Do Organisms Create Exact Copies Of Themselves?

The most basic event in reproduction is the creation of a DNA copy. Cells use biochemical reactions to build new copies of their DNA. This process is accompanied by the creation of additional cellular structures, and then the cell divides into two, with each daughter cell receiving one copy of the DNA and its own cellular components.

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While the DNA copying process is remarkably accurate, no biochemical reaction is perfectly reliable. Minor errors or variations can occur during copying. As a result, the generated DNA copies will be similar but may not be absolutely identical to the original or to each other. Some variations might be so significant that the new cell cannot survive, while others might lead to subtle differences in the daughter cells.

Thus, reproduction involves the creation of new individuals that are similar to the parent but also exhibit some variations. This inherent tendency for variation during reproduction is the foundation of evolution.

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The Importance Of Variation

Populations of organisms occupy specific ecological niches, relying on their reproductive ability to maintain their numbers. Consistency in DNA copying helps maintain the body design features suited to that niche, contributing to the stability of populations.

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However, ecological niches can change over time due to environmental factors (like temperature changes, water level variations, or natural disasters). If a population is perfectly suited to a particular niche, a drastic change in that niche could threaten the survival of the entire population.

Variation is useful for the survival of species over time. If there is variation within a population, some individuals might possess traits that make them better suited to survive in the altered environment. These variants will survive and reproduce, passing on their advantageous traits, while the less suited individuals die out. This differential survival and reproduction based on variation is a key mechanism of evolution and ensures the continuation of the species even when conditions change.

Modes Of Reproduction Used By Single Organisms

Organisms reproduce through various modes, which depend on their body design and complexity. Simpler organisms often use simpler reproductive methods.

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Reproduction from a single individual is known as asexual reproduction.

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Fission

For unicellular organisms (single-celled), cell division or fission is the method of reproduction, directly creating new individuals.

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Types of Fission:

• Binary fission: The parent cell splits into two equal daughter cells. Many bacteria and protozoa (like Amoeba) reproduce by binary fission. In organisms like Amoeba, the plane of division can be in any direction. In organisms with more body organisation, like Leishmania (causes kala-azar), binary fission occurs in a definite orientation relative to specific body structures.
• Multiple fission: The parent cell divides into many daughter cells simultaneously. This occurs in organisms like Plasmodium (malarial parasite).

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Fragmentation

Some simple multicellular organisms with relatively simple body organisation can reproduce by fragmentation.

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In fragmentation, the organism body breaks up into smaller pieces or fragments upon maturation, and each fragment grows into a complete, new individual. This is seen in organisms like Spirogyra (a type of filamentous algae).

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Regeneration

Some fully differentiated organisms have the ability to regenerate lost body parts or even grow into a complete new organism from a piece of their body.

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Regeneration involves specialised cells that can proliferate and differentiate to form various tissues and organs. Simple animals like Hydra and Planaria can reproduce through regeneration. If their body is cut into several pieces, each piece can develop into a complete organism. However, regeneration is not typically used as a primary mode of reproduction; most organisms do not depend on being broken into pieces to reproduce.

More complex organisms cannot reproduce through simple regeneration because their body organisation is complex, with specialised tissues and organs located at definite positions. Cell-by-cell division or simple growth from a fragment would not be practical for forming a complete, organised body in complex animals.

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Budding

Organisms like Hydra can also reproduce using a method called budding.

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In budding, an outgrowth called a bud develops on the parent body due to repeated cell division at a specific site. This bud develops into a tiny individual, which eventually detaches from the parent body when mature and becomes an independent new organism.

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Vegetative Propagation

Many plants can reproduce asexually using their vegetative parts (roots, stems, or leaves).

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Vegetative propagation is a method of asexual reproduction in plants where new plants develop from vegetative parts under appropriate conditions. This method is widely used in agriculture for commercially important plants.

Methods of Vegetative Propagation:

• Natural methods: Buds on potato tubers (eyes) or along the leaf margins of Bryophyllum can develop into new plants when placed in moist soil. Roots of some plants (like sweet potato) can give rise to new plants.
• Artificial methods: Techniques like layering and grafting are used to propagate plants like sugarcane, roses, grapes, and many fruit trees.

Advantages of Vegetative Propagation:

• Plants produced by vegetative propagation can bear flowers and fruits earlier than those grown from seeds.
• It makes propagation possible for plants that have lost the capacity to produce seeds (e.g., banana, orange, rose, jasmine).
• All plants produced are genetically identical to the parent plant, inheriting all its desirable characteristics (like fruit quality).

Tissue Culture: A modern technique of vegetative propagation where small pieces of plant tissue or cells from the growing tip are grown in an artificial nutrient medium. This allows for rapid multiplication of plants from a single parent in disease-free conditions. Commonly used for ornamental plants.

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Spore Formation

Some simple multicellular organisms, like fungi, reproduce through spore formation.

