Anaphase Definition
Anaphase is a stage during eukaryotic cell division in which the chromosomes are distributed to opposite poles of the cell. The stage before anaphase, metaphase, the chromosomes are drawn to the metaphase plate in the center of the cell.
Although the chromosomes were highly condensed at the beginning of cell division, they continue to condense through anaphase. Anaphase begins after the cell has passed the spindle formation checkpoint, which allows chromosomes, or chromatids, to separate.
When the microtubules that connect the chromosomes to the centrosomes shorten, the chromosomes are pulled toward the centrosome until they form a semicircle around it. In the next phase of cell division, the telophase, the cell rebuilds the nucleus and prepares for division.
The checkpoint of spindle formation occurs before anaphase can begin. This cellular mechanism ensures that all chromosomes are connected to microtubules and aligned on the metaphase plate.
Once this step occurs, the cell releases a signal that creates the anaphase-promoting complex, or APC, a substance that kickstarts the process of dividing homologous chromosomes or sister chromatids, depending on which cell cycle is occurring.
The APC, as seen in the graph below, will break down Securin, an inhibitory molecule that stops Separase from working. Once separase is released, it can act on the cohesins that hold chromatids together.
Cohesins are macromolecules made up of multiple proteins. When these proteins are broken down by separase, the chromatids come apart. In the first division of meiosis, homologous chromosomes are held together by cohesins, which are broken down during anaphase I.
Chromosomal Cohesion
The microtubules remain attached to the kinetochore after the cohesins break apart. The kinetochore microtubules then shrink toward the centrosomes (not shown), pulling the chromosomes apart. Non-kinetochore microtubules grow in the opposite direction, expanding the cell and further separating the chromatids.
Traditionally, sister chromatids are called sister chromosomes after they separate because they carry the same information and function independently in their new cells. After the chromosomes are completely separated, a nuclear envelope forms and the cytoplasm is divided in the final steps of cell division.
Anaphase In Mitosis
Mitosis is the process cells use to make exact copies of themselves. Mitosis creates two new daughter cells, each identical to the parent, from a single parent. Before mitosis, the chromosomes containing DNA are replicated and the replicated sister chromatids remain attached.
Before anaphase, chromosomes are condensed, spindle fibers form from microtubules, and chromosomes align on the metaphase plate. The sister chromatids begin to separate at the onset of anaphase when the separase begins to break the cohesin that holds them together.
Anaphase ends when telophase and cytokinesis begin, when the nuclear envelope reforms and the chromosomes begin to unwind. Once they have detached and the cells have divided, they can resume their own function. This marks the end of cell division and the beginning of interphase.
Anaphase In Meiosis
Anaphase I
Meiosis consists of two consecutive cell divisions between which no DNA is replicated. This means that a diploid organism containing two alleles for each gene is reduced to a haploid organism with only one allele on each gene.
These alleles are separated during anaphase I. Before meiosis, the DNA is duplicated, which in turn produces sister chromatids that are attached to each other as single chromosomes. These chromosomes have homologous pairs that contain the other alleles for the genes on the chromosome.
These chromosomes are also duplicated in sister chromatids. During meiosis I, the homologous chromosomes are separated.
The homologous chromosomes are attached to each other during prophase I of meiosis by similar cohesin molecules that bind sister chromatids. As the meiotic spindle adjusts during metaphase I, the cell ensures that each homologous pair of microtubules is attached from each side of the cell.
These microtubules then pull against each other to move the homologous pairs to the metaphase plate. While the pairs are connected, they can exchange genetic information in a process called recombination. When the homologous pairs are separated during anaphase I, the DNA variations are determined for different cells, ensuring variability in a population’s genetics.
Anaphase II
Meiosis is completed with a second division of each new daughter cell. The same stages occur, this time in a manner more similar to mitosis. The chromosomes, still made up of sister chromatids, align on the metaphase plate and must complete a checkpoint to enter anaphase II.
At the beginning of anaphase II, the sister chromatids are separated when the cohesin that binds them is released. The copies of the chromosome are then separated to their final destination. These new cells give rise to gametes that can fertilize a gamete of the opposite sex to produce new offspring.
When the cellular checkpoints that lead to anaphase I or anaphase II fail, the end result is too many copies of a chromosome (or many) in the final gamete. This condition, known as nondisjunction, can lead to fatal birth defects and other symptoms in many sexually reproducing animals.
In humans, these disorders include Down’s syndrome, Edwards’ syndrome, Klinefelter’s syndrome and Turner’s syndrome. These syndromes have a variety of developmental features that produce phenotypes that differ from the average.
