What happens in meiosis I?

During meiosis I, a critical phase in sexual reproduction, the chromosome number is halved while allowing for genetic variation through a process known as crossing over. At the start of meiosis, each chromosome consists of two sister chromatids, and as the homologous chromosomes pair up, sections of DNA can be exchanged between them. This recombination contributes to genetic diversity in the resulting gametes.

As the homologous chromosomes separate during the first meiotic division, the reduction division occurs, halving the chromosome number. For instance, in humans, where the diploid chromosome number is 46, meiosis I reduces this number to 23 chromosomes in each haploid gamete. The halving of the chromosome number is crucial for maintaining the species-specific chromosome count across generations, ensuring that when gametes fuse during fertilization, the diploid state is restored.

This integration of halving the chromosome number and introducing genetic variation through crossing over is fundamental to the process of meiosis, distinguishing it from mitosis, where chromosomes remain unchanged and the chromosome number is preserved. Thus, the essence of meiosis I is accurately captured by the choice that indicates both the halving of the chromosome number and the occurrence of crossing over.