Final answer:
In sexual reproduction, the reduction division stage is known as meiosis I, which reduces the ploidy level from diploid to haploid to create gametes.
Step-by-step explanation:
In sexual reproduction, the reduction division stage is known as meiosis I. This is the stage where the ploidy level is reduced from diploid (two sets of chromosomes) to haploid (one set of chromosomes), creating the gametes that are required for fertilization. The term 'reduction division' specifically refers to meiosis I, highlighting the halving of chromosome sets from the parental nucleus. It is important to distinguish meiosis I from meiosis II. While meiosis II also consists of a division process, it follows meiosis I and involves the separation of sister chromatids without further reduction in chromosome number. Therefore, the reduction division stage needed to maintain chromosome numbers through generations in sexual reproduction is meiosis I.
Meiosis is also compared to mitosis, as they both share similar mechanisms. However, meiosis includes one round of chromosome duplication and two rounds of nuclear division, resulting in haploid cells, whereas mitosis results in daughter nuclei that are genetically identical to the original parent nucleus and are at the same ploidy level.