- Mitosis phase is the shortest phase in cell cycle. Although mitosis is a continuous process,The mitosis itself involves the condensation and separation of the replicated chromosomes. Mitosis has been subdivided into five phases:
a. Prophase (condensation):-
A stage of chromosome condensation.
- beginning of prophase chromosomes appear as thin , filamentous uncoiled structures.
- Chromosomes become coiled, shortened and more distinct in prophase, which is of much longer duration than other stages.
- Nucleoli disappear.
- Each chromosome longitudinally splits into two sister chromatids. Double structure of each chromosome is visibleat late prophase.
- The duplicated chromosome subunits (each one called chromotid) join together at the centromeres.
- And two chromatids are attached to spindle tubules(Chromosomal fibers) with the help of protein plates called kinetochores a specialised structures develop on either surface of centromere of each chromosome.
- The kinetochore is the actual site of the insertion of the spindle threads and is a permanent part of the chromosome.
- The nuclear membrane disintegrates.When the nuclear membrane dissolves, there is no differentiation between cytoplasm and nucleoplasm. The chromosomes are attached to the spindles through their centromeres. Such a mitosis is called extra-nuclear mitosis or eumitosis. The mitosis takes place within the nuclear membrane and is called intranuclear mitosis or premitosis e.g.many protozoans and some animals cells The chromosomes move freely and proceed towards the equator.
- Microtubules A system of microtubules needed to move the chromosomes begins to form during prophase.The microtubules, also called spindle fibers, form from an area of the cell called the centrosome. During interphase, the cell has one centrosome but just before prophase, the centrosome duplicates, producing a second centrosome. During prophase, microtubules radiate from each centrosome. Some of the microtubules extend from one centrosome toward the other.
- Spindle fibers:-Spindle fibers extend from the poles to the equator.
- The spindle apparatus forms.The spindle apparatus which moves the chromosomes consists of two proteins actin and tubulin The entire complex of centrosomes and spindle fibers is called the spindle apparatus.
- The two centrosomes move to the opposite poles of the nucleus, elongating their + ends. At the end of prometaphase, the two centrosomes are at opposites poles of the cell, and some of the spindle microtubules are attached to the chromosome at the knietochore.
- The microtubule from one pole may attach to the knietochore first, and the chromosome and the chromosome begins to move toward the other pole aligning the chromosomes at the equatorial plane.
- Microtubules can only remain attached to a kinetochore where there is a force exerted on the chromosome from the opposite end of the cell.
- Kinetochore microtubules are spindle fibers that attach to the kinetochores and move the chromosomes to the center of the cell.
- The next phase (Metaphase) begins when the chromosomes become aligned in the center of the cell.
- The chromosomes line up in one plane to form the equatorial plate or metaphasic plate.
- Chromosome lies in the middle of the spindle apparatus and is perpendicular to its axis.
- In actuality, only the centromere lies on the equatorial plate, while the chromosome arms (chromatids) are directed away from the equator.
- Smaller chromosomes are usually central in position whereas the larger ones are peripheral
- At metaphase, the chromosomes are aligned on the cell’s midline. Approximately 15-35 mircotubules are attached to the kinetochore (by kinetochore microtubules).
- There are also two types of nonkinetochore microtubules:-
- Some microtubules radiate from the centrosome toward the metaphaseplate without attaching to chromosomes. Others are too short to reach the metaphase plate.
- Still others extened across the plate and overlap with nonkinetochore microtubules from the opposite pole of the cell.
- Anaphase(centromere separation and chromatid migration):-
- Anaphase begins when the centromeres divide and the spindle apparatus starts pulling the kinetochores to the opposite poles (progressive shortening of the microtubular spindle fibers pulls the chromosomes in opposite directions toward the poles).
- The daughter kinetochores move apart dragging the chromosomes ( each now a single strand) to the poles.The chromosomes appear in the shape of V,L, J or I
- Two cells begin to form. In anaphse the centromeres divide and two sister chromatids separate and move to the opposite ends of the cell.
- Microtubules pull a chromosome towards a pole by losing protein subunits are their centrosome and at the + end (attached to the kinetochore). The nonkintetochore microtubules are responsible for elongating the whole cell along the polar axis during anaphase. When the chromosomes split in anaphase, the chromosome number is doubled. For example, the number of chromosomes and chromatids during each phase in a pea (Pisum sativum) cell is:-
|Phase||# Chromosomes||# Chromatids|
Cytokinesis (division of the cytoplasm) begins in anaphase. Two cells will be produced as this process continues.
- Telophase (recovery of nuclear envelope and decondensation).Telophase begins when chromosomes reach the poles of the daughter cells. Many of the events in telophase are the reverse of prophase , but there are now two nuclei instead of one.
- Chromosomes decondense (uncoil)
- Nuclear membrane reappears around daughter nuclei
- Spindle Fibers become disorganized.the spindle apparatus breaks down
- The nucleolus reappears
- The cell pinches in the middle, beginning the formation of the two cells The new cell plate expanding centrofugally.
Each daughter cell gets the same complement of chromosomes and nucleoli as of the mother cell.During division, cell organelles like mitochondria, plastids, Golgi complex, lysosomes and the cytoplasmic matrix are distributed into the two daughter cells
- Cytoplasmic division (cytokinesis):-
Plant cells divide by formation of a new cell wall (cell plate ) between daughter nuclei At the end of the telophase, soon after the formation of daughter nuclei, the spindle fragments and gather at the equator region and form a barrel shaped structure called phragmoplast. The vacuoles of golgi complex enter the phragmoplast and release pectins into it. At the end of anaphase, Golgi-derived secretory vesicles carrying cell wall materials are transported to the equator of a dividing cell. Fusion of these vesicles gives rise to a membrane-bound compartment, the cell plate. The cell plate expands from the middle out (centrifugally) until it reaches the "zone of attachment" or division site on the mother cell wall. Cell plate gradually undergoes physical and chemical changes to form middle lamella thus dividing cytoplasm into parts. On both sides of the middle lamella the primary wall materials like cellulose and hemi cellulose are deposited.As a result two daughter cells are formed
The places where vesicles of the cell plate fail to fuse, the cytoplasmic contact between the daughter cells is maintained.Such cytoplasmic channels are lined by plasma membrane to form plasmodesmata
Function of mitosisCell division is a fundamental part of a plant's existence and development
- Growth:- The number of cells in a plant increases due to mitosis leading to growth.
- Asexual reproduction, regeneration and cell replacement:- A number of plants propagate (vegetatively)by mitotic divisions ofhe cells.Regeneration of lost parts, and cells replacement (tissue repair) occurs by mitosis in multicellular plants.
- As a result of mitosis, two nuclei are formed which have same number of chromosomes as the parent cell.
EndomitosisReplication of chromosomes without division of the nucleus is called endomitosis; it leads tothe formation polyploids.Endomitosis without separation of chromatids leads to polyteny