Cell division (Mitosis)
What is mitosis? Describe the process of mitosis in animal cells. Add a short note on the significance of mitosis. Explain the mitosis and it’s all stages of mitosis?
In this article we will discuss about the mitosis:1.Introduction of mitosis,2.factors that initiate mitosis, 3.phases of mitosis, 4.significance of mitosis, 5.variations of mitosis.
Introduction of mitosis
Factors that initiate mitosis
Phase of mitosis:-
Significance of mitosis
Variations of mitosis
* Free nuclear division
Fleming (1882) coined the term mitosis to describe the division of somatic cells. It is also known homeotypic division or equational division due to the equal, qualitative and quantitative distribution of chromatin material between the two newly formed daughter cells. Mitosis is the most common type of somatic cell division. Mitosis includes two consecutive processes: Karyokinesis (Gr. Karyon-Nucleas ,Kinesis -Movement) or nuclear division and Cytokinesis (Gr. Kytos-Cell;Kinesis -movement) or cytoplasmic division. At the end of mitosis, the parent cell is divided into two daughter cells which are identical to each other and identical to the mother cell as well. The genetic material is duplicated and equally distributed so that each daughter cell receives the same number of chromosomes as in the parent cell.
Factors that Initiate Mitosis :
Several factors initiate mitotic cell division. They are as follows:
- Specific changes in the physical and chemical organization of the cell.
- Dispersion and dispersal of the nuclear membrane.
- Duplication of chromosomes into chromatids.
- Several extrinsic factors such as nutrients, steroid hormones, auxins (in plant cells),epidermal growth factors (EGF) etc.facilitate the initiation of mitosis.
Karyokinesis is a continuous process. For the convenience of study, it is divided into the following phases.
Prophase marks the beginning of mitosis. It is the longest phase which may last from one to several hears Prophase is marked by the following events.
- The cell assumes a rounded shape.
- The viscosity of the cytoplasm increases many Fold.
- The chromatin fibres undergo dehydration and condensation to become short, thick, compact and distinct bodies called chromosomes.
- Each chromosome consists of two chromatids made up of newly replicated DNA molecules and united at a point called centromere.
- The nuclear membrane breaks off and the nucleoplasm mixes with the cytoplasm
- The nucleolus starts disappearing.
- The centrioles separate and move towards the opposite poles of the cell.
- The spindle fibres develop between the two poles of the dividing cell Fibres develop around the centrioles forming a star-shaped aster at each pole. Finally, spindle fibres extend between the aster and the centromeres of the chromatids constituting the mitotic apparatus.
- Due to the above changes, prophase is called the phase of disorganization.
Metaphase is shorter in duration than prophase but longer than anaphase; it lasts for about 20-30 minutes. It is characterized by the following events.
- The chromosomes become highly condensed. They are at their shortest and thickest physical state.
- The chromosomes move towards the equator of the spindle, a place half-way between the two poles.
- The chromosomes are arranged at the equator in such a manner that their centromeres lie in one plane with their arms towards the poles.
- The centromeres are attached to the poles by means of spindle fibres.
- Usually the small chromosomes lie in the middle of the equatorial plane whereas the large chromosomes occupy a peripheral position.
- Anaphase is the shortest phase of mitosis which last for about 6-12 minutes.
- The beginning of anaphase is marked by the separation of sister chromatids of each chromosome.
- The free chromatids now move towards their respective (opposite) poles of the cell by the contraction of the spindle fibres.
- The centromeres are pulled towards the poles with the arms of the chromosomes facing the equatorial plane.
- Depending upon the position of the centromere. the chromosomes appear ‘J’-or ‘U’- or ‘V’-shaped.
- At the end of anaphase, there is a full complement of chromosomes identical to the karyotype of the parent cell.
- The mitotic apparatus shows some ATPase activity that produces energy for the movement of chromosomes.
- Finally, a nuclear membrane starts appearing around each group of chromosome
- Telophase is regarded as the phase of reconstruction and reorganization. Some of the changes which occur during prophase are reversed during telophase.
- The chromosomes gradually uncoil and unfold to from thin,sender, thread-like structure called chromatin reticulum.
- The mitte spindle disappears and the nucleolus is reorganized.
- A new unclear membrane is completely formed from elements of ER and each group of chromosomes at both the poles.
- A shallow, cytoplasmic constriction appear on the cell membrane the imel of the equatorial plane.
- The daughter cells are thus restored to the Interphase condition.
Cytokinesis immediately follows telophase of karyokinesis. The cytoplasmic constriction of the cleavage furrow which developed during late telophase progressively deepens and finally joins to separate the rip daughter cells .The cytoplasmic constriction results due to the contraction of a protem ring at the level of equatorial plane. It has also been suggested that the furrow may result due to the lengthening of microtubules (actin and myosin) and the cell membrane accompanied by a contraction at the equator.
Significance of Mitosis:
Mitotic cell division is significant on account of the following reasons:
- In unicellular organisms like Amoeba, mitosis is the chief method of reproduction.
- Mitosis helps in the repair and replacement of aged and worn-out cells like RBC and epithelial cells.
- Regeneration of lost parts is a mitotic event. Repeated mitotic cell division of the unicellular zygote results in growth and development and ultimately the individual attains adulthood.
- The surface volume ratio or the size of any type. of cell remains constant due to cell division.
- Chromosomes carry hereditary information. Mitosis accounts for equal qualitative and quantitative distribution of chromatin material from the mother cell into the daughter cells.
- A constant chromosome number is maintained from one cell generation to the other by this kind of cell division.
- Mitosis plays an important role in reproduction.
- Mitosis is essential for embryonic development.
- Mitosis ensures genetic stability and genetic continuity.
Variations in Mitosis:
Several variations in mitotic division are known to occur in animal and plant cells. These are (i) amitosis, (ii) free nuclear division, and (iii) budding.
Amitosis is the most simple and direct method of cell division. It involves the following events.
- The nucleus elongates and constricts in the middle.
- The constriction gradually deepens and the nucleus becomes dumb-bell shaped in appearance.
- Finally, the nucleus separates into two parts.
- The division of the nucleus is soon followed by the appearance of a groove in the middle of the cell membrane which deepens and pinches off the cell into two daughter cells.
- The new cells are almost equal in size but the genetic material is not equally distributed between the two daughter cells.
- Amitosis is of rare occurrence It takes place only under certain conditions. The cells of total membrane and the cartilage cells in mammals,the pathogenic and degenerative cells and some upicellular protozoans divide by amitosis.
Free nuclear division:
During normal mitosis, the division of nucleas is followed by the division of cytoplasm. In free cell formation, the nucleus divides once or several times without cytokinesis. This is called free nuclear division which results in the formation of syncytium with several nuclei in a common cytoplasmic mass. Such cells are called coenocytes. In plants, free nuclear division is seen in endosperm cells. In animals, syncytium is commonly found in muscles.
Budding is a form of asexual reproduction. Budding is commonly seen in yeast and fungi. In budding. a small bulge-like outgrowth or protuberance is formed on the cell wall. The nucleus divides by amitosis which is pushed into the bulge along with cytoplasm. Budding is complete with the formation of a new cell wall.
In Hydra, the interstitial cells of epidermis divide rapidly to form a small protuberance. A hollow projection or bud develops from the protuberance containing ectoderm, merogloea, endoderm with an extension of the coelenteron. Later on it leads a self supporting, independent life. A bud is an identical copy of the parent.