Main Difference – Karyokinesis vs Cytokinesis
Karyokinesis and cytokinesis are two steps in the cell division. Karyokinesis is the division of the replicated genetic material in an equal manner between two daughter nuclei. A series of events are taken place during the karyokinesis which is collectively referred to as mitosis. Usually, during mitotic cell division, karyokinesis is followed by cytokinesis, the division of the cytoplasm. During cytokinesis, cytoplasm and organelles are equally divided. The main difference between karyokinesis and cytokinesis is that karyokinesis is the equal distribution of replicated genetic material between two daughter nuclei whereas cytokinesis is the approximately equal distribution of cytoplasm between the two daughter cells.
This article explores,
1. What is Karyokinesis
– Definition, Characteristics, Mechanism
2. What is Cytokinesis
– Definition, Characteristics, Mechanism
3. What is the difference between Karyokinesis and Cytokinesis
What is Karyokinesis
Karyokinesis is the equal distribution of genetic material between two nuclei, which is the first step of cell division. It is composed of a series of sequential events of chromosomal segregation, collectively referred to as mitosis. Mitosis is one of the two types of nuclear division that occurs in vegetative cells during asexual reproduction, in order to increase the number of cells in the population. The other type of nuclear division is meiosis, which is observed in germ cells during the production of gametes in sexual reproduction.
The mitotic phase is called the M phase of the cell cycle. Eukaryotic chromosomes are replicated during S phase of the interphase, which is the first phase of the cell cycle. Interphase is followed by the M phase. Replicated chromosomes contain two sister chromatids joined together by their centromeres. Two types of mitosis can be identified among organisms: open mitosis and closed mitosis. During the open mitosis in animals, nuclear envelope is broken down in order to separate the chromosomes. But in fungi, chromosomes are separated in the intact nucleus, which is called closed mitosis. An overview of mitosis is shown in figure 1.
An Overview of Mitosis
Mechanism of Karyokinesis
Replicated chromosomes are tightly coiled by chromosome condensation, exhibiting short, thick, thread-like structures during the interphase. Their centromeres are also attached to the kinetochores, which is an important type of proteins in nuclear division. Proteins required for the cell division are synthesized during the interphase, and cellular components including organelles increase their number.
Mitotic division takes place through four sequential phases: prophase, metaphase, anaphase and the telophase. During prophase, condensed chromosomes are aligned in the equatorial plate of the cell with the aid of forming spindle apparatus. Spindle apparatus is composed of three components: spindle microtubules, kinetochore microtubules and the kinetochore protein complexes. Kinetochore protein complexes are assembled with centromeres of the each chromosome. All microtubules in a cell are controlled by two centrosomes arranged at the opposite poles of the cell, forming the spindle apparatus. Kinetochore microtubules from each pole are attached to the centromere through the kinetochore protein complex.
During metaphase, kinetochore microtubules are contracted, aligning the individual bivalent chromosomes on the cell equator. Tension is generated on the centromere by further contracting kinetochore microtubules during the anaphase. This tension leads to the cleavage of cohesin protein complexes in the centromere, separating the two sister chromatids apart, producing two daughter chromosomes. During telophase, these daughter chromosomes are pulled towards the opposite poles by the contraction of the kinetochore microtubules. Phases of the mitosis along with the interphase is shown in figure 2.
Figure 2: Phases of Mitosis with Interphase
What is Cytokinesis
Cytokinesis is the division of the cytoplasm into two daughter cells, along with the two daughter nuclei, organelles, and cytoplasm. During the cell cycle of eukaryotes, karyokinesis is followed by the cytokinesis. The process of approximately equal division of the cytoplasm is called the symmetrical cytokinesis. On the contrary, during oogenesis, the ovum consists of almost all the organelles and the cytoplasm of the precursor germ cell, genocytes. Cells of the tissues like liver and skeletal muscle omit the cytokinesis by producing multi-nucleated cells.
In mitotic division, daughter cells enter the interphase after the completion of the cytokinesis. In meiotic division, gametes are used for the completion of the sexual reproduction after the completion of cytokinesis by fusing with the other type of the gametes in the same species.
