Why Is Anaphase Crucial in the Cell Cycle?
Anaphase is a sub-process of mitosis and meiosis processes of cell division. Anaphase is the third stage of both the processes and is concerned with the division and distribution of cell materials. This process precedes the telophase of mitosis and meiosis while it supersedes the metaphase in both.
Mitosis and meiosis vastly differ in functionality and process, but they share several similarities as well. This applies to the anaphase too.
Subsequently, we shall discuss the two phases of anaphase, the structural changes in the shapes of chromosomes during anaphase, the importance of the phase, and much more. However, the main focus shall lie on mitosis.
Before we discuss what is anaphase, let us understand mitosis and meiosis, followed by an introduction of the stages before the stage.
Mitosis and Meiosis
Mitosis is a cell division process where the parent cell divides itself into two cells identical in nature. It has four stages: prophase, metaphase, anaphase, and telophase. Meiosis also has all four phases. However, it produces four cells at the end. Each cell has half of the chromosomes of the parent cell.
Before the anaphase, the hormones already have themselves paired and replicated. They also align themselves along a plane, called the equator of the cell, via the microtubules of the centrosome. In other words, the cell prepares itself to be equally divided during the anaphase.
What is Anaphase?
The cell division processes of mitosis and meiosis have a subprocess called the anaphase, which ensures that all the daughter cells get their rightful share of chromosomes and other cell materials. The stage is often divided into two more stages, namely anaphase 1 and 2.
Let us discuss the structures involved in anaphase before we move on to the details of the separation.
Anaphase Structure
The anaphase involves several crucial events. The structural changes brought by it are of paramount importance to the mitosis process, which is characterized by the formation of identical cells.
A simplistic view of anaphase structure is as follows:
The spindle fibres form an oval around the chromatids, which are attached to them at the kinetochore.
The centrosomes are the spindle fibre poles and hence lie at extreme ends.
The chiasmas are separated into two independent chromosomes and move towards the poles.
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Different Structures Involved in Anaphase
The anaphase of mitosis involves the functioning of the following:
Centrosome:
They form the interpolar microtubules and the astral microtubules around the chromosomes, which facilitate the movement of the chromatids. The microtubules now form an oval shape instead of a circular one.
The Chiasma:
The chromosomes are in an X shape, called chiasma, aligned at the equatorial plane. The shapes of chromosomes during anaphase change to Y-like or V-like after being pulled to both the poles.
Centromeres:
They join the two pairs of sister chromatids through a protein. They are specialized strings of DNA that allow themselves to be attached to the spindle fibre. Their protein is broken down during anaphase, which allows movement.
Motor Proteins:
Motor proteins carry the chromatids and other proteins throughout the cell division phase
What Happens During Anaphase?
The main characteristic of the anaphase of mitosis is the separation of two sister chromatids and their migration to the poles of the cell. During the process, the centromeres readily divide to give birth to two chromosomes.
What happens during anaphase is that proteins break the substance that makes the two copies of chromosomes stick together at the centromere. Simultaneously, the two ends of the spindle fibre contract to create an oval shape.
The tension thus caused pulls both the chromatids apart, and the sister chromatids reach their respective poles. This is facilitated by motor proteins.
This process has two phases, anaphase 1 and 2. The main difference between anaphase 1 and anaphase 2 is the part that moves in the phase. In the former, the sister chromatids separate to allow movement. These chromatids move towards the spindle pole along the microtubule towards the spindle poles in anaphase 2.
Fun Fact
Motor proteins discussed earlier can actually walk through the cell structure. It is these motor proteins that carry out what happens during anaphase. Hence, they carry the chromatids away from the centre by literally "walking" along the microtubules.
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In the anaphase of mitosis, the Anaphase-Promoting Complex/Cyclosome (APC/C), commonly called separase, is released. It is this chemical that triggers the phase. This is a multi-subunit ubiquitin ligase that facilitates the chromatids to separate through protein degradation. This protein degradation is vital to facilitate a smooth movement of the chromosomes during anaphase 2.
FAQs on Anaphase Explained: Structure, Stages, and Role in Cell Division
1. What is anaphase, and what are its key events in mitosis?
Anaphase is the third and typically shortest stage of mitosis, following metaphase. Its primary function is to ensure the equal segregation of genetic material. The key events are:
Splitting of Centromeres: The protein complexes holding the sister chromatids together are cleaved, allowing them to separate.
Separation of Sister Chromatids: Once separated, each chromatid is now considered a full-fledged chromosome.
Movement to Opposite Poles: The newly separated chromosomes are pulled towards opposite ends of the cell by the shortening spindle fibres.
2. What is the main difference between anaphase and the preceding metaphase stage?
The main difference lies in the arrangement and state of the chromosomes. In metaphase, duplicated chromosomes, each consisting of two sister chromatids joined at a centromere, align along the cell's equator, known as the metaphase plate. In anaphase, the centromeres divide, and the sister chromatids separate, moving to opposite poles of the cell. Therefore, metaphase is characterized by chromosome alignment, while anaphase is characterized by chromosome separation and migration.
3. What distinguishes Anaphase I of meiosis from Anaphase II?
The key distinction is what separates. In Anaphase I of meiosis, it is the homologous chromosomes that separate and move to opposite poles, while the sister chromatids remain attached at their centromeres. This is a reductional division. In contrast, Anaphase II is similar to mitotic anaphase, where the sister chromatids separate at the centromere and move to opposite poles. This is an equational division.
4. What is the significance of the Anaphase Promoting Complex (APC) in initiating anaphase?
The Anaphase Promoting Complex (APC) is a crucial protein checkpoint that acts as a gatekeeper for the transition from metaphase to anaphase. Its significance lies in triggering the separation of sister chromatids. The APC targets a protein called securin for destruction. Securin normally inhibits an enzyme named separase. When securin is destroyed, separase becomes active and cleaves the cohesin proteins that hold the sister chromatids together, thus initiating anaphase.
5. What would be the genetic consequence for daughter cells if sister chromatids failed to separate properly during anaphase?
If sister chromatids fail to separate, an event known as non-disjunction, the genetic consequences are severe. It results in daughter cells with an incorrect number of chromosomes, a condition called aneuploidy. One daughter cell would receive an extra copy of a chromosome (trisomy), while the other would be missing that chromosome (monosomy). Such errors are often detrimental and can lead to genetic disorders or cell death.
6. How do spindle fibres pull the chromosomes apart during anaphase?
The pulling force is generated by kinetochore microtubules, which are the spindle fibres attached to the centromere region of each chromosome. These microtubules shorten through a process of depolymerisation (disassembly) at the kinetochore end. This shortening, powered by molecular motor proteins, effectively reels in the chromosomes, pulling them towards the opposite spindle poles of the cell.
7. Besides chromosome separation, what other structural change occurs in the cell during anaphase?
Besides the separation of chromosomes (known as Anaphase A), the cell itself begins to elongate. This process, called Anaphase B, involves the non-kinetochore microtubules. These microtubules, which do not attach to chromosomes, overlap at the cell's middle. They slide past each other, pushing the two poles of the cell further apart. This elongation helps to further separate the two sets of chromosomes and prepares the cell for its final division in telophase and cytokinesis.
