How Organisms Reproduce (Class X) PDF

# How do Organisms Reproduce (Class X) ## What is reproduction? Mention the two methods of reproduction. Reproduction is a process of production of new organisms from the existing ones of the same species. Two methods of reproduction are: * Sexual Reproduction: The production of a new organism fro...

# How do Organisms Reproduce (Class X) ## What is reproduction? Mention the two methods of reproduction. Reproduction is a process of production of new organisms from the existing ones of the same species. Two methods of reproduction are: * Sexual Reproduction: The production of a new organism from two parents by making use of their sex cells (gametes) * Asexual reproduction: The production of a new organism from a single parent without the involvement of sex cells (gametes) ## State the importance (significance) of the process of reproduction. * It is very necessary for the perpetuation or continuity of the species. * Increase in the population of a species. * Helps in the transmission of characters from parents to offspring (inheritance) ## Do organisms produce exact copies of themselves during reproduction? Organisms look similar because their body designs are similar. If body designs are to be similar, the blueprints for these designs should be similar. The chromosomes in the nucleus of a cell contain information for inheritance of features from parents to the next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. The DNA in the cell nucleus is the information source for making proteins. A basic event in the reproduction is the creation of a DNA copy. Cells use biochemical reactions to build copies of their DNA. The process of DNA copying will have variations each time. Hence the DNA copies generated will be similar but may not be identical to the original. ## What is the importance of variations? * Variations increase the survival chances and do not allow the species to extinct. * Accumulation of variations over a period of time gives rise to new species (evolution). * Successful variations are selected in evolution. ## Why is variation beneficial to the species but not necessarily for the individuals? Variations can be beneficial to the species than individual because sometimes for a species, the environmental conditions change so drastically that their survival becomes difficult. E.g.: If the temperature of the water increases suddenly by global warming then most of the bacteria living in that water would die. Only few variants that are resistant to heat would be able to survive. However, if these variants were not there, then the entire species of bacteria would have been destroyed. Thus these variants help in the survival of the species. ## What is the importance of DNA copying in reproduction? (OR) Why is DNA copying an essential part in the process of reproduction? DNA found in the chromosomes in the nucleus of the cell has the information to make the proteins which lead to the body design of the organism. If a similar individual has to be reproduced, the DNA should replicate to make an exact copy of itself. Therefore DNA copying is an essential part in the process of reproduction. ## List down the different methods of asexual reproduction. Explain each method in brief. The different methods of asexual reproduction are 1. **Fission:** In unicellular organisms, the parent organism splits to form two or many new organisms. * **Binary fission:** The parent cell divides into two new individuals. * E.g. Amoeba and Leishmania. * In Amoeba - During favourable conditions, when the parent cell is mature and has reached its maximum size of growth, then the nucleus lengthens and divides into two in any indefinite plane. This is followed by the division of the cytoplasm resulting in two daughter cells that are identical to the parent cell. * In Leishmania (causal organism of kala-azar)- binary fission takes place in a definite plane i.e. longitudinally with respect to its flagella at one end. * **Multiple Fission:** The parent cell divides into many daughter cells simultaneously. * E.g. Plasmodium (Malarial parasite) * In plasmodium, during unfavourable conditions, a protective layer called cyst develops around the parent cell. Within the cyst, the nucleus of the parent cell divides several times to form a number of nuclei that collects little bits of cytoplasm around them and develops into a a number of daughter cells. When favourable conditions return the cyst breaks open to release the daughter cells. 2. **Budding:** In budding, a small part of the body of the parent organism grows out as a bud which then detaches and becomes a new organism. * E.g: Hydra and Yeast. * In hydra, a bud develops as an outgrowth due to repeated cell divisions at one specific site. These bud develops into tiny individuals and when fully mature, detach from the parent body and become new independent individuals. * In yeast, a small outgrowth called bud develops on the outer cell wall of the parent yeast cell. The nucleus of the parent cell divides into two and one part moves to the bud. When fully developed the bud detaches itself from the parent cell to live as a separate organism. 3. **Fragmentation:** Simple multicellular organisms break up into many pieces upon maturation. These fragments grow into new individuals. * E.g. Spirogyra * The difference between fission and fragmentation is that in fission, a unicellular organism breaks up to form two or more daughter organisms while in fragmentation, a multicellular organism breaks up to form two or more daughter organisms. 4. **Regeneration:** Many fully differentiated organisms have the ability to give rise to new individuals from their body parts even if they are cut or broken up into many pieces. This is known as regeneration. Regeneration is carried out through specialized cells. * E.g: Animals like Hydra and Planaria which can be cut into number of pieces and each piece grows into a complete organism. 