Describe in words what happens to the chromosomes (or chromatids) in each phase of the eukaryotic cell cycle.

The DNA content of a cell can be described using the following terms:

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  Haploid (n): The term haploid describes the number of different types of chromosomes in a cell. Human cells contain 23 different types of chromosomes. So, for human cells, n =23.

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  Diploid (2n): The term diploid describes a cell that has two copies of each type of chromosome. Since a haploid human cell contains 23 different types of chromosomes, a diploid human cell would contain 23 pairs of chromosomes for a total of 46 chromosomes. So, for human cells, 2n = 46.

The eukaryotic cell cycle is divided into four phases:

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  Gap 1 (G1): During the G1 phase, cells are diploid (2n). So, in human cells there are 23 pairs of chromosomes in each cell. Each chromosome has one set of chromatids.

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  Synthesis (S): During the S phase, each chromosome replicates its DNA. Each chromosome now contains two sets of chromatids. So, in human cells, there are still 23 pairs of chromosomes, but each chromosome is now composed of two complete copies of itself held together by the centromere. This means the cell now has a DNA content equal to 4n.
  

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  Gap (G2): During the G2 phase, the DNA content of the cell remains at 4n. Therefore, in humans, there are still 23 pairs of chromosomes with two sets of chromatids.

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  Mitosis (M): During mitosis (M), the chromosomes condense and prepare for cell division. The DNA content remains at 4n until cell division. When the cell divides, the two sets of chromatids on each chromosome split apart and are given to the daughter cells. Each daughter cell now has 23 pairs of chromosomes, each consisting of one set of chromatids. This means that the DNA content of the daughter cells is now back to 2n.