Q.What is the significance of meiosis?

Meiosis matters for two big reasons in sexually reproducing organisms.

• Keeping the chromosome number constant: Meiosis halves the chromosome number while making gametes, so each gamete is haploid. When two gametes fuse at fertilisation, the full diploid number is restored, keeping the species' chromosome number steady across generations. Without this halving, the number would double every generation. …

Meiosis is far more than a way of simply cutting the chromosome number in half — that halving is exactly what allows sexually reproducing species to keep their chromosome number steady, and the reshuffling it brings about supplies the variation that evolution needs.

Keeping the chromosome number constant

At first this seems contradictory, because the process itself reduces the chromosome number. The resolution is in the life cycle as a whole:

• Meiosis reduces the chromosome number by half while producing the gametes, so each gamete carries only the haploid set.
• At fertilisation, two gametes fuse and the diploid number characteristic of the parent is restored.

If the number were not halved before fertilisation, fusion of gametes would double the chromosome number in every generation. Meiosis is precisely what prevents this and lets the number stay fixed generation after generation.

Generating genetic variation

Meiosis also increases the genetic variability that passes from one generation to the next. This variation arises during the process itself:

• Crossing over exchanges segments between homologous chromosomes, producing new combinations of genetic material. …

Step 1 — Notice the question has two genuinely separate parts to build the answer around: keeping chromosome number constant, and generating variation.

Step 2 — Work out the chromosome-number argument. If gametes were diploid, fertilisation would double the chromosome number every generation; meiosis halves it in the gametes so fertilisation restores, not doubles, the parental number. …