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Why do organisms reproduce?

Reproduction is not an essential life process but yet all animals and plants reproduce and spend their energy in nurturing their young ones.

Reproduction is done to ensure the continuity of their species on earth by passing on similar characteristics to the new individuals through DNA (Deoxyribose Nucleic Acid) present in the cell nucleus.

Why is there variation in different individuals?

DNA essentially is a blueprint of body design and is the information source for making proteins. When a cell divides this information too if copied onto the new cell leading to inheritance of body design and the working information.

However, the new individuals are not the exact copies of their parents and they have some variation in them because of errors in DNA copying.

These errors or variations arise due to different combination of protein forming information present in the DNA.

These variations are useful for survival of the individual and species over time by ensuring evolution of favorable traits. But if there are too many errors in copying of DNA information then the new individual will find it difficult to work with the cell structure it has inherited and such an individual will die.

The surviving cells are similar to, but subtly different from each other. This inbuilt tendency for variation during reproduction is the basis for evolution which helps the new organism adapt to changes in the environment.

Types of Reproduction

Asexual Reproduction

Sexual Reproduction

Single organism is required

Two organisms are required

Gametes are not formed

Gametes are formed

​Individual is identical to parent

Individual is genetically similar but not identical

Helpful in rapid multiplication

Useful for generation of more variations

Normally found in lower/simple organisms

Normally found in higher/complex organisms

Modes of Asexual Reproduction

1. Fission:

In unicellular organisms, the parent cell divides into daughter cells. This process of creation of new individuals is by splitting of parent cell is called Fission.

  • Binary Fission: The splitting of the parent cell in two cells during division is called as binary fission. Example: In Amoeba and Leishmania (causes Kala Azar)

  • Multiple Fission: The splitting of the parent cell simultaneously into many cells during division is called as multiple fission. Example: In Plasmodium

2. Fragmentation:

The organism breaks-up into smaller pieces upon maturation, each piece develops into new individual. It is common in simple organisms.

Example: Spirogyra.

3. Regeneration:

If organism is somehow cut or broken into many pieces, each piece grows into a complete organism. It is carried out by special cells in a organized sequence that further develop into special tissues and cell types performing specific functions. The new organism is developed from the body parts of the parent organism.

Regeneration is not the same as reproduction. Most animals normally don’t depend on being cut up to reproduce.

Example : Planaria, Hydra.

4. Budding:

A bud is formed using regenerative cells which develops into tiny individual. It detaches from parent body upon maturation and develops into separate individual.

Example: Hydra

Vegetative Propagation:

There are many plants in which parts like the root, stem and leaves develop into new plants under appropriate conditions.

1. By roots: E.g., dahlias, sweet potato.

2. By stem: E.g., potato, ginger.

3. By leaves: E.g., bryophyllum (leaf notches bear buds which develop into plants).

Artificial methods:

Cutting and Grafting - In Rose and Mango Tree

Layering - Jasmine

Tissue culture: New plants are grown by using growing tip of a plant. These growing cells are kept in a culture medium leads to the formation of callus. Callus is then transferred to hormone medium which causes growth and differentiation. E.g., ornamental plants, orchid.

Sexual Reproduction

When reproduction takes place as a result of the fusion of male and female gametes is called sexual reproduction.

Fusion of gametes is called fertilizations which results in variation.

Sexual Reproduction in Plants

• Flowers are the reproductive organs of plants.

• A typical flower consists of four main whorls namely sepals, petals, stamen and pistil.

Types of Flowers

• Bisexual flower : Both male and female reproductive parts are present. E.g., Hibiscus, mustard.

• Unisexual flower : Either male or female reproductive part is present. E.g., Papaya, watermelon.

Structure of Flower

Process of Seed Formation

Pollen grains, produced in the anther, are transferred to the stigma of same flower (self pollination) or stigma of another flower (cross pollination) through agents like air, water or animals.

Pollen grains germinate and form pollen tubes which pass through style to reach upto the ovules present in ovary.

The fusion of male and female gametes is called fertilization. Zygote is produced inside the ovary.

Zygote divides to form embryo. Ovule develops thick coat and changes into seed gradually.

Ovary changes into fruit and other parts of flower fall off.

The seed germinates to form a plant under suitable conditions such as air, moisture etc.

Reproduction in Human Beings

  • Puberty – Reaching Sexual Maturation Preparation of Human Body to Reproduce!

During teenage years, often called as adolescent age changes happen in human body which are different from simple body enlargement that happens in the preceding period. As the general rate of body growth begins to slowdown, reproductive tissues begin to mature. These changes help individuals showcase and identification of sexual maturity and fitness for reproduction.

