Final answer:
Meiosis includes chromosome duplication and crossing over, followed by two rounds of division to ensure genetic variability and maintain the diploid state after fertilization. Independent assortment and crossing-over contribute to over eight million possible gamete combinations in humans, providing significant genetic diversity.
Step-by-step explanation:
In meiosis, after DNA replication, homologous chromosomes (each consisting of two sister chromatids) undergo crossing over, which introduces genetic variation. This process precedes two rounds of nuclear division: meiosis I, which separates the homologous chromosomes, and meiosis II, which separates sister chromatids. The result is four genetically unique haploid daughter cells, each with half the number of chromosomes as the original diploid cell.
In humans, due to independent assortment and crossing over, there are millions of possible combinations of chromosomes, contributing to genetic diversity. This complexity ensures genetic variation in populations, which is beneficial for evolution and adaptation.
Independent assortment during metaphase I further increase variability by arranging homologous chromosomes randomly at the metaphase plate, resulting in over eight million possible alignments in humans without considering the added variability from crossing over.
Lastly, the purpose of reducing the chromosome number in gametes is to maintain the diploid state upon fertilization, where two haploid gametes combine to form a new organism with the correct number of chromosomes.