Chapter Notes

How do Organisms Reproduce?

Reproduction is the biological process by which new individual organisms—offspring—are produced from their "parents". While other life processes like nutrition, respiration, and excretion are essential for an individual organism to stay alive, reproduction is not. Instead, reproduction is vital for the continuation of a species. It is because organisms reproduce that we see large populations of them, allowing us to notice their existence.

Do Organisms Create Exact Copies of Themselves?

When organisms reproduce, they create new individuals that look very similar to themselves. This similarity exists because the fundamental "blueprints" for their body designs are passed down from one generation to the next.

The Role of DNA

• The blueprint for an organism's body design is contained within the chromosomes found in the nucleus of a cell.
• This information is stored in the form of DNA (Deoxyribo Nucleic Acid) molecules.
• DNA is the master information source for making proteins. Different proteins lead to different body designs.
• Therefore, the most basic event in reproduction is the creation of a copy of the cell's DNA.

The Process of Copying

1. DNA Copying: A cell uses a series of chemical reactions to build a copy of its DNA. This results in two copies of the DNA.
2. Cellular Apparatus: Simply pushing one DNA copy out of the cell wouldn't work, as it needs a complete cellular structure to maintain life. Therefore, DNA copying is followed by the creation of an additional cellular apparatus.
3. Cell Division: The cell then divides, and the two DNA copies separate, each with its own cellular structure. This results in the formation of two new cells from one.

Are the Copies Identical?

No biochemical reaction is 100% accurate. This means that the process of copying DNA will inevitably have small variations each time.

• The new DNA copies will be very similar, but not absolutely identical, to the original.
• Sometimes, a variation can be so drastic that the new cell cannot function and dies.
• However, many other variations are minor and allow the new cells to survive. These surviving cells are similar to, but subtly different from, each other.
• This natural tendency for variation during reproduction is the foundation for evolution.

The Importance of Variation

Populations of organisms live in specific, well-defined places in an ecosystem called niches. Their ability to survive and reproduce in that niche depends on their body design features, which are maintained by consistent DNA copying.

However, niches can change due to factors beyond an organism's control, such as changes in temperature, water levels, or even catastrophic events like meteorite impacts.

• If a population were perfectly suited to a niche and had no variation, a drastic change in that niche could wipe out the entire population.
• If some variations exist within the population, a few individuals might have traits that allow them to survive the new conditions.

Therefore, variation is useful for the survival of a species over time, even if it doesn't necessarily benefit a single individual.

Modes of Reproduction Used by Single Organisms

The method an organism uses to reproduce depends on its body design. Reproduction that involves only a single parent to create a new generation is called asexual reproduction.

Fission

For unicellular (single-celled) organisms, cell division, or fission, is the mode of reproduction.

• Binary Fission: The parent cell splits into two equal halves, creating two new individuals. This is common in bacteria and protozoa.
  • In Amoeba, this splitting can happen in any plane.
  • In organisms with more organized bodies, like Leishmania (which causes kala-azar), binary fission occurs along a definite orientation because of structures like its whip-like tail.
• Multiple Fission: The parent cell divides into many daughter cells simultaneously. This is seen in the malarial parasite, Plasmodium.

Fragmentation

In multi-cellular organisms with a relatively simple body organisation, like Spirogyra, a simple method called fragmentation works.

• Upon maturation, the organism simply breaks up into smaller pieces, or fragments.
• Each of these fragments then grows into a new, complete individual.

This method is not practical for complex multi-cellular organisms because they are not just a random collection of cells. They have specialised cells organised into tissues and organs that must be placed at definite positions, which cannot be achieved by simple cell-by-cell division.

Regeneration

Many fully differentiated organisms have the amazing ability to give rise to new individuals from their body parts. This is called regeneration.

• If a simple animal like Hydra or Planaria is cut into several pieces, each piece can grow into a complete organism (see Fig. 7.3).
• This process is carried out by specialised cells that multiply rapidly to form a large mass of cells. From this mass, different cells change to become various cell types and tissues in an organised sequence.

Budding

Organisms like Hydra use regenerative cells for reproduction in a process called budding.

1. A bud develops as an outgrowth at a specific site on the parent's body due to repeated cell division.
2. This bud grows into a tiny individual.
3. When fully mature, it detaches from the parent body and becomes a new, independent individual (see Fig. 7.4).
4. Yeast also reproduces through budding.

Vegetative Propagation

This is a type of asexual reproduction found in many plants where new plants develop from parts like the root, stem, or leaves under appropriate conditions.

• Methods: Techniques like layering and grafting use vegetative propagation to grow plants like sugarcane, roses, and grapes for agricultural purposes.
• Examples:
  • Buds on the surface of a potato (which is a stem) can grow into new plants.
  • Buds produced in the notches along the leaf margin of Bryophyllum can fall on the soil and develop into new plants (see Fig. 7.5).

