Do organisms create exact copies?

Why reproduce at all?

No living thing lives for ever. A goat grows old, a mango tree dies, a bacterium may be eaten or dried up. Yet goats, mango trees and bacteria are still here. They survive as species because each individual makes new individuals of the same kind before it dies. This is reproduction.

Reproduction is not needed to keep one organism alive. An animal can live without ever having young. But without reproduction, its kind would die out.

Copying the instructions

Every cell holds a set of instructions in its DNA, inside the nucleus. The DNA decides what proteins the cell makes, and so what the body looks like and how it works. A new individual needs its own copy of these instructions.

So the basic event of reproduction is the copying of DNA. The cell makes a second copy of its DNA. Then the cell builds extra structures around it, and divides, so that each new cell gets one full copy.

Copies are never quite exact

The copying of DNA is done by chemical reactions in the cell. These reactions are very good, but no reaction is perfect. Now and then a copy comes out a little different from the original.

• Most of these changes are small. The new cell still works well.
• A few changes are so large that the new cell cannot survive. Such copies die out.
• So the copies that survive are similar to the parent, but not identical.

These small differences are called variations. They appear in every kind of reproduction.

Why variation helps

A species fits its place in nature, its niche. But the niche can change. The water in a pond may warm up, a new disease may arrive, or the rain may fail for years.

Picture a pond full of bacteria of one kind. Suppose a few of them carry a variation that lets them stand more heat. In a normal year this makes no difference. Then a hot summer heats the pond, and most of the bacteria die. The few with the heat-resistant variation survive and multiply. The species lives on.

If every individual were an exact copy, one change in the niche could wipe out the whole species. Variation gives the species a chance to survive. It is also the basis of evolution, which you will meet in a later class.

Watch how quickly small changes build up below. Even a small chance of change in each copy affects most cells after many generations.

Try it in Python

The program copies a short piece of imaginary DNA, 30 letters long, for six generations. Each letter has a 1 in 100 chance of being copied wrongly. It counts the cells that differ from the first one.

Practise with new numbers

Common mistakes

• "Asexual reproduction gives exact copies." The DNA copying is never perfect, so even asexual copies show small variations.
• "Variation always harms an organism." Most variations are small and harmless. A few turn out useful when the surroundings change.
• "An organism needs to reproduce to stay alive." It does not. Reproduction keeps the species alive, not the individual.
• "A variation is planned to help." Variations arise by chance during copying. Some just happen to be useful later.

Remember these

• Reproduction makes new individuals of the same kind, so the species survives.
• Its basic event is the copying of DNA, followed by the division of the cell.
• DNA copying is never perfect, so copies are similar but not identical. The differences are variations.
• Variation helps a species to survive when its niche changes, and it is the basis of evolution.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A banana plant lives its whole life without making a new plant. Does it die because of this? What does its kind lose?

2. What is copied at the very start of reproduction, and where is it found in the cell?

3. Two daughter cells come from one parent cell. Why may they differ a little from each other?

4. A disease kills most rice plants of one kind, but a few survive. How can variation explain the survivors?

5. Why would a species with no variation at all be in danger?