7 min readConcept sheet1-mark MCQ and assertion-reason; 2 or 3-mark questions on the importance of DNA copying, variation and germ cells

DNA copying, variation and why sexual reproduction

Reproduction begins with DNA copying. Copies are never perfect, so small variations arise; sexual reproduction mixes DNA from two parents and creates even more variety.

DNAFound in the chromosomes of the nucleus; holds the information for making proteins

Basic event in reproductionMaking a copy of the DNA

Germ cells (gametes)Have one set of chromosomes: half the DNA of body cells

Value of variationHelps a species survive when its surroundings change

What happens when a cell copies its DNA

1
DNA in the chromosomes of the nucleus holds the information for making proteins, and so for the body design.
2
The cell uses chemical reactions to build two copies of its DNA.
3
The cell also makes extra cell parts (cellular apparatus) for the new cell.
4★
The two DNA copies separate, each with a set of cell parts, and the cell divides into two.
5★
Copying is not perfectly accurate, so the new cells are similar but not identical: this is variation.

★ key step

Why both sameness and variation matter

Each kind of organism fits its own place in nature, its niche, because of its body design. So DNA copying must be consistent, or the body design would break down. Still, small variations are useful. Suppose the water of a pond becomes very hot: most of its bacteria die, but a few variants that resist heat survive and multiply. So variation helps a species survive over time, even if it does not help every individual.

Asexual vs sexual reproduction

AsexualOne parent. Offspring are almost identical to the parent; variation comes only from small DNA copying errors. Quick, and needs no partner.

SexualTwo parents. DNA from two individuals is combined, giving new combinations of traits and much more variation.

Why germ cells have half the DNA

If each parent passed on its full DNA, the offspring would have double the DNA, and the amount would keep doubling in every generation.
So special cells in the reproductive organs make germ cells (gametes) with only one set of chromosomes, half the usual DNA.
When a male and a female germ cell join (fertilisation), the zygote gets the normal amount of DNA again.
Usually one germ cell is small and motile (the male gamete) and the other is large, with stored food (the female gamete).

In real life

Brothers and sisters look alike but never exactly the same, because each gets a different mix of DNA from the parents. Farmers grow many varieties of rice and mango because plants with different traits suit different soils and weather.

Exam angle

2 marks: “Why is DNA copying an essential part of reproduction?” DNA carries the information for the body design. Copying it passes this design to the new individual; small copying errors also give variation.

2 marks: “Why is variation useful for a species but not always for an individual?” When conditions change, some variants survive, so the species continues. A particular variation may not help, or may even harm, the individual that has it.

3 marks: “Why do germ cells have half the amount of DNA? What would happen if they did not?” So that the zygote formed at fertilisation has the normal amount. Otherwise the DNA would double in every generation and the body design would not be kept.

Remember it

Copy, but not perfectly: tiny DNA copying errors make variation; two parents mix DNA for even more.