How Do Organisms Reproduce Notes - Asexual Sexual Reproduction, Human Reproduction and Contraception

• Reproduction is the production of offspring that are biologically similar to the parent organism.
• An individual organism does not need reproduction to maintain its own life, but it is necessary for the existence and continuity of each species. Therefore, an organism expends significant energy to create more individuals.

DO ORGANISMS CREATE EXACT COPIES OF THEMSELVES?

• Chromosomes in the cell nucleus contain DNA (Deoxyribonucleic Acid). It holds the information (blueprints) for body designs that are inherited from parents to offspring.
• DNA is the information source for making proteins. If the information is changed, different proteins will be made, leading to changes in body designs. Therefore, the creation of a DNA copy is the basic event in reproduction.
• A reproducing cell creates two DNA copies and additional cellular apparatus. Then, they separate, and the cell divides into two.
• In each process of DNA copying, some variations are created. As a result, DNA copies will be similar but not identical to the original.
• If large variations occur, the new DNA copy may not work with the cellular apparatus, causing the newborn cell to die. If variations are mild, cells can survive.
• Variation during reproduction is the basis of evolution.

THE IMPORTANCE OF VARIATION

• Consistent DNA copying maintains body design, helping the organism to use a particular niche. Thus, reproduction is linked to the stability of populations of species.
• Niches can change due to many reasons, such as changes in temperature, water levels, or meteorite impacts.
• If a niche is drastically altered, the population not suited to that niche may be wiped out. Individuals with variations may survive. For example, consider a bacterial population in temperate waters. If the water temperature increases due to global warming, most bacteria die, but the few variants resistant to heat survive. Variation is thus useful for the survival of species over time.

MODES OF REPRODUCTION USED BY SINGLE ORGANISMS

• The modes of reproduction by various organisms depend on their body design.
• Creation of new generations from a single individual is called asexual reproduction.
• Types of asexual reproduction: Fission, fragmentation, regeneration, budding, vegetative propagation, and spore formation.

1. FISSION

• It is the cell division of unicellular organisms to form new individuals. E.g., many bacteria and protozoa.
• It is of two types: Binary fission and Multiple fission.

1. Binary Fission

• In this, the parent cell divides into two daughter cells. E.g., Amoeba, Leishmania.
• In Amoeba, binary fission takes place in any plane.
• Some unicellular organisms have complex body organisation. E.g., Leishmania (cause kala-azar), have a whip-like structure. Here, binary fission occurs in relation to these structures.

2. Multiple Fission

• In this, some single-celled organisms divide into many daughter cells. E.g., Plasmodium (malarial parasite).

2. FRAGMENTATION

• In this, an organism simply breaks up into smaller pieces (fragments), and they grow into new individuals. E.g., Spirogyra (greenish, filamentous multicellular structures seen in ponds or lakes).
• All multicellular organisms cannot simply divide cell-by-cell because the cells are organised as tissues and organs. Here, different cell types perform different functions. So, reproduction is also performed by a specific cell type. It can grow, proliferate, and make other cell types.

3. REGENERATION

• In this, a differentiated organism is cut into many pieces, and they grow into separate individuals. E.g., Hydra and Planaria.
• During regeneration, some specialised cells proliferate to form a cell mass. From this, different cells develop to form various cell types and tissues.
• However, regeneration is not the same as reproduction, since most organisms do not normally depend on regeneration for reproduction. Also, many organisms use this only to regenerate lost parts.

4. BUDDING

• In this, a new organism is developed from a small part of the parent’s body. E.g., Hydra, Yeast, etc.

Budding in Yeast

• Dissolve about 10 gm sugar in 100 mL of water.
• Take 20 mL of this solution in a test tube and add some yeast granules. Put a cotton plug on the mouth of the test tube and keep it for 1–2 hours in a warm place.
• Put a drop of yeast culture from the test tube on a slide and cover with a coverslip. Observe under a microscope. We see yeast reproducing by forming buds.
• Organisms such as Hydra use regenerative cells for budding. In Hydra, a bud develops as an outgrowth due to repeated cell division at one specific site. Buds develop into tiny individuals and, when fully mature, detach from the parent body and become new individuals.

5. VEGETATIVE PROPAGATION

• In this, plant parts like roots, stems, and leaves develop into new plants under appropriate conditions.

Advantages

• It is used in layering or grafting to grow plants like sugarcane, roses, or grapes for agricultural purposes.
• Plants raised by this method can bear flowers and fruits earlier than those produced from seeds.
• Seedless progenies can be produced. E.g., banana, orange, rose, jasmine, etc.
• All plants are genetically similar to the parent plant.

