Final answer:
Diploid cells become haploid during meiosis I at the stage of anaphase I when homologous chromosomes are separated. Telophase I concludes with the formation of two daughter cells that are already haploid, which then enter meiosis II to further divide into four distinct haploid gametes.
Step-by-step explanation:
Diploid cells first become haploid during meiosis in the phase known as anaphase I. It is at this point that homologous chromosomes separate and are pulled to opposite poles of the cell, reducing the chromosomal number by half. Cells remain diploid at the end of telophase I, but by the completion of telophase II, the division process is complete, and four haploid cells are formed.
Cytokinesis, which typically occurs concurrently with telophase, finalizes the cell division. Thus, after telophase I and cytokinesis, the two daughter cells are indeed haploid, each containing one set of chromosomes. However, these haploid cells must undergo a second round of division, meiosis II, where sister chromatids are separated to produce four distinct haploid gametes.