Advantages of Vegetative Propagation

• Plants raised this way can bear flowers and fruits earlier than those grown from seeds.
• It allows for the propagation of plants that have lost the ability to produce seeds, such as bananas, oranges, and jasmine.
• All plants produced are genetically similar to the parent plant, ensuring they have all its desired characteristics.

Tissue Culture

A modern technique where new plants are grown from a few cells or tissue removed from the growing tip of a parent plant.

1. The cells are placed in an artificial medium, where they divide rapidly to form a small group of cells called a callus.
2. The callus is transferred to another medium containing hormones that promote growth and differentiation.
3. The resulting plantlets are placed in soil to grow into mature plants. This technique can be used to grow many plants from a single parent in disease-free conditions and is common for ornamental plants.

Spore Formation

In many simple multi-cellular organisms, like the bread mould Rhizopus, reproduction occurs through spore formation.

• The tiny blob-on-a-stick structures seen on mouldy bread are called sporangia, which are involved in reproduction.
• The sporangia contain cells, or spores, which can develop into new individuals.
• Spores are covered by thick walls that protect them from harsh conditions until they land on a moist surface where they can begin to grow (see Fig. 7.6).

Sexual Reproduction

Unlike asexual reproduction, sexual reproduction is a mode of reproduction that involves two individuals (a male and a female) to create a new generation.

Why the Sexual Mode of Reproduction?

While DNA copying is the source of variation, it is a fairly slow process. Sexual reproduction provides a way to speed up the generation of variation, which is crucial for the survival of a species.

• Combining Variations: Each individual in a population has a unique pattern of accumulated variations from previous generations. Sexual reproduction combines the DNA from two different individuals.
• New Combinations: This process creates brand new combinations of variants in the offspring. Since these variations come from living, successful parents, it's assured they don't have immediately harmful effects. This increased variation enhances the species' ability to adapt to changing environments.

The Chromosome Number Problem

If the offspring is simply a combination of DNA from two parents, it would end up with twice the amount of DNA as its parents. This would mess up the cell's control mechanisms.

The Solution: Meiosis

• Complex multi-cellular organisms have solved this problem by having specialised reproductive organs with cells that undergo a special type of cell division called meiosis.
• Meiosis produces special germ-cells, called gametes, which have only half the number of chromosomes and half the amount of DNA compared to the normal body cells.
• When a male gamete and a female gamete combine during sexual reproduction, the normal number of chromosomes and DNA content is restored in the new individual.

Specialisation of Gametes

As organisms become more complex, their gametes also specialise:

• The female gamete is typically large and contains food stores to nourish the developing embryo.
• The male gamete is smaller and motile (able to move) to reach the female gamete.

Sexual Reproduction in Flowering Plants

In angiosperms (flowering plants), the reproductive parts are located in the flower. The main reproductive parts are the stamens and the pistil.

• Parts of a Flower:

  • Stamen: The male reproductive part. It produces yellowish pollen grains, which contain the male germ-cells.
  • Pistil (or Carpel): The female reproductive part, found in the center of the flower. It consists of three parts:
    1. Stigma: The sticky terminal part that receives pollen.
    2. Style: The middle, elongated part connecting the stigma to the ovary.
    3. Ovary: The swollen bottom part that contains ovules. Each ovule has an egg cell (the female gamete).
  • Petals and Sepals: Petals are often brightly coloured to attract pollinators, while sepals protect the flower in its bud stage.

• Types of Flowers:

  • Unisexual Flower: Contains either stamens or a pistil (e.g., papaya, watermelon).
  • Bisexual Flower: Contains both stamens and a pistil (e.g., Hibiscus, mustard).

The Process of Reproduction in Plants

1. Pollination: The transfer of pollen grains from the anther (part of the stamen) to the stigma of a pistil.
   • Self-pollination: Pollen is transferred within the same flower.
   • Cross-pollination: Pollen is transferred from one flower to another by agents like wind, water, or animals.
2. Fertilisation: After pollen lands on a suitable stigma, a tube grows out of the pollen grain, travels through the style, and reaches an ovule in the ovary. The male germ-cell then travels down this tube and fuses with the female gamete (egg cell) inside the ovule. This fusion is called fertilisation, and it results in the formation of a zygote.
3. Post-Fertilisation:
   • The zygote divides several times to form an embryo within the ovule.
   • The ovule develops a tough coat and becomes a seed.
   • The ovary grows rapidly and ripens to form a fruit.
   • Other parts like petals, sepals, and stamens usually shrivel and fall off.
4. Germination: The seed contains the future plant (embryo). Under appropriate conditions (water, warmth, air), the embryo develops into a seedling. This process is known as germination.

