CBSE Class 10 Science How Do Organisms Reproduce Notes Set 06

These CBSE Class 10 Science notes for Chapter 7, How Do Organisms Reproduce, cover DNA copying and variation, the modes of asexual reproduction (fission, fragmentation, regeneration, budding, spore formation and vegetative propagation), sexual reproduction in flowering plants, the human male and female reproductive systems, fertilisation, the placenta and menstruation, and reproductive health including contraception and sexually transmitted diseases.

Important Notes

Reproduction and the Importance of DNA Copying

Reproduction is the process by which an organism produces new individuals of its own kind. It is not necessary for the survival of an individual organism (unlike nutrition, respiration or excretion), but it is essential for the continuity of the species.

The nucleus of a cell contains DNA (deoxyribonucleic acid) in the form of chromosomes. DNA carries the information for making proteins, and the proteins decide the body design. So, reproduction involves making a copy of DNA.

1. Cells copy their DNA with the help of chemical reactions. The two copies must then be separated, so an additional cellular apparatus is also created, and the cell divides.
2. No biochemical reaction is absolutely accurate, so the DNA copies are similar but not identical. This difference is called variation.
3. Variations help some individuals survive when the environment changes (for example, heat resistant bacteria in a warming pond), so variations are useful for the survival of the species and they are the basis of evolution.
4. Reproduction keeps the number of individuals of a species stable and passes the body design from one generation to the next.

Modes of Asexual Reproduction

In asexual reproduction a single individual gives rise to new individuals without the formation and fusion of gametes. The offspring are genetically almost identical to the parent (they are clones). Simple organisms with a simple body design generally reproduce asexually.

Fission

In binary fission, as in Amoeba, the cell divides into two nearly equal daughter cells. In Leishmania, which has a whip-like structure at one end, the division takes place in a definite plane with respect to it. In multiple fission, as in Plasmodium, the parent forms a cyst, the nucleus divides repeatedly, and many daughter cells are released when conditions become favourable.

Binary fission in Amoeba

Fragmentation and Regeneration

In fragmentation, for example in the filamentous alga Spirogyra, the filament breaks into pieces on maturity and each piece grows into a new filament. In regeneration, simple animals like Hydra and Planaria can grow back missing parts, and if the body is cut into pieces, each piece can develop into a complete organism. Regeneration is carried out by specialised cells which divide to make a mass of cells; these cells then change into the various cell types and tissues in an organised sequence. Complex organisms cannot reproduce this way because their cells are organised into specialised tissues and organs. Regeneration is not the same as reproduction, because most organisms do not depend on being cut up to reproduce.

Budding

In Hydra, regenerative cells divide repeatedly at one site and form an outgrowth called a bud. The bud grows into a small Hydra and finally detaches from the parent to live independently. Yeast also reproduces by budding when it has nutrition such as sugar solution.

Budding in Hydra

Spore Formation

The thread-like structures on a moist slice of bread are the hyphae of the bread mould Rhizopus. Erect hyphae bear round structures called sporangia, which contain spores. The spores are covered by thick walls, which protect them until they land on a moist surface where they germinate. On maturity the sporangium bursts and releases the spores into the air. Because the spores are very many and light, they spread easily, which is why the mould spreads fast on moist bread.

Spore formation in Rhizopus

Vegetative Propagation

In vegetative propagation new plants are obtained from the root, stem or leaf of the parent plant. It may be natural (the plant does it by itself) or artificial (done by gardeners and farmers).

1. Natural: by roots (sweet potato, dahlia), by stems (potato, ginger, turmeric), by leaves (Bryophyllum, whose leaf notches carry buds that grow into new plants when the leaf falls on moist soil).
2. Artificial: cutting (rose, sugarcane), layering (jasmine: a branch is bent and covered with moist soil while attached to the parent, and roots grow from the buried part), grafting (the cut shoot, or scion, of one plant is joined to a rooted plant, the stock, of another) and tissue culture (new plants are grown from cells of the growing tip in an artificial medium; the cells form a mass called callus, which is shifted to a medium with hormones, and the plantlets are planted in soil).
3. Advantages: plants that do not produce viable seeds (banana, orange, rose, jasmine) can be grown; the plants flower and fruit earlier; it is easier and cheaper; the plants are genetically identical to the parent, so good qualities are preserved.
4. Disadvantages: there is no genetic variation, so no new varieties arise; and a disease can affect all the plants because they are alike.

Sexual Reproduction

In sexual reproduction two individuals (male and female) are generally involved. Each produces special reproductive cells called gametes. The male gamete is small and motile, and the female gamete is large and carries stored food. The two gametes fuse (fertilisation) to form a zygote, which develops into the new organism.

1. Gametes are formed by a special cell division, meiosis, which halves the number of chromosomes and the amount of DNA. The fusion of the two gametes restores the full number, so the chromosome number stays the same in each generation (23 + 23 = 46 in humans).
2. Combining the DNA of two individuals gives new combinations, so sexual reproduction produces more variation than asexual reproduction. The variations help the species to survive and evolve.

