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Introduction
At the cellular level, cell division is the driving pressure of reproduction. Except for germ cells, that have half the variety of chromosomes, most eukaryotic cells divide in a way that keeps the ploidy or number of chromosomes the identical. So, cell division of types-mitosis and meiosis. Mitosis generally takes place in vegetative or somatic cells, While meiosis is essential for germ cells.
Mitosis meaning
- Flemming (1882) gave the term mitosis (Gr.: Mitos-thread + osis-state). However, mitosis was first observed in plants by E. Strasburger (1875) and in animals by W. Flemming (1879).
Mitosis or mitotic division is meant for multiplication of cells. Development of complete organisms from zygote is by means of mitotic cell division. So mitosis is necessary for maintenance, growth, repair and continuity of life.Mitosis generally takes place in vegetative or somatic cells. In this process, one parent cell divides into two daughter cells, where chromosome number remains the same as in parent cell, i.e., daughter cells exactly resemble parent cells both quantitatively as well as qualitatively. So mitosis leads to an increase in the number of cells without any change in genetic make-up.
Interphase
A cellular spends a length of its growth in interphase before starting up mitosis. whilst in interphase, it is going via the following levels:
The G1 phase is the time earlier than when DNA synthesis starts off.
S phase: that is the time while DNA synthesis takes region.
The G2 phase takes place between the end of DNA synthesis and the start of prophase.
Phases of Mitosis
The actual mitotic-phase (M-phase) is completed in two steps
(A) Karyokinesis: i.e., division of the nucleus into two.
(B) Cytokinesis: Karyokinesis is followed by cytokinesis, i.e., division of cytoplasm into two cells or wall formation.
(A) Karyokinesis-
Karyokinesis (division of nucleus) consists of the following four stages:
(i) Prophase
(ii)Metaphase
(iii) Anaphase
(iv) Telophase
(i) Prophase: After interphase, prophase comes, which is the first and longest stage of mitosis.
Prophase is the most active of all the stages of M-phase. The chromosomes appear as thin, filamentous, uncoiled structures, which later on become coiled, shortened, thickened. These two sister chromatids are attached only at a point called centromere or kinetochore or primary constriction. Nuclear membrane breaks down to form membranous vesicles. Nucleolus disappears or gets dispersed.Centrosome (particularly in animal cells) divides into two centrioles, which move to opposite poles. Spindle is also formed at late prophase (or early metaphase).
(ii) Metaphase: At this stage, the chromosomes become maximally distinct due to further contraction and thus size of the chromosomes is measured at mitotic metaphase. Due to attachment of spindle fibres at the centromeres of chromosomes, the chromosomes are arranged in the center or at equator or equatorial plate or metaphase plate due to their active movements. In animal cells, spindle apparatus is formed due to division of centrosome or centriole, but in plants, spindle is still formed even in absence of centrosome in plant cells. Mechanism of the formation of spindle is, however, not clear in plant cells.
(iii) Anaphase: This is the shortest stage of mitosis. Chromosomes divide at the point of centromere or kinetochore and thus two sister chromatids are formed, which are now called chromosomes. These sister chromatids or future chromosomes or daughter chromosomes now move towards the opposite poles of spindle and this movement is due to repulsion between centromeres, contraction of spindle fibres.Different shapes of chromosomes are observed at anaphase (V or J or L or I-shaped), depending upon position of kinetochore or centromere.
(iv) Telophase:Chromosomes reach opposite poles of the spindle.Chromosomes now become decondensed, uncoiled and not clear in outline (fuzzy). Disintegration of spindle fibres occurs at this stage and nuclear membrane reappears around each group of chromosomes. Nucleoli also make their appearance, thus forming two daughter nuclei.
(B) Cytokinesis-Karyokinesis results in formation of two nuclei inside a cell and now it is followed by division of cytoplasm (Cytokinesis), thus forming two cells (daughter cells). It has four stages- initiation, contraction, membrane insertion and completion.
Cytokinesis is by 2 methods: It’s considered as the sixth phase of mitosis.
(i) Cell furrow method: This is characteristic of animal cells. Due to the absence of rigid cell walls here, the more flexible plasma membrane forms the outer layer of the cell. A circular constriction or invagination appears at center or equator, which deepens gradually and finally two daughter cells are separated.
(ii) Cell plate method: This is characteristic of plant cells. Here, vesicles provided by Golgi apparatus unite to form phragmoplasts, which join to form cell plates. Cell plate is first laid down in the center and then proceeds towards the periphery (i.e., centrifugal plate-formation). Cell wall materials are now laid down on both sides of the cell plate and thus forming two daughter cells.
Conclusion
Congratulations to you for taking the first step towards your journey of success by completing this topic. Mitosis is a very important chapter from an exam point of view as it explains the important concepts of various states of cell division and their function. Mitosis or mitotic division is meant for multiplication of cells. Development of complete organisms from zygote is by means of mitotic cell division. So mitosis is necessary for maintenance, growth, repair and continuity of life
