Quick answer: The basic event in reproduction is the creation of a DNA copy. The cell must also make an additional cellular apparatus, so that each DNA copy has the machinery it needs to work. DNA copying is never perfectly accurate, so small differences called variations appear. Variation is useful for the survival of the species over time: if the environment changes, some variants may survive even if the rest of the population dies.
When a bacterium divides, the two new bacteria look and work almost exactly like the parent. Offspring of all organisms broadly resemble their parents. What makes this possible? And why are offspring never exactly identical?
The answer lies in DNA, the information stored in the cell's chromosomes, and in the way it is copied.
In CBSE Class 10 Science, Chapter 7 (How do Organisms Reproduce?), questions such as "What is the basic event in reproduction?", "Why is DNA copying accompanied by the creation of an additional cellular apparatus?" and "Why is variation beneficial to the species but not necessarily to the individual?" are frequently asked in board exams.
What You Will Learn
- What DNA is and where it is found
- Why the creation of a DNA copy is the basic event in reproduction
- Why DNA copying must be accompanied by the creation of an additional cellular apparatus
- Why DNA copying is not perfectly accurate, and how this causes variation
- Why variation is important for the survival of a species
1. The Basic Event in Reproduction: Making a DNA Copy
The chromosomes in the nucleus of a cell contain information for passing on features from parents to the next generation, in the form of DNA (deoxyribonucleic acid).
- DNA is the information source for making proteins.
- If the information changes, different proteins are made, and the body design can change.
Key point: The basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions to build copies of their DNA.
2. Why Is an Additional Cellular Apparatus Needed?
Common board question: Why is DNA copying accompanied by the creation of an additional cellular apparatus?
Answer:
- DNA only carries information. On its own, it cannot carry out life processes.
- To use this information, a cell needs its cellular apparatus: the cytoplasm, cell organelles and cell membrane.
- So, when a cell reproduces, it makes a copy of its DNA and an additional set of cellular apparatus.
- The cell then divides into two, so that each new cell has its own DNA copy and its own cellular apparatus.
Analogy: DNA is like a recipe. A recipe on its own cannot make food; you also need a kitchen, tools and ingredients. Similarly, DNA needs the cell's apparatus to work.
3. Why DNA Copying Is Never Perfect: The Origin of Variation
No chemical reaction is completely reliable. So each time DNA is copied, there are likely to be small errors.
| Type of change in the DNA copy | Result |
|---|---|
| Small changes | The new cell survives. It is similar to, but slightly different from, the parent. These differences are variations. |
| Very large changes | The new DNA may not work with the cellular apparatus it inherits, so the new cell may die. |
Key point: Because DNA copying is not perfectly accurate, offspring are similar to, but not exactly the same as, their parents. These small differences are the basis of variation.
4. Why Is Variation Beneficial to the Species?
Common board question: Why is variation beneficial to the species, but not necessarily to the individual?
Every population lives in a particular habitat, or niche, and its body design suits that niche. But conditions in a niche can change in ways the organisms cannot control, for example a rise in temperature, a drought or a change in water level.
Example (from NCERT): Imagine a population of bacteria living in temperate water.
- If all the bacteria are identical and the water temperature rises sharply because of global warming, most or all of them may die, and the population could be wiped out.
- If there is variation, a few bacteria may be able to tolerate heat. These variants survive and reproduce, so the species continues.
Answer: Variation is useful for the survival of a species over time, because some variants may survive if the environment changes drastically. A particular variation may not help an individual in its present environment, but it can help the population survive in the future.
5. Summary Table
| Concept | What happens | Why it matters |
|---|---|---|
| DNA copying | The cell makes a copy of its DNA | It is the basic event in reproduction |
| Additional cellular apparatus | The cell makes new organelles, cytoplasm and membrane | DNA needs this machinery to work |
| Copying errors | Small changes appear in the DNA copy | These create variation |
| Variation | Offspring differ slightly from parents | Helps the species survive changes in its niche |
6. Exam Tips and Common Mistakes
Exam Tip: For "why is variation beneficial", use the bacteria and rising water temperature example and explain that only heat-tolerant variants survive.
Exam Tip: Use the exact phrases "creation of a DNA copy" and "additional cellular apparatus" in your answers.
Common Mistake: Saying variation always helps the individual. It helps the species survive over time; a particular variant may gain no benefit in its present environment.
Common Mistake: Saying DNA copying is perfectly accurate. It is not; small errors are the source of variation.
Frequently Asked Questions
What is the basic event in reproduction?
The creation of a DNA copy is the basic event in reproduction.
Why is DNA copying accompanied by the creation of an additional cellular apparatus?
DNA only carries information. Each new cell also needs its own cellular apparatus, such as cytoplasm and organelles, to use that information and carry out life processes.
Why is DNA copying not completely accurate?
No chemical reaction is completely reliable, so small errors occur when DNA is copied. These lead to variations.
Why is variation beneficial to the species but not necessarily to the individual?
If the environment changes drastically, some variants may be able to survive and reproduce, so the species continues. A particular variation may give no advantage to an individual in its current environment.
What happens if the changes during DNA copying are very large?
The new DNA may not work with the cellular apparatus, and the new cell may die.