The correct option is Meiosis producing germ-cells with half the chromosomes.
Explanation
In sexually reproducing organisms, the maintenance of a constant chromosome number across generations depends on the alternation between meiosis and fertilization. Somatic cells are typically diploid (2n), meaning they contain two sets of chromosomes. To prevent the doubling of chromosomes in every generation, the gametes involved in fertilization must be haploid (n).
Option Analysis
- Meiosis producing germ-cells with half the chromosomes is Correct: Meiosis is a specialized form of cell division known as reduction division. It occurs in germ cells to produce gametes (sperm and ova) that contain half the number of chromosomes (n) of the parent cell. When fertilization occurs, the fusion of two haploid gametes restores the original diploid chromosome number (2n) in the zygote. This mechanism ensures that the species-specific chromosome number remains stable over generations.
- Mitosis in germ cells is Incorrect: Mitosis is an equational division where daughter cells inherit the exact same number of chromosomes as the parent cell. If germ cells produced gametes via mitosis, the gametes would be diploid (2n). Fertilization would then result in a tetraploid (4n) zygote, leading to an exponential increase in chromosome number with each generation.
- Binary fission of zygote is Incorrect: Binary fission is a method of asexual reproduction common in unicellular organisms (e.g., bacteria, amoeba). It involves the division of a parent cell into two identical daughter cells and does not involve the formation of gametes or the restoration of ploidy required in sexual reproduction.
- Spore formation in somatic tissue is Incorrect: Spore formation is generally a method of asexual reproduction found in fungi, algae, and plants. While spores can be part of a sexual cycle (e.g., in alternation of generations), the option mentions "somatic tissue," which implies vegetative growth via mitosis, not the reduction division required to maintain chromosome counts in sexual reproduction.
Key Takeaway: Meiosis ensures the production of haploid gametes, counterbalancing the doubling effect of fertilization, thereby maintaining the specific chromosome number of a species across generations.