AMITOSIS or Direct Nuclear Division :

It is also called direct cell division .In this type of division  there is no differentiation of chromosomes and spindle. The nuclear envelope does not degenerate. The nucleus elongates and constricts in the middle to form two daughter  nuclei.The is followed by a centripetal constriction of the cytoplasm to form two daughter  cells.It is not a regular method of division because it does not divide the nuclear matter equitably. The nuclei formed are frequently of unequal size.It occurs in metabolic nucleus (e.g.,meganucleus of Paramecium)of some protozoa. The growth  of embryonic membrane of some vertebrates is due to amitosis . It also occurs  in diseases cells,  certain protists and degenerating cells of higher plants. Some authors include cell division  of monerans ( e.g. bacteria ) under amitosis due to absence  of spindle formation. As compared  to  amitosis, other types of divisions ( mitosis and meiosis) are called indirect cell divisions.

CEll Division:

Cell division  is a means of multiplication in the unicellular  organisms. In multicellular organisms, it brings about embryonic development and growth, and also plays a role in repair and maintenance of the body, and also  in reproduction, both asexual  and sexual 

Cell division was first studied by Prevost  and Dumas ( 1824),when they described the cleavage of the zygote of frog. The chromosomes occupy the central position in cell division. As the vehicle of heredity, they determine the characteristics of the cell and its progeny. 

Mode of Cell division 

Depending on the manner in which the nucleus divides, three different  kinds of cell division are recognized.

(i) Direct nuclear division  or Amitosis 

(ii) Indirect nuclear division or Mitosis 

(iii) Reduction division  or Meiosis

In each case, division of the nucleus, called karyokinesis, occurs before the division of the cytoplasm, termed cytokinesis.


Discovery: Amitosis was first discovered by Remak² in 1841 in the red blood cells of chick embryo and later described  by Fleming ( 1882).


Amitosis ³ is very simple.It  occurs without the formation of spindle and appearance of chromosomes. The nuclear envelope remains intact. Amitosis is often called direct division .The  nucleus of a cell elongates and develops a constriction round its middle. The construction gradually  deepens and finally cuts the nucleus into two daughter  nuclei. A similar construction appears in the  cytoplasm between  the  two daughter  nuclei and divides the parent cell into two daughter  cells, each with  a nucleus. The daughter  cells receive approximately equal amounts  of nuclear and cytoplasmic materials

Amitosis is rare, probably because it is not an exact method of cell division. It takes place in certain  specialised  cells, such as those in the mammalian cartilage, in the growing embryonic membranes  some vertebrates , in the degenerating cells of higher plants, in the diseased  tissues, and in the old tissues.The macronucleus of ciliates, such as Paramecium ,divides by Amitosis. Some authors regard bacterial  cell division  as  amitosis because no spindle is formed in it. 

Wha is the purpose of cell division?

The purpose of cell division is to ensure the growth, development, and maintenance of organisms. It is necessary for a variety of biological processes, including the replication of genetic material, tissue repair, and regeneration. Cell division also plays a crucial role in reproduction, as it enables the production of sperm and eggs. Additionally, cell division is responsible for the formation of multicellular organisms from a single fertilized egg cell through the process of embryonic development. Overall, cell division is essential for the survival and functioning of living organisms.

Characteristics  features of Amitosis 

Amitosis, also known as direct division or fragmentation, is a type of cell division that occurs in some single-celled organisms and certain specialized cells in multicellular organisms. Unlike the more common types of cell division, such as mitosis and meiosis, amitosis does not involve the formation of a spindle apparatus or the separation of chromosomes. Instead, the cell simply splits into two daughter cells without undergoing the complex process of nuclear division. 

Here are some characteristic features of amitosis:

1): Lack of spindle formation: Unlike mitosis or meiosis, amitosis does not involve the formation of a spindle apparatus, which is responsible for segregating the chromosomes during cell division.

2):Absence of chromosome condensation: In amitosis, there is no visible condensation or organization of chromosomes as seen in mitosis. The genetic material is not replicated or distributed in an orderly manner.

 3):Splitting of the cell: Instead of undergoing nuclear division followed by cell division, the cell directly splits into two daughter cells through a process of simple division or fragmentation. This results in the formation of genetically identical daughter cells.

4):Limited occurrence: Amitosis is relatively rare and occurs in specific situations, such as the division of specialized cells in certain tissues like muscle fibers or the division of unicellular organisms like bacteria and some protists.

