Question

You are studying a species of single-celled yeast. For the certain group you are studying, you notice that there are 7 groups of chromosomes on the metaphase plate during mitosis. Do you think these are haploid cells? Diploid cells? Is it not possible to determine? Explain your answer.

Answer 1

Yeast exists in two phases: haploid and diploid, both of which can reproduce by mitosis. Mitosis is an equational division where the daughter cells receive the same chromosomal complement as the parental cell. This is achieved by chromosomal duplication before entering the mitotic phase. Thus, for a haploid cell with N chromosomes, the chromosomal content is 2N at the onset of mitosis. These 2N chromosomes arrange at the metaphase plate during metaphase after which they are pulled by the spindle fibers to each pole before the cell divides and each daughter cell receives half the content or N chromosomes.

Similarly, for a diploid cell with 2N chromosomes, there is a chromosomal duplication event leading to 4N chromosomes after which the cell enters the mitosis phase. At metaphase, these 4N chromosomes are arranged as tetrads or bivalents at the metaphase after which the sister chromatids are pulled into each daughter cell leading to 2N chromosomes in each daughter cell and thus the restoration of the chromosomal number.

Now for the cell group under investigation, there are 7 chromosomal pairs during metaphase or 14 chromosomes. Hence each daughter cell will receive 7 chromosomes. If this would be a diploid it would be the cell had 3.5 chromosomes as N which cannot be correct. Hence the cell is haploid with N=7.