Chapter 7 · How do Organisms Reproduce?
NCERT Class 10 Science Chapter 7 Intext Questions (Page 119)
All 5 questions of NCERT Class 10 Science Chapter 7 Intext Questions (Page 119) — How do Organisms Reproduce? — in the book's order, each with its answer and a step-by-step solution.
- Q1How does binary fission differ from multiple fission?Step-by-step solution →
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Binary fission splits the parent into two equal cells after one nuclear division; multiple fission gives many small cells after repeated nuclear divisions.
The two differ in how many daughter cells are produced and in how the nucleus behaves before the cell splits. (i) Number of daughter cells: in binary fission the parent cell divides into two equal halves, so one parent gives exactly two daughter cells; in multiple fission the parent cell divides into many daughter cells simultaneously. (ii) Division of the nucleus: in binary fission the nucleus divides only once and the cytoplasm then divides; in multiple fission the nucleus divides again and again inside the parent cell while the cytoplasm stays undivided, and only afterwards does the cytoplasm divide around each nucleus. (iii) Size of the daughter cells: binary fission gives two comparatively large daughters, while multiple fission shares the parent among many, so each daughter cell is very small. (iv) Examples: binary fission is seen in Amoeba, where the splitting can take place in any plane, in Leishmania, where it occurs in a definite orientation in relation to the whip-like structure at one end of the cell, and in many bacteria and protozoa; multiple fission is seen in the malarial parasite Plasmodium. - Q2How will an organism be benefited if it reproduces through spores?Step-by-step solution →
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Spores let an organism produce many offspring without a partner, spread them widely, and survive harsh conditions inside a thick wall.
Reproducing through spores benefits the organism in three ways, one for each feature of a spore. (i) Numbers: a single sporangium contains a large number of spores and every one of them can develop into a new individual, so one parent starts very many offspring at once, and does so asexually, without needing a partner. (ii) Dispersal: spores are microscopic and light, so wind, water and animals carry them far away. New individuals therefore grow in fresh places and do not have to compete with the parent for food and space, which is how a mould that cannot move spreads from one slice of bread to another. (iii) Survival of bad conditions: each spore is covered by a thick wall, which protects it until it comes into contact with another moist surface, where it can begin to grow. The organism can therefore live through drought, heat and shortage of food in the resting spore stage instead of dying, and start growing again the moment conditions become favourable. - Q3Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?Step-by-step solution →
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Complex bodies are organised and their cells differentiated, so a cut piece cannot rebuild the rest; germ cells reproduce instead.
Because a complex organism is not a random collection of cells and no piece of it carries what regeneration needs. (i) Its specialised cells are organised as tissues, its tissues into organs, and its organs have to lie at definite positions so that the organ systems work together. A piece cut from such a body holds only a fragment of this arrangement, with parts of some organs and none of others, so it cannot carry out digestion, respiration, circulation and excretion by itself and cannot stay alive. (ii) Its cells are fully differentiated for particular jobs and cannot start turning into other cell types. Regeneration in Hydra and Planaria is carried out by specialised cells that proliferate into a mass of cells from which the various cell types and tissues then arise in an organised sequence of development, and an ordinary piece of a complex body contains no such cells. (iii) Even in the animals that can regenerate, regeneration is not the same as reproduction, since no organism can depend on being cut up in order to reproduce. For all these reasons reproduction in complex organisms is made the function of one specific cell type, the germ cells produced in specialised reproductive organs, from which a new individual develops in the proper order from the beginning. - Q4Why is vegetative propagation practised for growing some types of plants?Step-by-step solution →
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Plants flower and fruit earlier, seedless plants can be multiplied, and the parent's characteristics are kept unchanged.
Vegetative propagation is practised because it gives a grower what raising the plant from seed cannot. (i) Plants raised by vegetative propagation bear flowers and fruits earlier than those produced from seeds, since they start from a piece of mature tissue instead of from a single fertilised cell, so the crop is ready sooner. (ii) It makes possible the propagation of plants that have lost the capacity to produce seeds, such as banana, orange, rose and jasmine; for these there is no other way of raising new plants at all. (iii) All the plants produced are genetically similar enough to the parent to have all its characteristics, so a chosen variety with a particular taste, colour or yield can be multiplied without change, which seeds could not guarantee. (iv) Only one parent plant is needed and methods such as layering and grafting are simple and cheap, so large numbers of plants of sugarcane, roses and grapes can be raised quickly for agricultural purposes. - Q5Why is DNA copying an essential part of the process of reproduction?Step-by-step solution →
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DNA is the blueprint of body design, so each new cell needs its own complete copy to form a similar individual.
Because a new individual cannot be built without its own complete set of instructions. Reproducing organisms create individuals that look very much like themselves, and they look alike because their body designs are alike; if the body designs are to be similar, the blueprints behind them must be similar. That blueprint is the DNA in the cell nucleus, which is the information source for making proteins, and different proteins would give an altered body design. The parent cell cannot simply hand over its own DNA, because it would then be left with none, so before dividing it uses chemical reactions to build a copy. This gives two complete sets, one for each of the two cells formed, and every new cell therefore starts life with the full instructions for making the parent's body design. Copying the DNA is also inseparable from the rest of cell division: keeping one copy and pushing the other out would fail, since the copy pushed out would have no organised cellular structure for maintaining life processes, so the copying is accompanied by the creation of an additional cellular apparatus and the two copies then separate, each with its own. This is why the creation of a DNA copy is called the basic event in reproduction. It also has a second consequence, that the copying is never perfectly accurate, and the resulting variations are the basis of evolution.