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In spore formation, specific reproductive structures called sporangia produce numerous microscopic spores. Each spore is covered by a thick protective wall that helps it survive unfavourable conditions. When the spore lands on a suitable moist surface, it germinates and grows into a new individual.

This allows the organism to produce a large number of offspring from a single parent, and the protective walls of spores help them survive and disperse to new locations.

Sexual Reproduction

Sexual reproduction is a mode of reproduction that involves the participation of two individuals, typically male and female, to create a new generation. This process results in offspring that are genetically distinct from either parent.

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Why The Sexual Mode Of Reproduction?

A key advantage of sexual reproduction over asexual reproduction is the generation of greater variation in the offspring.

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While DNA copying mechanisms in asexual reproduction introduce some variations, the process of combining DNA from two different individuals in sexual reproduction creates new combinations of accumulated variations. Each offspring of sexual reproduction has a unique combination of genetic material inherited from both parents.

This increased variation is highly beneficial for the survival of the species. As discussed earlier, variation increases the chances that some individuals in a population will have traits enabling them to survive and adapt to changing environments, thus ensuring the continuity of the species.

However, combining DNA from two individuals creates a challenge: if each parent contributes a full set of chromosomes, the offspring would have twice the amount of DNA as the parents, and the amount would double in each generation. To solve this, organisms that reproduce sexually produce specialised reproductive cells called germ-cells or gametes (male and female gametes) through a special type of cell division called meiosis. Meiosis reduces the number of chromosomes (and the amount of DNA) in germ-cells by half compared to the normal body cells.

When the male and female gametes fuse during fertilisation, the normal number of chromosomes and DNA content is restored in the resulting single cell, the zygote, which then develops into a new individual.

In simpler organisms, the two gametes may be similar. But in more complex organisms, the gametes are specialised: the female gamete (egg) is typically large and contains food stores, while the male gamete (sperm) is smaller and often motile.

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Sexual Reproduction In Flowering Plants

In angiosperms (flowering plants), the reproductive parts are located within the flower. A typical flower has sepals, petals, stamens (male reproductive part), and pistil (female reproductive part).

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Reproductive Parts of a Flower:

• Stamen: The male reproductive part. Consists of the anther (produces pollen grains containing male germ-cells) and the filament (stalk).
• Pistil: The female reproductive part, located in the center. Consists of the stigma (sticky tip that receives pollen), style (stalk connecting stigma to ovary), and ovary (swollen base containing ovules). Each ovule contains a female germ-cell (egg cell).

Flowers can be unisexual (contain either stamens or pistil, e.g., papaya, watermelon) or bisexual (contain both, e.g., Hibiscus, mustard).

Process of Sexual Reproduction in Flowering Plants:

1. Pollination: Transfer of pollen grains from the anther to the stigma. Can be self-pollination (pollen to stigma of the same flower) or cross-pollination (pollen to stigma of a different flower). Pollination is often carried out by agents like wind, water, or animals.
2. Fertilisation: After landing on a suitable stigma, a pollen grain germinates and grows a pollen tube down through the style to reach the ovule in the ovary. The male germ-cell from the pollen travels down the tube and fuses with the female gamete (egg cell) inside the ovule. This fusion of germ-cells forms the zygote.

Post-fertilisation Changes:

• The zygote within the ovule divides repeatedly to form an embryo.
• The ovule develops a tough protective coat and matures into a seed.
• The ovary grows rapidly and ripens into a fruit.
• Other parts of the flower (sepals, petals, stamens, style, stigma) usually wither and fall off.

The seed contains the embryo (future plant) and stored food. Under favourable conditions, the seed germinates, and the embryo develops into a seedling. Seed formation is advantageous for plants as it protects the embryo and provides stored food for germination and initial growth, and allows for dispersal to new locations.

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Reproduction In Human Beings

Humans reproduce sexually. Sexual maturation is a gradual process that occurs during adolescence (puberty), enabling the body to reproduce.

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Changes at Puberty: Around the early teenage years, the body undergoes significant changes beyond general growth, indicating sexual maturation. Some changes are common to both sexes (hair growth in armpits and genital area, oilier skin, pimples), while others are sex-specific:

• Girls: Increase in breast size, darkening of nipple skin, beginning of menstruation.
• Boys: New thick hair growth on the face (beard, moustache), voice cracking/deepening, occasional enlargement and erection of the penis.

These changes are signals that the reproductive tissues are maturing. While general body growth slows down, the body's resources are directed towards the maturation of specialised reproductive organs and the production of germ-cells (sperms and eggs). These changes also make individuals identifiable as sexually mature to others, facilitating mating.

In humans (mammals), internal fertilisation and internal development of the baby occur within the mother's body for a long period, followed by breastfeeding. The female reproductive organs and breasts mature to support pregnancy and nurturing the offspring.

