Anaphase is the stage of cell division in which duplicate genetic material present in the nucleus of the parent cell is separated and distributed equally into two identical daughter cells.
During the preceding phase, metaphase, sister chromatids (replicated chromosomes) are aligned at the center of the cell along the metaphase plate.
In anaphase, each pair of sister chromatids separates, resulting in two identical but now independent chromosomes.
The separation of chromosomes is carried out by mitotic spindle fibers, also known as microtubules, which are attached to the chromosomes at both ends of the cell.
The chromatids separate simultaneously at the centromere, which allows each chromosome to move independently.
Once separated, each chromosome is pulled by the spindle microtubules toward opposite poles of the cell.
The primary function of anaphase is to ensure that each daughter cell receives an identical and complete set of chromosomes before the cell enters the final stage of division, telophase.
What happens during anaphase?
Anaphase begins with the activation of the anaphase-promoting complex (APC), which brings an end to metaphase and initiates the transition into anaphase.
The anaphase-promoting complex functions by tagging securin, a regulatory protein involved in controlling the metaphase-to-anaphase transition.
APC adds ubiquitin molecules to securin, marking it for degradation and allowing APC to act as an inhibitory chaperone.
Securin normally inhibits the enzyme separase, which is a type of protease responsible for chromatid separation.
When securin is destroyed, separase is released and becomes active.
Active separase breaks down the cohesin proteins that hold the sister chromatids together at the centromere.
Multiple types of microtubules generate the forces required for chromatid separation, including astral microtubules, kinetochore microtubules, and interpolar microtubules.
The breakdown of cohesin leads to splitting of the centromere, allowing sister chromatids to separate.
Kinetochore microtubules then pull the separated sister chromatids toward opposite poles of the cell.
As they move, the separated sister chromatids typically adopt a V- or Y-shaped appearance at each pole of the cell.
Astral and interpolar microtubules contribute to stretching, elongation, and overall shaping of the cell, causing it to take on an oval form.
Once separated, the sister chromatids are considered individual sister chromosomes.
These sister chromosomes contain identical genetic information but function independently as components of newly forming cells.
Successful completion of anaphase allows the cell to progress into the next phase of the cell cycle, known as telophase.
Anaphase in mitosis
Anaphase in mitosis begins when sister chromatids start to separate due to the activation of an enzyme called separase.
Separase functions by breaking the cohesin proteins that hold the sister chromatids together at the centromere.
Once the cohesion is removed, the spindle microtubules attached to the chromatids pull them toward opposite poles of the cell.
As the sister chromatids move apart, they are now considered individual chromosomes migrating to opposite ends of the cell.
Astral microtubules contribute by anchoring the spindle apparatus to the cell cortex, helping to position the spindle correctly.
Interpolar microtubules interact with each other and push the spindle poles further apart.
The combined action of astral and interpolar microtubules causes the cell to elongate and lengthen, giving it an oval shape during anaphase.
Anaphase in meiosis
Anaphase in meiosis occurs in two consecutive stages known as anaphase I and anaphase II.
During meiosis, there is no DNA replication between these two divisions, which results in the reduction of chromosome number.
As a result of this process, a diploid cell containing two alleles for each gene is reduced to haploid cells that contain only a single allele for each gene.
In anaphase I, homologous chromosomes separate and move toward opposite poles of the cell, while the sister chromatids of each chromosome remain attached to each other.
During this stage, the chromosomes are still connected to the spindle microtubules and migrate as intact units.
Anaphase II involves the actual separation of sister chromatids at the centromere.
In this second stage, the sister chromatids split into single chromatids, which then move toward opposite poles of the cell.
Anaphase I
During anaphase I, the kinetochore microtubules shorten, which pulls the homologous chromosomes toward opposite poles of the cell.
At the same time, the non-kinetochore microtubules begin to lengthen, pushing the centrosomes further apart.
As a result of these microtubule dynamics, the cell elongates in preparation for division at the center.
Throughout this phase, the cohesin proteins around the centromere remain intact and protected.
This protection is provided by a protein called Shugoshin, also known as the “guardian spirit.”
Shugoshin prevents the separation of sister chromatids, ensuring that only homologous chromosomes are segregated during anaphase I.
Anaphase II
Anaphase II occurs after metaphase II, marking the stage in which the remaining centromeric cohesins are no longer protected by Shugoshin.
These unprotected cohesins are cleaved, allowing the centromeres to divide.
As a result, the sister chromatids separate from each other.
Once separated, the chromatids are individually referred to as sister chromosomes.
The newly separated sister chromosomes are then pulled by spindle fibers and move toward opposite poles of the cell.
Reference
Expii. Anaphase: Definition & Diagrams. Retrieved from https://www.expii.com/t/anaphase-definition-diagrams-10161
Lumen Learning. Biology for Majors. (Lumenlearning.com)
Newcastle University Teaching Wiki. Anaphase. Retrieved from https://teaching.ncl.ac.uk/bms/wiki/index.php/Anaphase
Nature Scitable. Anaphase. Retrieved from https://www.nature.com/scitable/definition/anaphase-179/
Wikipedia. Meiosis — Anaphase I. Retrieved from https://en.wikipedia.org/wiki/Meiosis#Anaphase_I
Wikipedia. Anaphase. Retrieved from https://en.wikipedia.org/wiki/Anaphase
Khan Academy. Phases of Mitosis. Retrieved from https://www.khanacademy.org/science/biology/cellular-molecular-biology/mitosis/a/phases-of-mitosis
PHSchool.com. Anaphase (BioCoach Activity). Retrieved from http://www.phschool.com/science/biology_place/biocoach/mitosisisg/anaphase.html