Reproduction in Human Beings

Humans use a sexual mode of reproduction. The ability to reproduce begins during a period of adolescence called puberty, when the body undergoes significant changes and reaches sexual maturity.

Changes at Puberty

• Changes in both Boys and Girls:
  • Growth of thick hair in new parts of the body, like armpits and the genital area.
  • Thinner hair appears on legs, arms, and face.
  • The skin becomes oily, sometimes leading to pimples.
• Changes in Girls:
  • Breast size begins to increase.
  • The onset of menstruation.
• Changes in Boys:
  • Growth of new thick hair on the face (beard and moustache).
  • The voice begins to crack and deepen.
  • The penis occasionally becomes enlarged and erect.

These changes are signals that the body is becoming sexually mature and are triggered by hormones.

Male Reproductive System (see Fig. 7.10)

The male reproductive system is designed to produce and deliver sperm.

• Testes: A pair of organs located outside the abdominal cavity in a pouch called the scrotum. They produce the male gametes, called sperms, and the male sex hormone, testosterone. The scrotum keeps the testes at a slightly lower temperature than the rest of the body, which is necessary for sperm formation.
• Accessory Glands: Glands like the seminal vesicles and the prostate gland add their secretions to the sperm. This fluid makes the transport of sperm easier and provides nutrition.
• Vas Deferens: A tube that carries sperm from the testes.
• Urethra: A common passage for both sperm and urine, which runs through the penis.
• Penis: The external organ that delivers sperm into the female's body.

Female Reproductive System (see Fig. 7.11)

The female reproductive system is designed to produce eggs, receive sperm, and nurture a developing foetus.

• Ovaries: A pair of organs that produce the female gametes, called eggs or ova. When a girl is born, her ovaries already contain thousands of immature eggs. At puberty, these eggs begin to mature. Ovaries also produce female sex hormones.
• Oviduct (Fallopian Tube): A pair of thin tubes that carry the egg from the ovary to the uterus. Fertilisation occurs here.
• Uterus: An elastic, bag-like structure where the fertilised egg implants and develops into a foetus.
• Cervix: The lower, narrow part of the uterus that opens into the vagina.
• Vagina: A muscular tube that receives sperm during sexual intercourse and serves as the birth canal.

Fertilisation and Development in Humans

1. During sexual intercourse, sperms are deposited into the vagina.
2. They travel up through the cervix and uterus to reach the oviduct.
3. If an egg, released from the ovary, is present in the oviduct, one sperm may fuse with it. This is fertilisation.
4. The fertilised egg, now a zygote, begins to divide and forms an embryo.
5. The embryo travels to the uterus and gets implanted in its thick lining, where it continues to grow.
6. The embryo gets nutrition from the mother's blood via a special tissue called the placenta. The placenta is a disc embedded in the uterine wall that allows for the transfer of glucose, oxygen, and other nutrients from mother to embryo, and the removal of waste products from the embryo to the mother.
7. The embryo develops organs and becomes a foetus. The development of the child inside the mother's body takes about nine months.
8. The child is born as a result of rhythmic contractions of the muscles in the uterus.

What happens when the Egg is not Fertilised?

If fertilisation does not occur, the egg lives for about one day. The uterus prepares itself every month to receive a fertilised egg by making its lining thick, spongy, and rich in blood supply.

• If there is no fertilisation, this lining is no longer needed.
• The lining slowly breaks down and comes out through the vagina as blood and mucous.
• This cycle, which takes place roughly every month, is known as menstruation. It usually lasts for about two to eight days.

Reproductive Health

Sexual maturation is a gradual process. Being physically mature does not necessarily mean a person is mentally or emotionally ready for sexual acts or for having and bringing up children.

Sexually Transmitted Diseases (STDs)

Many diseases can be transmitted through sexual contact because it involves an intimate connection of bodies.

• Bacterial Infections: Gonorrhoea and syphilis.
• Viral Infections: Warts and HIV-AIDS.
• Using a condom for the penis during sex can help prevent the transmission of many of these infections.

Contraception

Pregnancy makes major demands on a woman's body and mind. If she is not ready for it, her health can be adversely affected. Contraceptive methods are ways to avoid pregnancy.

• Mechanical Barriers: Prevent sperm from reaching the egg.
  • Condoms on the penis.
  • Similar coverings worn in the vagina.
• Hormonal Methods: Change the hormonal balance of the body so that eggs are not released.
  • These are usually taken as oral pills. They can have side-effects.
• Intra-Uterine Devices (IUDs): Devices like the loop or copper-T are placed in the uterus to prevent pregnancy. They can sometimes cause irritation.
• Surgical Methods: These are permanent methods that block the transport of gametes.
  • In males, the vas deferens is blocked (vasectomy).
  • In females, the fallopian tube is blocked (tubal ligation). Surgery itself carries risks if not performed properly.