hens, crows), reptiles (like lizards), amphibians (like frogs), and insects (like butterflies, moths).

Eggs laid by oviparous animals vary. Bird eggs have hard shells, while frog eggs, for instance, are delicate and covered by a jelly-like layer for protection.

Young Ones To Adults

After being born (viviparous) or hatching from eggs (oviparous), young individuals grow and develop until they reach adulthood. In some animals, the young stage looks significantly different from the adult form and undergoes dramatic changes during development. This transformation is called metamorphosis.

Examples of animals undergoing metamorphosis include:

• Silkworm: The life cycle involves distinct stages: egg $\rightarrow$ larva (caterpillar) $\rightarrow$ pupa $\rightarrow$ adult moth. The caterpillar and pupa look very different from the adult moth.
• Frog: The life cycle includes stages like egg $\rightarrow$ larva (tadpole) $\rightarrow$ adult frog. Tadpoles live in water, have gills, and a tail, looking very different from the adult frog which lives on land and in water, has lungs, and no tail. The tadpole transforms into an adult frog capable of jumping and swimming.

In humans, the process of development is different. The baby born already possesses body parts similar to those of an adult, though they grow in size and mature. Humans do not undergo metamorphosis in the same way as insects or amphibians.

Asexual Reproduction

In contrast to sexual reproduction, asexual reproduction involves only a single parent. A new individual develops from a part of the parent body or through processes like cell division without the fusion of gametes. This mode of reproduction is common in very small animals and microscopic organisms.

Two common methods of asexual reproduction are:

• Budding: Observed in organisms like Hydra. New individuals develop from small projections or bulges that form on the parent body. These bulges, called buds, grow and eventually detach from the parent to become independent new organisms. This method involves a single parent.
• Binary Fission: A type of asexual reproduction seen in single-celled organisms like Amoeba. The process begins with the nucleus of the parent cell dividing into two. Following the nuclear division, the cytoplasm and the rest of the cell body divide into two parts, with each part receiving one nucleus. This results in the formation of two genetically identical daughter cells from a single parent cell.

Besides budding and binary fission, there are other methods of asexual reproduction in the living world.

Story of Dolly, the Clone

Cloning is the process of creating an exact genetic copy of a cell, part of a living organism, or a complete organism. The first successful cloning of a mammal, a sheep named Dolly, was achieved by Ian Wilmut and his team at the Roslin Institute in Scotland in 1996. Dolly was the first mammal clone.

The cloning of Dolly involved taking a cell from the mammary gland of one sheep (a Finn Dorsett sheep) and an egg cell from another sheep (a Scottish blackface ewe). The nucleus was removed from the egg cell of the Scottish blackface ewe. The nucleus from the mammary gland cell of the Finn Dorsett sheep was then inserted into this enucleated egg cell. This reconstructed egg was implanted into the uterus of the Scottish blackface ewe (the surrogate mother). The egg developed into an embryo and then a foetus, resulting in the birth of Dolly.

Despite being born from the Scottish blackface ewe, Dolly was genetically identical to the Finn Dorsett sheep from which the nucleus was taken, as the nucleus contains the genetic material. Dolly did not inherit characteristics from the Scottish blackface ewe because its nucleus was removed from the egg. Dolly was a healthy clone and even produced offspring through normal sexual reproduction. She died in 2003 due to a lung disease.

Since Dolly, attempts have been made to clone other mammals, but the process remains complex and often results in abnormalities or death before or shortly after birth.

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Question 10. Complete the crossword puzzle using the hints given below.

Across

1. The process of the fusion of the gametes.

6. The type of fertilisation in hen.

7. The term used for bulges observed on the sides of the body of hydra.

8. Eggs are produced here.

Down

2. Sperms are produced in these male reproductive organs.

3. Another term for in vitro fertilisation.

4. These animals lay eggs.

5. A type of fission in amoeba.

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