The ploidy of a cell during Anaphase is a common point of confusion in biology. The answer depends entirely on the type of cell division occurring—Mitosis or Meiosis—and, in Meiosis, which division stage is being discussed. Anaphase is characterized by the physical separation and movement of chromosomes, which temporarily alters the chromosome count within the single dividing cell.
Defining Ploidy: Haploid, Diploid, and Chromosome Counting
Ploidy refers to the number of complete sets of chromosomes in a cell, represented by \(n\). A haploid cell (\(n\)) contains one complete set of chromosomes, such as the 23 chromosomes found in human gametes. A diploid cell (\(2n\)) contains two complete sets, one inherited from each parent, such as the 46 chromosomes found in most human somatic cells.
Accurately counting chromosomes requires counting the number of functional centromeres present. The centromere is the constricted region where sister chromatids are joined after DNA replication. A duplicated chromosome consists of two sister chromatids held by one centromere, counting as one chromosome. Once sister chromatids separate during Anaphase, however, each newly separated chromatid is considered an individual chromosome because it possesses its own centromere.
Anaphase in Mitosis: Maintaining the Diploid State
Mitosis is the process of cell division that produces two genetically identical daughter cells, primarily for growth and tissue repair. Anaphase in Mitosis begins with the cleavage of proteins holding the sister chromatids together at the centromere. The sister chromatids then separate and are pulled toward opposite poles of the cell by spindle fibers.
Before Anaphase, a human diploid cell has 46 chromosomes, each consisting of two chromatids. When these 46 sister chromatid pairs separate, the total chromosome number temporarily doubles to 92, as there are now 92 individual centromeres moving apart. Despite this temporary doubling, the cell remains functionally diploid (\(2n\)). This is because the cell contains the genetic information for two complete sets of chromosomes, which will be equally distributed to the two resulting daughter cells.
Anaphase in Meiosis: The Reduction Division
Meiosis is a specialized cell division process that produces four haploid gametes through two sequential divisions: Meiosis I and Meiosis II. Meiosis I is known as the reduction division because it is where the ploidy level is halved.
Anaphase I
Anaphase I is distinct from mitotic Anaphase because homologous chromosomes separate and move to opposite poles, while the sister chromatids remain attached at their centromeres. In a human cell, 23 pairs of homologous chromosomes separate, with one full set of 23 duplicated chromosomes moving to each pole.
Because only one set moves to each pole, the cell transitions from diploid to haploid (\(n\)), even though each chromosome still consists of two chromatids. The cells formed after Meiosis I are considered haploid, containing 23 duplicated chromosomes.
Anaphase II
Anaphase II occurs in the two haploid cells produced by Meiosis I and closely resembles Anaphase in Mitosis. During this stage, the centromeres divide, and the sister chromatids separate, moving to opposite poles.
Since the cell entering Meiosis II is already haploid (containing 23 chromosomes), the separation of sister chromatids does not change the ploidy level. This separation ensures that the resulting four daughter cells each receive 23 individual, unduplicated haploid chromosomes.
Why Anaphase Is Confusing
The confusion regarding Anaphase’s ploidy stems from the distinction between the absolute chromosome number and the cell’s ploidy level (the number of chromosome sets). In Mitotic Anaphase and Anaphase II, the physical separation of sister chromatids causes the temporary doubling of the chromosome count within the single cell. For instance, a human cell in Mitotic Anaphase temporarily has 92 chromosomes moving apart.
Ploidy is defined by the number of complete sets of genetic information that will be packaged into the final daughter nuclei, not the transient count during the separation phase. Therefore, Mitotic Anaphase is diploid (\(2n\)) because it leads to two diploid daughter cells. Anaphase I is the stage where the cell transitions to haploid (\(n\)), and Anaphase II is a haploid stage that separates the final chromatids to produce unreplicated haploid chromosomes. The answer is that Anaphase is diploid in Mitosis and haploid in both stages of Meiosis.