1. Starting with Diploid Cells:
* Body cells (somatic cells) in sexually reproducing organisms are diploid, meaning they have two sets of chromosomes (one from each parent).
2. Meiosis I: Reducing the Chromosome Number
* Homologous chromosome pairing: Meiosis I begins with the pairing up of homologous chromosomes. These are chromosome pairs that carry the same genes but may have different versions of those genes (alleles).
* Crossing over: During this pairing, homologous chromosomes exchange genetic material through a process called crossing over. This increases genetic diversity.
* Separation of homologous chromosomes: Meiosis I culminates in the separation of homologous chromosomes, resulting in two daughter cells, each with half the number of chromosomes as the original cell (haploid).
3. Meiosis II: Separating Sister Chromatids
* Sister chromatid separation: Meiosis II is similar to mitosis but occurs in haploid cells. Each chromosome, now consisting of two identical sister chromatids, is separated.
* Four haploid gametes: The final result of meiosis II is four haploid gametes (sperm or egg cells) that each contain a single set of chromosomes.
4. Fertilization: Restoring Diploid Number
* Fusion of gametes: During fertilization, a haploid sperm cell fuses with a haploid egg cell.
* Diploid zygote: The fusion of these gametes results in a diploid zygote (fertilized egg) with a full set of chromosomes, half from the mother and half from the father.
In summary: Meiosis reduces the chromosome number from diploid to haploid during gamete formation, and fertilization restores the diploid number in the offspring. This ensures that each generation of sexually reproducing organisms maintains a constant number of chromosomes.
