KARYOKINESIS: STAGES AND THEIR CHARACTERISTICS - BIOLOGY - 2025

The cariocinesis is a term used to refer to the process of dividing the core. Mitosis involves cell division and two stages are distinguished in this phenomenon: karyokinesis and cytokinesis - division of the cytoplasm.

The fundamental structure that carries out this process, and is considered its “mechanical agent”, is the mitotic spindle. This is made up of microtubules and a series of associated proteins that divides it into two poles, where the centrosomes are located.

Source: Lordjuppiter, from Wikimedia Commons

Each centrosome is considered a non-membrane-delimited cellular organelle and consists of two centrioles and a surrounding substance, known as pericentriolar material. A peculiar characteristic of plants is the absence of centrioles.

There are a number of drugs that are capable of truncating karyokinesis. Among them are colchicine and nocodazole.

Stages of karyokinesis

The term karyokinesis comes from the Greek roots cario which means nucleus, and kinesis which translates as movement. Thus, this phenomenon refers to the division of the cell nucleus, that is, the first phase of mitosis. In some books, the word karyokinesis is used synonymously with mitosis.

In general, karyokinesis includes the equal distribution of genetic material to the two daughter cells, resulting from the mitotic process. Later, the cytoplasm is also distributed to the daughter cells, in the event of cytokinesis.

Cell cycle phases

In the life of a cell, several phases can be distinguished. The first is the M phase (M of mitosis), where the genetic material of the chromosomes has been duplicated and they are separated. This step is where karyokinesis occurs.

This is then followed by the G ₁ phase, or gap phase, where the cell grows and makes the decision to begin DNA synthesis. Next comes the S phase or synthesis phase, where DNA duplication occurs.

This stage involves the opening of the helix and the polymerization of the new strand. In the G ₂ phase, the accuracy with which the DNA was replicated is verified.

There is another phase, G ₀, which may be an alternative for some cells after the M phase - and not the G ₁ phase. In this stage, many of the body's cells are found, performing their functions. The mitosis phase, which involves the division of the nucleus, will be described in more detail below.

Prophase

Mitosis begins with prophase. At this stage the condensation of genetic material occurs, and very well defined chromosomes can be observed - since the chromatin fibers are tightly wound.

Furthermore, the nucleoli, regions of the nucleus that are not bounded by membrane, disappear.

Prometaphase

In the prometaphase, fragmentation of the nuclear envelope occurs and, thanks to them, the microtubules can penetrate the nuclear area. They begin to form interactions with the chromosomes, which by this stage are already highly condensed.

Each chromatid of the chromosome is associated with a kinetochore (the structure of the spindle and its components will be described in detail later). Microtubules that are not part of the kinetochore interact with the opposite poles of the spindle.

Metaphase

The metaphase lasts almost a quarter of an hour and is considered the longest stage of the cycle. Here the centrosomes are located on opposite sides of the cell. Each chromosome is attached to microtubules that are radiating from opposite ends.

Anaphase

In contrast to metaphase, anaphase is the shortest stage of mitosis. It begins with the separation of the sister chromatids in a sudden event. Thus, each chromatid becomes a complete chromosome. The elongation of the cell begins.

When anaphase ends, there is an identical set of chromosomes at each pole of the cell.

Telophase

In telophase the formation of the two daughter nuclei begins and the nuclear envelope begins to form. The chromosomes then begin to reverse the condensation and become increasingly lax. Thus ends the division of the nuclei.

The mitotic spindle

The mitotic spindle is the cellular structure that enables karyokinesis and mitosis events in general. This begins its formation process in the cytoplasmic region during the prophase stage.

Structure

Structurally, it is composed of microtubule fibers and other proteins associated with them. It is believed that at the time of assembly of the mitotic spindle, the microtubules that are part of the cytoskeleton disassemble - remember that the cytoskeleton is a highly dynamic structure - and provide the raw material for the elongation of the spindle.

Training

Spindle formation begins at the centrosome. This organelle is made up of two centrioles and the pericentriolar matrix.

The centrosome functions throughout the cell cycle as an organizer of cellular microtubules. In fact, in the literature it is known as the microtubule organizing center.

At the interface, the only centrosome that the cell has undergoes replication, obtaining a pair as the final product. These remain close together, close to the nucleus, until they separate in prophase and metaphase, as microtubules grow from them.

At the end of the prometaphase, the two centrosomes are located at opposite ends of the cell. The aster, a structure with a radial distribution of small microtubules, extends from each centrosome. Thus, the spindle is made up of centrosomes, microtubules, and asters.

Function

In chromosomes, there is a structure called the kinetochore. This is made up of proteins and they are associated with specific regions of the genetic material in the centromere.

During prometaphase, some of the microtubules of the spindle adhere to the kinetochores. Thus, the chromosome begins to move towards the pole from which the microtubules extend.

Each chromosome undergoes forward and backward movements, until it manages to settle in a middle region of the cell.

In metaphase, the centromeres of each of the duplicated chromosomes are located in a plane between both poles of the mitotic spindle. This plane is called the metaphase plate of the cell.

Microtubules that are not part of the kinetochore are responsible for promoting the process of cell division in anaphase.

References

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