Cells with a full set of chromosomes are referred to as diploid or 2n, whereas cells with half the chromosomes are haploid or n. At which stage(s) of meiosis I are the cells diploid and at which stage(s) are they haploid

Explanation:

During prophase I,  chromosomes get condensed. Each of the chromosomes gets in pair with its homologous one. They do so to make the crossing-over possible, a stage where they interchange their parts → 2n

During metaphase I, each of the homologous pairs is driven to the equatorial plane, where they randomly line up → 2n

During anaphase I, occurs the independent separation of homologous chromosomes that migrate to opposite poles of the cell. This separation generates different chromosomal combinations in the daughter cells. There are two alternatives per homologous pair → 2n  

In telophase I, half of the chromosomes are already in one of the poles, while the other half is on the other pole. Each group of chromosomes has now half the number of the original cell. The nuclear membrane forms again in each pole → n

Finally, occurs cytokinesis, which involves the invagination of the cell membrane and cytoplasmic division.

The two new cells are ready for meiosis II.

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