Difference Between Cytokinesis and Karyokinesis

Introduction

Eukaryotic cells multiply by progressing through the four phases of the cell cycle: G₁, G₂, S, and M. DNA replication occurs during the S phase, followed by the separation of chromosomes (karyokinesis) in the M phase and cell division (cytokinesis) during the G₁ and G₂ growth stages. The G₁ phase is further divided into early G₁ (post-mitotic G₁), mid-G₁, where most cell growth happens, and late G₁, during which final preparations for DNA replication are made. The G₂ phase is thought to be important for regulating chromosome replication and assembling the mitotic spindle.

Cells that are not actively dividing may undergo terminal differentiation, enter senescence, or experience apoptosis to permanently leave the cell cycle. Alternatively, they may pause in a temporary quiescent state known as G₀ if they contain G₁ DNA, although quiescence can sometimes occur in the G₂ phase (G₂ arrest).

Explain Karyokinesis and Cytokinesis

Karyokinesis: During the first stage of cell division, called karyokinesis, genetic material is evenly divided between two nuclei. Mitosis is the term used to describe a series of chromosomal separation events. In nonsexual reproduction of vegetative cells, mitosis, one of two forms of nuclear division, occurs to increase the number of cells.

The second form of nuclear division, meiosis, takes place in germ cells during sexual reproduction to produce gametes. The stages of karyokinesis, or mitosis, include prophase, prometaphase, metaphase, anaphase, and telophase, which lead to the division of the cell’s nucleus.

Karyokinesis Stages

Prophase: Mitosis starts when the number of chromosomes, which were duplicated in the S phase, condenses, making them more visible under a microscope as distinct structures. At this point, a structure called the centriole is duplicated, and the two daughter centrioles move to opposite ends of the cell, where they begin to form the mitotic spindle, often composed of microtubule proteins.

• Prometaphase: In this stage, the chromosomes, consisting of paired sister chromatids connected at a structure called the centromere, begin to migrate toward the center of the cell. Meanwhile, the centrioles continue to organize the mitotic spindle, acting as a network of fine fibers.
• Metaphase: At this point, the 46 human chromosomes are lined up along the metaphase plate, an imaginary plane at the cell’s center, perpendicular to the spindle’s poles. Each sister chromatid pair is positioned with one chromatid on either side of the plate, connected by the centromere.
• Anaphase: During anaphase, the spindle fibers pull the sister chromatids apart, moving them toward opposite poles of the cell. A cleavage furrow starts to form, signaling the beginning of cytokinesis. At each end of the cell, a complete set of 46 chromatids gathers.
• Telophase: After the chromosomes have been divided and relocated, the cell re-forms a nuclear membrane around each set of chromosomes. The genetic material begins to unwind and decondense. Telophase is essentially the reverse of prophase, and it also marks the progression into early cytokinesis.

Significance of Karyokinesis

Karyokinesis is a vital process for living organisms as it ensures that every cell, apart from reproductive cells, can regenerate. This guarantees the proper functioning of cells and tissues, allowing them to grow, develop, and repair within a set time frame.

As a result, karyokinesis supports the growth of multicellular organisms, the development of the body, and the repair of tissues that suffer damage. Thanks to this complex process, cells can persist for extended periods and transfer their traits to future generations. The daily replacement of dead cells happens naturally and continuously.

Cytokinesis: The final step in eukaryotic cell division, called cytokinesis, involves the division of the cytoplasm, organelles, and cell membrane to form two daughter cells. Although cytokinesis typically takes place at the end of mitosis, right after telophase, it is an independent process.

In most animals, cytokinesis starts during late anaphase or early telophase to ensure complete separation of the chromosomes. The same spindle apparatus that handled chromosome separation also drives the visible cytokinesis processes within the cell. The spindle components responsible for breaking down the chromosomes at the end of cell division help reorganize the chromosomes within the two newly formed cells.

Cytokinesis Stages

•  Stage 1: The focus of this initial phase is the formation of the cleavage furrow, the area where actin filaments will begin to constrict the cell. The spindle, which ensures chromosomes are evenly divided between the two nuclei, marks this region. The spindle helps organize the cell internally, connecting the plasma membrane and nuclei. Astral microtubules, another set of tiny structures on the spindle, interact with the cell membrane and guide the arrangement of actin fibers at the site of the future cleavage furrow.
• Stage 2: Once the location of the cleavage furrow is identified, actin filaments gather in the area. Numerous other proteins, including myosin, are also drawn to this site, allowing the actin filaments to contract as the contractile ring begins to form.
• Stage 3: Myosin, a type of mechanical protein known as a molecular motor, plays a key role here. While myosin is important for muscle contraction in the body, in a single cell, it contracts the contractile ring. Powered by ATP, this action squeezes the cell at its center, helping the two daughter cells take shape.
•  Stage 4: Once the cell membrane is constricted as far as possible, it eventually breaks apart completely. The membrane quickly splits at the breaking point, resulting in two identical daughter cells, each fully capable of functioning on its own.

Significance of Cytokinesis

After mitosis and each of the two meiotic divisions, two separate nuclei exist within a single cell. When a cell divides into multiple daughter cells, a process called cytokinesis ensures that each daughter cell contains one nucleus. Cytokinesis begins during the nuclear division phase known as anaphase and continues until telophase. A ring of protein fibers, known as the contractile ring, encircles the center of the cell, just beneath the plasma membrane.

As the contractile ring pulls inward, it stretches the plasma membrane at the center of the cell, forming a cleavage furrow. Ultimately, the contractile ring contracts, resulting in the formation of two distinct cells, each enclosed by its own plasma membrane.

Karyokinesis and Cytokinesis Difference

 Summary

The cell cycle includes two phases that involve cell division: karyokinesis and cytokinesis. Karyokinesis refers to the equal separation of replicated genetic material into two daughter nuclei. This phase encompasses a series of events collectively known as mitosis. Following karyokinesis, cytokinesis occurs, which is the division of the cytoplasm. During cytokinesis, both the cytoplasm and organelles are divided between the two new cells.