Chapter 7 · How do Organisms Reproduce?

NCERT Class 10 Science Chapter 7 Intext Questions (Page 114)

All 2 questions of NCERT Class 10 Science Chapter 7 Intext Questions (Page 114) — How do Organisms Reproduce? — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   What ​is ​the importance ​of ​DNA copying ​in ​reproduction?
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   DNA copying passes the body design to offspring, keeping populations stable, and its inexact copying creates variation, the basis of evolution.

   DNA copying is important for two reasons. (i) It passes on the body design. The DNA in the nucleus is the information source for making proteins, and the proteins settle the body design of the organism. Before a cell divides it therefore builds a copy of its DNA, so that each new cell receives a complete blueprint and can build the same body design as the parent. This is what makes the offspring belong to the same species and is why reproduction gives stability to populations of a species. The copy is accompanied by the creation of an additional cellular apparatus, since a DNA copy pushed out on its own would have no cellular structure for maintaining life processes and would simply die. (ii) It produces variation. Copying is carried out by chemical reactions, and no bio-chemical reaction is absolutely reliable, so the copies are similar but not identical to the original. Very drastic changes kill the new cell, but milder ones survive, so the daughter cells are similar to yet subtly different from each other. This inbuilt tendency for variation during reproduction is the basis of evolution and it lets some members of a species survive when the niche changes.

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2. Q2
   Why ​is variation beneficial ​to ​the ​species ​but not necessarily for the individual?
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   A variation seldom helps an individual in its usual surroundings, but when conditions change drastically, varied members may survive and keep the species going.

   Variation appears in a single individual, but survival is counted over a whole population. For the individual, a variation is usually of no advantage: in the conditions the population already lives in, the parent's body design works, so a difference from it brings nothing extra, and a drastic variation may even leave the new DNA copy unable to work with the cellular apparatus it inherits, so that the cell dies. For the species the picture is different, because niches change for reasons beyond the control of the organisms, such as a rise or fall in temperature, a change in water level or a meteorite hit. If a population of identical individuals suited to one niche were faced with a drastic change, every member would fail in the same way and the population could be wiped out. Because its members differ, there is some chance that a few of them can survive the new conditions and reproduce, so the species continues. The chapter's example makes the split plain: among bacteria living in temperate waters, resistance to heat does nothing for the few that carry it while the water is cool, but if global warming raises the temperature most of the bacteria die and only those heat-resistant variants survive and grow further. Variation is thus an insurance policy for the species, and most individuals pay the premium without ever collecting on it.

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