• Potato tuber has buds for vegetative propagation.

• Cut a potato into small pieces such that some pieces contain a notch or bud and some do not.
• Place them on some wet cotton on a tray.
• After a few days, potato pieces with buds give rise to fresh green shoots and roots.

• In Bryophyllum, buds produced in the notches along the leaf margin fall on the soil and develop into new plants.

Tissue Culture

• It is a method of growing tissues or cells from a plant in an artificial medium. Cells are divided rapidly to form a callus (a small group of cells). It is transferred to another medium containing hormones for growth and differentiation. Then, the plantlets are placed in the soil. They grow into mature plants.
• Thus, many plants can be grown in disease-free conditions. This technique is commonly used for ornamental plants.

6. SPORE FORMATION

• It is present in organisms like bread mould (Rhizopus).
• Keep a slice of wet bread in a cool, moist, and dark place.
• Observe it for a week.
• A layer of white mass of thread-like structures is seen on the slice. It is the hyphae of Rhizopus.
• Hyphae include some blob-on-a-stick structures. They are involved in reproduction. The blobs (sporangia) contain cells (spores) that can develop into new Rhizopus.
• Spores are covered by thick walls that protect them until they come into contact with a moist surface and can begin to grow.

SEXUAL REPRODUCTION

Why Sexual Mode of Reproduction?

• It is the reproduction in which both sexes (male and female) are needed to produce new generations.
• The errors during DNA copying create variations.
• Variations do not protect all individuals in a population but help for survival of the species. If the DNA copying mechanisms are less accurate, the resultant DNA cannot work with cellular apparatus and will die.
• Sexual reproduction can generate more variations and speed up the making of new variants by combining DNA copies from two individuals.
• All these variations are accumulated and inherited from generation to generation. It produces individuals with different patterns of variations in a population.
• New generations will not have twice the amount of DNA due to a process of specialised cell division called meiosis. It occurs in sex organs.
• During meiosis, gametes (germ cells) are formed in which the number of chromosomes and amount of DNA are half compared to the non-reproductive cells.
• Gametes from two individuals combine to form a zygote that becomes a new individual. It re-establishes the chromosome number and DNA content in the new generation.
• In very simple organisms, the two germ cells are almost similar. In complex organisms, one germ cell is large and contains food stores. It is called the female gamete. The other is small and motile, called the male gamete.

Sexual Reproduction in Flowering Plants

• Reproductive parts of angiosperms are located in the flower.
• Parts of a flower: sepals, petals, stamens, and pistil.

• Sepals: Outermost part which protects the bud.
• Petals: Brightly colored to attract pollinators.
• Stamen: Male reproductive part, formed of anther and filament. It produces yellowish and sticky pollen grains. Pollen grains produce male germ cells.
• Pistil: Female reproductive part. It has three parts:
• Ovary: The swollen bottom part. It contains ovules. Each ovule has an egg cell (female gamete).
• Style: Middle elongated part.
• Stigma: Terminal part which may be sticky.

• Unisexual flower: Contains either stamens or pistil. E.g., papaya, watermelon.
• Bisexual flower: Contains both stamens and pistil. E.g., Hibiscus, mustard.
• Pollination: It is the transfer of pollen from stamen to the stigma. It is two types:
• Self-pollination: Transfer of pollen in the same flower.
• Cross-pollination: Transfer of pollen from one flower to another by agents like wind, water, or animals.

• After pollination, a pollen tube grows out of the pollen grain, travels through the style, and reaches the ovary. The male germ cell from the pollen grain reaches the female germ cells through this tube.

• Male germ cell fuses (fertilization) with the female gamete in the ovule to form a zygote.
• 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.
• Petals, sepals, stamens, style, and stigma shrivel and fall off.
• Seed contains the embryo (future plant) which develops into a seedling under suitable conditions (germination).
• A cut-open germinated seed (e.g., Bengal gram, chana) shows the seedling parts.

REPRODUCTION IN HUMAN BEINGS

• In early teenage years, the appearance and proportions of the body change, and new features and sensations appear. These changes indicate sexual maturation.
• All these changes occur slowly and gradually.
• The pattern and duration of these changes vary among different people, resulting in different patterns of hair growth, size, and shape of breasts or penis.
• Up to the adult stage, the body's resources are mainly used for growth, not for maturation. Then the growth rate slows down, and reproductive tissues begin to mature. This period during adolescence is called puberty.
• Changes during puberty are signals of sexual maturity so that other individuals can identify them for mating.

