Question

The karyotype of an organism is described by 2n=50. Assume a cell of this organism undergoes a complete normal cell cycle including mitosis. A) Determine the haploid number during G2 of the cell cycle. Describe in your own words how you determined this value. Please note: a value without a written explanation will not be graded. B) How many chromatids would you expect to find during prophase of mitosis in this organism? Describe in your own words how you determined this value. Please note: a value without a written explanation will not be graded. C) Determine the ploidy and total chromosome number in each of the resulting two daughter cells during G1 of the cell cycle. Explain in your own words how you determined these values. Please note: values without a written explanation will not be graded.

Answer 1

2n = 50
Diploid chromosome number = 50
Haploid chromosome number, n = 25
Number of homologous pairs of chromosomes = 25 pairs

A. The DNA replication occurs during the S-phase.
During the G2-phase, the genome content would be doubled, but the chromosome number does not change. This is due to the fact that the duplicated chromosomes are still attached at the parent centromere.
So, the total number of chromosomes = 50
Haploid number = 25
Total number of sister chromatids = 100

B. Total number of sister chromatids during the mitotic prophase = 100
Each chromosome contains two sister chromatids.
50 chromosomes = 100 chromatids

C. Mitosis is an equational division. i.e. it produces identical cells with the same chromosome number as that of the parent cell.
Number of chromosomes in daughter cells during the G1 phase = 50
Ploidy = 2n