Mechanism of Cytokinesis
The main difference between plant cell and animal cell cytokinesis is the formation of new cell wall surrounding the daughter cells in plants. In plant cells, a cell plate is formed in the middle of the parent cell with the aid of microtubules and vesicles. Phragmoplast is the microtubule array, supporting and guiding the cell plate formation. Vesicles containing proteins, carbohydrates, and lipids are trafficked into the midzone of the phragmoplast by microtubules. Vesicles are fused with microtubules, forming a tubular-vesicular network. The deposition of cell wall components like cellulose, hemicellulose, and pectin leads to the maturation of the cell plate. This cell plate grows towards the cell membrane (centrifugal).
In animal cells, a cleavage furrow is formed between the two daughter cells. The formation of cleavage furrow begins at the edges of the cell (centripetal) in animal cell cytokinesis. Thus, midbody formation can be identified only in the animal cell cytokinesis. Animal cell cytokinesis is tightly regulated by signal transduction pathways. ATP is required for the contraction of actin and myosin II proteins. Animal cell cytokinesis is shown in figure 3.
Figure 3: Animal Cell Cytokinesis
Difference Between Karyokinesis and Cytokinesis
Definition
Karyokinesis: Nuclear division is known as karyokinesis.
Cytokinesis: The division of the cytoplasm is known as cytokinesis.
In Cell Division
Karyokinesis: Karyokinesis is the initial step in the cell division.
Cytokinesis: Cytokinesis is the final step in cell division.
Function
Karyokinesis: The nucleus of the cell is divided into two daughter nuclei during karyokinesis.
Cytokinesis: The cytoplasm of the parent cell is divided into two daughter cells curing cytokinesis.
Correspondence
Karyokinesis: During karyokinesis, daughter chromosomes are divided into two daughter nuclei.
Cytokinesis: During cytokinesis, two daughter nuclei are divided into two daughter cells.
Distribution
Karyokinesis: During the karyokinesis, equal distribution of genetic material takes place.
Cytokinesis: During cytokinesis, approximately equal distribution of cellular organelles along with the cytoplasm takes place.
Complexity
Karyokinesis: Karyokinesis is a sequential process involving the complex segregation of genetic material.
Cytokinesis: Cytokinesis is a comparatively simple process, which involves approximately equal distribution of cytoplasm.
Events
Karyokinesis: Spindle formation and movement of chromosomes occur during karyokinesis.
Cytokinesis: Formation of the cell plate or cleavage furrow occurs during cytokinesis.
Other Names
Karyokinesis: Karyokinesis is usually referred to as the mitotic division.
Cytokinesis: Cytokinesis is referred to as the cytoplasmic division in both mitotic and meiotic cell divisions.
Sequence
Karyokinesis: Karyokinesis is sometimes followed by cytokinesis. In mitosis, karyokinesis is followed by cytokinesis. But, in meiosis 1, karyokinesis is followed by another karyokinesis, not by the cytokinesis.
Cytokinesis: Cytokinesis does not occur without karyokinesis.
Conclusion
Karyokinesis and cytokinesis are two steps in the cell division. Karyokinesis is also known as mitosis. During mitosis, replicated chromosomes in the parent nucleus are equally divided into two daughter nuclei through several sequential events, known as phases. Prophase, metaphase, anaphase and telophase are the four phases of the nuclear division. The equal segregation of chromosomes in the parent nucleus into two daughter nuclei is ensured by the spindle apparatus. Karyokinesis is followed by the cytokinesis in the mitotic division. In plant cells, the cytoplasm of the parent cell is divided by the formation of a cell plate in the middle of the parent cell. In animal cells, a cleavage furrow is formed by the plasma membrane, separating the two daughter cells. The main difference between karyokinesis and cytokinesis is the distribution of materials during the two processes.
Reference:
1.”Mitosis.” Wikipedia. Wikimedia Foundation, 12 Mar. 2017. Web. 13 Mar. 2017.
2.”Cytokinesis.” Wikipedia. Wikimedia Foundation, 12 Mar. 2017. Web. 13 Mar. 2017.
Image Courtesy:
1. “Major events in mitosis” By Mysid – Vectorized in CorelDraw by Mysid from . (Public Domain) via Commons Wikimedia
2. “Nuclear envelope breakdown and reassembly in mitosis”By Ya-Hui Chi , Zi-Jie Chen and Kuan-Teh Jeang – The nuclear envelopathies and human diseases (CC BY 2.0) via Commons Wikimedia
3. “Cytokinesis eukaryotic mitosis” By Cytokinesis_procariotic_mitosis.svg: LadyofHats – Cytokinesis_procariotic_mitosis.svg (Public Domain) via Commons Wikimedia
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