5):Rapid cell division: Amitosis can be a rapid process compared to other forms of cell division, as it does not involve the elaborate steps of chromosome separation and spindle formation.

It is important to note that amitosis is not as prevalent or well-studied as mitosis or meiosis, and the exact mechanisms and functions of amitosis are not fully understood in all cases.

Amitosis Functions and Significance of Amitosis

The functions and significance of amitosis can vary depending on the context in which it occurs. Here are some potential functions and significance of amitosis:

1): Regeneration : In certain organisms, amitosis may play a role in tissue regeneration or repair. For example, in some species of planaria, amitosis allows for the rapid regeneration of lost body parts.

2): Asexual  reproduction: Amitosis is involved in asexual reproduction in some unicellular organisms, such as bacteria and some protists. Through amitotic division, these organisms are able to produce genetically identical offspring without the need for sexual reproduction.

3): Developmental  processes: In certain developmental processes, amitosis may contribute to the growth and differentiation of specific cell types. For instance, the division of muscle fibers during tissue growth and repair is believed to occur through amitosis.

4): Cellular homeostasis: Amitosis can be involved in maintaining proper cell numbers and ratios in certain tissues. For example, in the lens of the eye, amitosis helps to balance the growth and shedding of lens cells to maintain clear vision.

5): Abnormal conditions:  Amitosis can occur in abnormal or pathogenic conditions, such as certain types of cancer. The uncontrolled or excessive amitotic division of cells in cancer can contribute to tumor growth and progression.

It is worth noting that amitosis is generally considered a less precise and controlled type of cell division compared to mitosis or meiosis. Therefore, its functions and significance may vary depending on the specific organism or tissue in which it occurs. Further research is required to fully understand the importance of amitosis in different biological contexts.

Examples of Amitosis 

Here are some examples of amitosis in different organisms:

1): Bacteria: Amitosis is the primary method of cell division in bacteria. During amitosis, the bacterial cell simply splits into two daughter cells without undergoing the complex process of mitosis. This allows bacteria to reproduce rapidly and efficiently.

 2): Protozoa:Some protozoa, such as the malaria parasite Plasmodium, undergo amitosis during their life cycle. It is involved in the replication of parasite cells within the host's red blood cells.

3): Planaria:Planaria are flatworms capable of regenerating lost body parts, such as the head or tail. This process is facilitated by amitosis, where the cells in the regenerating region divide directly to form all the necessary tissues, including muscles, nerves, and organs.

4): Lens cells:  In the lens of the eye, amitosis occurs to maintain the balance of cell numbers. New lens cells are continuously added from a specialized region called the lens equator, while older cells in the center of the lens are lost through amitotic division and shedding.

 5): Cancer cells: In certain types of cancer, amitosis can occur as a result of genetic abnormalities or mutations. The uncontrolled and excessive division of cancer cells through amitosis contributes to tumor growth and progression.

These examples highlight the diverse roles of amitosis in different organisms, including reproduction, regeneration, tissue homeostasis, and disease development.

What is the importance of Amitosis?

The importance of amitosis can be evaluated through its various roles in different organisms:

1):Rapid reproduction: Amitosis allows bacteria to reproduce quickly and efficiently, contributing to their ability to colonize new environments and adapt to changing conditions. This is crucial for their survival and growth.

2): Regeneration: Amitosis plays a critical role in the regenerative abilities of certain organisms, such as planaria. By directly dividing cells to form new tissues, amitosis facilitates the process of regrowing lost body parts and restoring organismal functionality.

3): Maintenance of tissue structure: In specific tissues, such as the lens of the eye, amitosis is involved in maintaining the balance of cell numbers. This helps to preserve the precise tissue architecture and functionality necessary for normal physiological processes.

4): Disease progression: Though not always beneficial, amitosis can be important in understanding disease development. Cancer cells, for example, often undergo abnormal amitotic divisions, which contribute to tumor growth and metastasis. Studying and targeting amitosis in cancer cells can potentially lead to therapeutic interventions.

5): Evolutionary perspectives: Amitosis occasionally occurs in various organisms, even though mitosis is the predominant form of cell division. This suggests that amitosis may have evolutionary significance, potentially providing advantages in specific environmental conditions or facilitating the emergence of new traits.

Overall, the importance of amitosis lies in its contributions to reproduction, regeneration, tissue homeostasis, disease progression, and potential evolutionary advantages. Studying amitosis can enhance our understanding of cellular processes and provide valuable insights into various biological phenomena.


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