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Male Reproductive System

The male reproductive system is designed to produce male germ-cells (sperms) and deliver them to the site of fertilisation.

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Components of the Male Reproductive System:

• Testes: Primary male reproductive organs, located outside the abdominal cavity in the scrotum. Produce sperms and the hormone testosterone. The scrotum's external location maintains a lower temperature necessary for sperm formation. Testosterone regulates sperm formation and secondary sexual characteristics in boys.
• Vas deferens: Tube that carries sperms from the testes. It joins with a tube from the urinary bladder, forming the urethra.
• Urethra: Common passage for both sperms and urine, opening to the outside through the penis.
• Accessory Glands: Seminal vesicles and prostate gland. Add their secretions to the vas deferens. These secretions form semen, a fluid that provides a medium for sperm transport and nourishes the sperms.
• Penis: Organ used for transferring sperms into the female reproductive tract during sexual intercourse.

Sperms are tiny cells containing genetic material and a tail for motility.

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Female Reproductive System

The female reproductive system is designed to produce female germ-cells (eggs), receive sperms, provide a site for fertilisation, and nurture the developing baby.

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Components of the Female Reproductive System:

• Ovaries: Primary female reproductive organs. Produce egg cells and hormones (oestrogen, progesterone). Girls are born with thousands of immature eggs in their ovaries. One egg matures and is released each month from puberty onwards.
• Oviducts (Fallopian tubes): Thin tubes connecting the ovaries to the uterus. The egg is carried from the ovary to the uterus through the oviduct. Fertilisation typically occurs in the oviduct.
• Uterus (Womb): An elastic, muscular, bag-like structure where the fertilised egg (zygote) implants and develops into a baby. The uterus prepares its lining monthly to receive a potential embryo.
• Cervix: Narrow lower part of the uterus that opens into the vagina.
• Vagina: A muscular tube that receives sperms during sexual intercourse and serves as the birth canal during childbirth.

Fertilisation and Pregnancy:

• Sperms enter through the vagina during sexual intercourse, travel upwards, and may meet the egg in the oviduct.
• Fusion of sperm and egg (fertilisation) forms a single cell called a zygote.
• The zygote divides repeatedly and forms a ball of cells (embryo) while travelling down to the uterus.
• The embryo implants in the thickened, blood-rich lining of the uterus.
• Inside the uterus, the embryo continues to grow and develop into a foetus, with organs and body parts forming.
• The foetus receives nourishment and oxygen from the mother's blood through a special tissue called the placenta, which is embedded in the uterine wall. Waste substances from the foetus are also transferred to the mother's blood via the placenta.
• Development inside the uterus takes approximately nine months (gestation period).
• Childbirth occurs through rhythmic contractions of the uterine muscles.

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What Happens When The Egg Is Not Fertilised?

If the egg released by the ovary is not fertilised by a sperm, it survives for about one day. The uterus, which has prepared its lining monthly to receive a fertilised egg, no longer needs this thickened lining.

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The uterine lining slowly breaks down and is shed from the body through the vagina as blood and mucus. This process is called menstruation. Menstruation occurs roughly every month in sexually mature females who are not pregnant and typically lasts for two to eight days.

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Reproductive Health

Reproductive health involves physical, mental, and social well-being related to reproduction. Sexual maturation enables the body for sexual acts and potential reproduction, but readiness also involves mental and social factors.

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Health Concerns related to Sexual Activity:

• Sexually Transmitted Diseases (STDs): Diseases transmitted through sexual contact. Can be bacterial (gonorrhoea, syphilis) or viral (warts, HIV-AIDS). Prevention methods include using physical barriers like condoms during sex.
• Pregnancy: Sexual intercourse always carries the potential for pregnancy. Pregnancy has major physical and mental demands on the woman.

Contraception (Methods to Avoid Pregnancy): Various methods have been devised to prevent pregnancy:

• Mechanical barriers: Prevent sperm from reaching the egg (e.g., condoms, diaphragms, cervical caps).
• Hormonal methods: Change the hormonal balance to prevent egg release (e.g., oral contraceptive pills). Can have side effects.
• Intrauterine Devices (IUDs): Devices like loops or copper-T placed in the uterus to prevent implantation or fertilisation. Can sometimes cause irritation or side effects.
• Surgical methods: Block the passages for gametes. Vasectomy (blocking vas deferens in males) prevents sperm transfer. Tubectomy (blocking fallopian tubes in females) prevents egg from reaching the uterus. Surgical methods are generally effective long-term but involve surgical risks.
• Abortion: Surgical or medical termination of a pregnancy. Used for unwanted pregnancies but can be misused for sex-selective abortion, negatively impacting the male-female sex ratio in society. Prenatal sex determination is prohibited by law.

Reproductive health also involves responsible decision-making regarding sexual activity, family planning, and protecting oneself and others from STDs. Given the impact of population size on living standards, family planning and birth control are significant considerations in public health.

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