Theses changes include change in proportion of body parts, appearance of new features and sensations. These changes occur over a period of months and years and may happen at different times in different individuals.

The achievement of sexual maturity signifies the ability of the organism to produce specialized germ cells called gametes such as sperm (in men) and ovum (in women) which have to fuse together either inside or outside the body for formation of zygote which develops into a new individual.

Changes common in both Boys and Girls during Puberty

- Growth of thick hair in pubic areas such as armpits and genital areas between the thighs

- Thinner hair appear on legs, arms and face

- Skin becomes oily leading to acne/pimples

Changes only in Girls during Puberty

- Increase in size of breasts

- Darkening of skin around nipples

- Menstruation begins

- Widening of Hip region

Changes only in Boys during Puberty

- Thich hair on face

- Protrusion of Adams apple and cracking of voice

- Enlargement and erection of penis during daydreams or at night.

Male Reproductive System

Formation of Germ Cells takes place outside the abdominal cavity in Testes which is located in sac called scrotum. The testes are located outside the body because formation of sperm requires a temperature lower than the body temperature.

Testes is also responsible for secretion of hormone called testosterone which is responsible for regulation of sperm production as well as development of secondary sexual characteristics during puberty.

The sperm produced is stored in epididymis where it matures and is then transported via a tube called as vas-deferens. Along the way, glands like seminal vesicle and prostate add their secretions to sperms providing the sperms fluidity and nutrition for better mobility towards the female germ cell.

Female Reproductive System

The female germ cell or gamete is produced in the ovaries which unlike testes is located inside the abdominal cavity. They are also responsible for secretion of hormone Oestrogen which is responsible for development of secondary sexual characteristics along with regulation of production of ovum.

The pair of ovaries in a female have thousands of immature eggs which are present at the time of birth and are released at the start of puberty each month by one of the ovaries.

The egg is carried to the uterus/womb through a thin duct called oviduct/fallopian tube. It is here that the fertilizations of egg takes place if a sperm is able to fuse with it.

The two tubes, one from each ovary unite into an elastic bag like structure called uterus which opens into the vagina through the cervix.

Once the egg is fertilized by the sperm in the fallopian tube it is called as zygote and it starts dividing to form an embryo. The embryo is implanted on the thickening and blood rich walls of the uterus where it continues to grow into a foetus.

The foetus receives nutrition from the mother through a special tissue called placenta which is a disc like structure embedded in the uterine wall and connects to the foetus through a tube like structure called umbilical cord connected to the foetus at the belly button.

The placenta is rich in villi which provides a large surface area for exchange of glucose, oxygen to the foetus and removal of waste products from the foetus into the mother’s blood.

The development of a human baby inside the womb takes place around nine months and this period is referred to as the gestation period. The child upon maturation is pushed out of the womb due to rhythmic contractions of the muscles in the uterus.

What happens when the egg is not fertilized?

The uterus prepares itself to receive a fertilized egg every month. Its walls thicken and become spongy to provide nutrition and support to the embryo. But if the fertilizations hasn’t taken place then there is no need for thick lining which breaks down and is released through the vagina in the form of blood and mucous. This cycle takes place roughly every month and is called as menstruation and usually lasts for 2 to 8 days.

Reproductive Health

Some degree of sexual maturation doesn’t mean the body or the mind of the individual is ready to bear and raise a child. There are various pressures from the society, friends, family and government on whether to have children or not making informed choices relatively difficult.

Sexual contact also being an intimate connection can lead to transmission of diseases such as:

Bacterial Infections: Syphilis and Gonorrhoea

Viral Infections: HIV-AIDS and Warts

In order to prevent sexually transmitted diseases and unwanted pregnancies many contraceptive methods have been devised such as:

Mechanical Barrier Methods: By use of condoms by males and females prevent the fusion of gametes. A loop or a copper T can also be placed in the uterus but it may have side effects such as irritation.

Oral/Chemical Contraceptives: By use of chemical pills that later the hormonal balance of the body preventing the release of eggs or implantation of fertilized egg onto the uterine wall. These methods may also have some side effects as they alter the hormonal balance of the body.

Surgical Methods: The vas-deferens in males can be surgically blocked through a process known as vasectomy and similarly the fallopian tube in females can be blocked through a process known as tubectomy. These methods are relatively safe in the long run but do carry the risk of infections during the period of surgery.

Surgeries can also be used to remove early unwanted pregnancies.

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