How do Organisms Reproduce?Do Organisms Create Exact Copies of Themselves?
Organisms of one kind look alike because their body designs are alike. Similar body designs need similar blueprints, so at its most basic level reproduction means making copies of the blueprint of the body design.
In Class IX you learnt where that blueprint is kept. The chromosomes in the nucleus of a cell carry the information for passing features from parents to the next generation, in the form of DNA (deoxyribonucleic acid) molecules. DNA is the information source for making proteins. If the information changes, different proteins are made, and different proteins eventually lead to altered body designs.
So the basic event in reproduction is the creation of a DNA copy. Cells build copies of their DNA using chemical reactions. But a copy on its own is not enough. If the cell kept one copy and simply pushed the other one out, the copy outside would have no organised cell around it to carry out life processes. So DNA copying goes hand in hand with making an additional cellular apparatus — a second set of cell parts — before the cell divides.
Put the events of a dividing cell in order, first at the top.
Each of the two new cells now has a full set of DNA, and its own cell parts to run its life processes.
Similar, but Not Identical
The two new cells are certainly similar. But are they absolutely identical? That depends on how accurately the copying reactions work — and no biochemical reaction is absolutely reliable. So every time DNA is copied, a few small changes can creep in.
The parent cell's DNA is shown as a strip of letters. Copy it six times, then compare each copy with the parent, letter by letter, and tap the three surviving cells whose DNA is not an exact copy.
Some changes are so drastic that the new DNA copy cannot work with the cellular apparatus it inherits, and the newborn cell simply
This built-in tendency for variation during reproduction is the basis for evolution. You will see more of it in the next chapter, Heredity.
The Importance of Variation
Every population of organisms fills a well-defined place in its ecosystem — its niche — and uses its ability to reproduce to stay there. Consistent DNA copying keeps the body design features that let the organism use that niche. So reproduction is linked to the stability of populations of a species.
But niches can change for reasons beyond the organisms' control. Temperatures on earth can rise or fall, water levels can change, a meteorite can strike. If every member of a population is suited to one niche and that niche changes drastically, the whole population can be wiped out. If a few individuals carry some variations, there is a chance that they will survive.
A population of bacteria lives in a pond of mild water, and global warming starts to heat it. Try both ponds: one where every bacterium is an identical copy, and one with a few purple heat-resistant variants. Complete all three missions.
That is what would happen to bacteria living in temperate waters: if global warming raised the water temperature, most of them would die, but the few variants resistant to heat would survive and grow further. Variation is useful for the
Notice that variation did nothing for the bacteria that died. A variation helps the species only because some individuals happen to carry the right one when the niche changes.