Changes Common to Boys and Girls

• Thick hair grows in armpits and the genital area, which becomes darker in color.
• Thinner hair appears on legs, arms, and face.
• Skin frequently becomes oily, which may cause pimples.

Changes in Girls

• Breast size increases, and the skin of the nipples darkens.
• Menstruation begins.

Changes in Boys

• Thick hair grows on the face.
• Voice begins to crack.
• Erection of the penis occurs during daydreams or at night.

(a) Male Reproductive System

• It consists of two testes and ducts to deliver germ cells.

• Testes: In these, germ cells (sperms) are formed.
• Testes are located in the scrotum because sperm formation requires a lower temperature than body temperature.
• Testes also secrete a hormone, testosterone, which regulates the formation of sperms and changes in boys during puberty.
• Ducts: Include the vas deferens to deliver sperms, which unites with the urethra coming from the urinary bladder. Thus, the urethra forms a common passage for sperms and urine.
• Along the path of the vas deferens, glands like the prostate and seminal vesicles release fluid secretions. These are mixed with sperms to form semen, which makes sperm transport easier and provides nutrition.
• A sperm consists of genetic material and a long tail that helps it move toward the female germ cell (egg).

(b) Female Reproductive System

• It consists of two ovaries, two oviducts, uterus, vagina, etc.

• Female germ cells (ovum or egg) are made in the ovaries. They also produce some hormones.
• The ovaries of a newly born girl contain thousands of immature eggs. At puberty, some of them start maturing. One egg is produced every month by one of the ovaries. It is carried from the ovary to the womb (uterus)—elastic and bag-like—through a thin oviduct (fallopian tube).
• The uterus opens into the vagina through the cervix.
• During sexual intercourse, sperms are transferred into the vagina through the penis.
• Sperms travel from the vagina via the uterus to reach the oviduct, where they meet the egg, leading to fertilization. This forms a zygote (fertilized egg), which divides to form an embryo. The embryo implants in the lining of the uterus, where growth continues, and organs develop into a foetus.
• The uterus prepares every month to receive and nurture the embryo. The lining becomes thick and spongy with a rich blood supply to nourish the embryo.
• Placenta: This is a disc of special tissue embedded in the uterine wall, connecting the foetus to the mother. It contains villi on the embryo’s side, surrounded by blood spaces on the mother’s side. Villi increase the surface area.

Functions of Placenta

• The embryo gets nutrition from the mother’s blood.
• Transports glucose and oxygen from the mother to the embryo.
• Removes wastes from the embryo into the mother’s blood.

• Development of the child inside the mother’s body takes about 9 months. The child is born as a result of rhythmic contractions of the muscles in the uterus.

What Happens When the Egg is Not Fertilized?

• If the egg is not fertilized, it lives for about one day.
• The lining of the uterus slowly breaks and comes out through the vagina as blood and mucus. This cyclic process occurring every month is called menstruation. It lasts for 2-8 days.

Reproductive Health

• A woman’s body and mind should be ready for sexual activity, pregnancy, and bringing up children. If not, her health will be adversely affected.
• There are many sexually transmitted diseases, e.g., gonorrhoea and syphilis (bacterial infections) and warts and HIV-AIDS (viral infections). They can be prevented by using a condom for the penis during sex.

Contraceptive Methods to Avoid Pregnancy

• Mechanical barriers: They prevent the meeting of sperm with the egg, e.g., condoms or coverings worn in the vagina.
• Oral pills: They change the hormonal balance of the body, so eggs are not released, and fertilization does not occur. They have side effects due to hormonal imbalance.
• Loop or copper-T: These are placed in the uterus to prevent pregnancy. They cause side effects due to irritation of the uterus.
• Surgical methods: Fertilization is prevented by blocking gamete transport. In males, the vas deferens is blocked to prevent sperm transfer. In females, the fallopian tube is blocked to prevent the egg from reaching the uterus. Surgical methods have long-term safety but can cause infections and other problems if not performed properly.

• Surgery can also be used to remove unwanted pregnancies. However, it is misused for illegal sex-selective abortion (female foeticide). Due to this, the female-male sex ratio has declined, so prenatal sex determination is prohibited.
• By reproduction, the population is increased. The birth and death rates in a given population determine its size. Population explosion adversely affects people’s standard of living. However, inequality in society is the main reason for poor standards of living.