5. **Spore formation:** Spores are microscopic asexual reproductive bodies protected by a hard protective coat that enables them to survive under unfavourable conditions. * E.g: Rhizopus (bread mould) * Rhizopus produce spores in blob like structure called sporangia that arise singly at the tip of thread like structures called hyphae. Inside the sporangium, the nucleus divides repeatedly and gets surrounded by cytoplasm to form spores. At maturity, the sporangial wall breaks down to release the spores. When they settle on moist surface, the protective wall ruptures and each spore germinates to form a new fungus. 6. **Vegetative propagation:** New plants are grown from the vegetative parts of the plant such as stem, root, and leaf is called vegetative propagation. These are of two types * **Natural vegetative propagation:** * Through stems: E.g- potato, onion, sugarcane, ginger * Through roots: E.g-guava, carrot, radish * Through leaves : E.g Bryophyllum- forms many buds on its margin which give rise to new plants. * **Artificial vegetative propagation:** * Stem-cutting: A small part of the plant is removed and buried in the soil to develop into a new plant E.g. rose, grapes, sugarcane, banana * Grafting : Two cut stems of two different plants - one with roots (stock) and the other without roots (scion) are joined together to grow as a single plant showing characteristics of both the original plants E.g. apple, peach, apricot, pear * Layering : A branch of a plant is pulled towards the ground and covered with moist soil till it produces roots to become a new plant. E.g. jasmine, strawberry, bougainvillea. ## How does binary fission differ from multiple fission? Binary fission occurs during favourable conditions. It consists of divisions of the nucleus followed by the division of cytoplasm producing two cells each having a daughter nucleus. * E.g. Amoeba Multiple fission occurs during unfavourable conditions. In multiple fission, the nucleus of the parent cell undergoes repeated divisions to produce many daughter nuclei. This is followed by the division of the cytoplasm forming several daughter nuclei in the parent cell. In favorable conditions when the parent cell membrane ruptures all the daughter individuals are released. * E.g. Plasmodium ## How will an organism be benefitted if it reproduces through spores? * Spores have thick walls that protect them from unfavorable conditions. * It is a simpler and faster mode of reproduction * Spores being small and light, can be easily dispersed through wind, water and animals hence can be dispersed easily. ## Can you think why more complex organisms cannot give rise to new individuals through regeneration? * Complex organisms have highly degree of organization in their body. * There are specific organs to do specific function. * There is a division of labor in the body of complex organisms. * Regeneration is carried out by specialized cells, which are not present in complex organisms. ## Why is vegetative propagation practiced for growing some types of plants? (Advantages) * In plants which do not produce viable seeds new plants can be produced by this method. * Vegetative propagation is quicker and mature plants can be raised very quickly than those produced from seeds. * Genetically similar plants can be produced and can preserve the desirable characters of the parent plant. * Many plants can be grown from just one plant. ## List down the disadvantages of asexual reproduction. * As offsprings are genetically identical, any disease in the parent can destroy the whole population. * The progeny (offspring) of asexual reproduction show little variations. This reduces the chances of survival and evolution of species. * No new desirable characters as there is no variation. ## How does reproduction help in providing stability to the population of species? * Reproduction helps in the transfer of genetic material from one generation to other (inheritance) through DNA copying. * During DNA copying there are considerable variations, which helps the population to tide over adverse environmental conditions and adapt to changing environment. ## How are the modes of reproduction different in unicellular and multicellular organisms. **Unicellular organisms** have no separate tissue for reproduction (as they are made of single cell) Hence they reproduce asexually by fission or budding (as in yeast). **Multicellular organisms** have separate tissues / systems for reproduction. So they can reproduce by both asexual and sexual reproduction. ## Write a short note on Tissue culture. (Micro propagation) Tissue culture is a technique of growing plants from a small piece of plant tissue. It is a method of asexual reproduction. The technique involves taking a small piece of plant tissue, culturing it on a sterilized medium containing nutrients and hormones. The cells in the tissue divide and grow to form a callus. The callus is then transferred to another medium where it differentiates into roots, shoots and leaves. The plantlet is then transferred to soil where it continues to grow into a mature plant. * **Advantages of tissue culture:** * It is a rapid method of propagation. * Plants produced by tissue culture are disease-free. * The technique can be used to produce plants from a single parent plant. * It can be used to produce plants from plants that are difficult to reproduce by other methods. * **Disadvantages of tissue culture:** * The technique is expensive. * It requires specialized skills and equipment. * Plants produced by tissue culture may be more susceptible to disease.