Sexual Reproduction in Flowering Plants

The reproductive parts of a flowering plant are in its flower. A typical flower has four whorls: sepals (protect the bud), petals (coloured, attract insects), stamens (male parts: the anther makes pollen grains and the filament holds the anther) and the carpel or pistil (female part: the stigma receives pollen, the style connects the stigma to the ovary, and the ovary contains ovules, each with an egg cell). A flower with only stamens or only carpels is unisexual (papaya, watermelon); a flower with both is bisexual (hibiscus, mustard).

Longitudinal section of a flower

Pollination and Fertilisation

1. Pollination is the transfer of pollen grains from the anther to the stigma. In self-pollination the pollen goes to the stigma of the same flower or another flower of the same plant; in cross-pollination it goes to a flower of a different plant of the same species. Wind, water, insects and other animals carry the pollen.
2. When a pollen grain lands on a suitable stigma, it grows a pollen tube through the style to the ovary. The tube carries the male germ cell to the ovule, where it fuses with the female germ cell (egg). This fusion is fertilisation and it gives the zygote.
3. After fertilisation the zygote divides to form an embryo inside the ovule. The ovule develops a tough coat and becomes the seed, the ovary ripens into the fruit, and the petals, sepals, stamens, style and stigma wither and fall off.
4. A seed contains the future plant (the embryo) with the plumule (future shoot), the radicle (future root) and the cotyledons (stored food). Under suitable conditions it grows into a seedling, a process called germination.

Germination of pollen on the stigma and fertilisation

Sexual Reproduction in Human Beings

Humans reproduce sexually. At puberty (adolescence) the reproductive organs mature. In boys, the testes release the hormone testosterone, which brings thick hair on the face, cracking of voice and other changes. In girls, the ovaries release oestrogen, which brings breast development, widening of hips and the start of menstruation (menarche), usually at 10 to 12 years. General body growth slows as sexual maturation takes place.

Male Reproductive System

Female Reproductive System

Fertilisation, Implantation and Placenta

1. Sperms enter the vagina and travel upwards to the oviduct. If a sperm meets the egg, they fuse and form the zygote (fertilisation).
2. The zygote divides repeatedly to form an embryo, which sinks into the soft lining of the uterus and gets embedded. This is implantation.
3. A disc-like tissue, the placenta, develops between the uterine wall and the embryo. It has villi on the embryo's side and blood spaces on the mother's side, giving a large surface area. Glucose, oxygen and other nutrients pass from the mother's blood to the embryo, and the embryo's wastes pass to the mother's blood.
4. The development takes about nine months (about 40 weeks, called the gestation period). The baby is born by strong rhythmic contractions of the muscles of the uterus.

Menstruation

Every month one egg is released and the lining of the uterus becomes thick and spongy, with many blood vessels, to receive a fertilised egg. If the egg is not fertilised, it lives for about one day, and the lining breaks down and leaves through the vagina as blood and mucus. This is menstruation, which occurs roughly every month and lasts for about two to eight days. Menstruation begins at puberty (menarche) and stops at about 45 to 50 years of age (menopause).

Reproductive Health

Reproductive health means a state of physical, mental and social well-being in all matters related to reproduction. Sexual maturity does not mean that the body or mind is ready for sexual acts or for bringing up children. Reproductive health helps to prevent sexually transmitted diseases, avoid unwanted pregnancies and keep mother and child healthy.

Methods of Contraception

Methods of preventing pregnancy are called contraception. They also help to control population growth and to space children.

Sexually Transmitted Diseases

Diseases that spread through sexual contact with an infected person are called sexually transmitted diseases (STDs). They can be avoided by safe sex, mainly by using a condom.

Sex Ratio and Population Size

1. The number of females per 1000 males is the sex ratio. A balanced sex ratio is needed for a healthy society.
2. Female foeticide (killing of the unborn girl child) has lowered the sex ratio in some regions, and the misuse of ultrasound to find the sex of the baby is the main cause. Prenatal sex determination is therefore prohibited by law in India.
3. Population size depends on the birth rate and the death rate. If the birth rate is higher, the population grows, so family planning and contraception are needed.

Key Points to Remember

1. Spirogyra reproduces by fragmentation, Planaria by regeneration, Hydra and yeast by budding, Rhizopus by spores, Plasmodium by multiple fission and Amoeba by binary fission.
2. Bryophyllum grows from leaf buds, potato from stem eyes, sweet potato from roots, and rose from cuttings.
3. Pollination comes before fertilisation; the ovule becomes the seed and the ovary becomes the fruit.
4. Fertilisation takes place in the oviduct and implantation in the uterus.
5. Only a condom protects from STDs; Copper-T and oral pills do not.