Mitosis is the process of cell division in which a single cell divides into two identical daughter cells, hence called equational division. This process is divided into four stages of nuclear division (karyokinesis).
1) Prophase
2) Metaphase
3) Anaphase
4) Telophase
Prophase:
- This is the first stage of karyokinesis of mitosis.
- Prophase is marked by the initiation of condensation of chromosomal material (which becomes visible under a microscope).
- The centrosome moves towards opposite poles of the cell. Chromosomes are seen to be composed of two chromatids attached together at the centromere.
- Each centrosome radiates out microtubules called asters. The two asters together with spindle fibers form mitotic apparatus.
- Cells at the end of prophase, when viewed under the microscope, do not show Golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope.
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Metaphase:
- The complete disintegration of the nuclear envelope marks the start of the metaphase.
- Chromosomes are highly condensed in this stage, because of that we can clearly observe the structure and morphology of chromosomes under a microscope.
- During metaphase, the chromosomes align along the equator of the cell (The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate). This is also called Congression.
- Congression: Arrangement of all chromosomes at equatorial plate/metaphasic plate.
- The spindle fibers attach to the centromeres of the chromosomes and prepare to pull them apart.
The key
features of metaphase are:
- Spindle fibers attach to kinetochores of chromosomes.
- Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibers to both poles.
Q) How many spindle fibers are present in metaphase?
A) 96
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Anaphase:
- During anaphase, the spindle fibers shorten and pull the sister chromatids apart
The key events in this phase are:
- Centromeres split and chromatids separate.
- Chromatids move to opposite poles.
Telophase:
This is the final stage of karyokinesis. During this stage, the chromosomes reach the opposite ends of the cell and begin to uncoil. The spindle fibers begin to break down and a new nuclear envelope forms around each set of chromosomes.
The key events are:
- Chromosomes cluster at opposite spindle poles and their identity is lost as discrete (seperate) elements.
- Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei.
- Nucleolus, Golgi complex and ER reform.
Cytokinesis: (Cytoplasmic division)
This is the final stage of the cell division, the cytoplasm of the cell divides, resulting in the formation of two daughter cells.
In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the center dividing the cell cytoplasm into two (Contractile ring composed of actin and myosin filaments forms around the cell, pinching the cell in two).
In plant cells,
- Cytokinesis occurs differently due to the presence of cell wall. Instead of cleavage, a cell plate forms in the center of the cell, dividing the cytoplasm into two separate compartments.
- The cell plate gradually develops into a new cell wall that separates the two daughter cells (cell wall represents the middle lamella between the walls of two adjacent cells).
- At the time of cytokinesis, organelles like mitochondria and plastids get distributed between the two daughter cells.
In some organisms karyokinesis is not followed by cytokines as a result of which multinucleate condition arises leading to the formation of syncytium (Ex: Liquid endosperm in coconut).
Importance of Mitosis:
It allows the cells to divide, grow, repair and reproduce and is essential for the survival and functioning of all living organisms.
Extra points:
- Kinetochores: Small disc-shaped structures at the surface of the centromeres, these structures serve as the sites of attachment of spindle fibers to the chromosomes.
- Cytokinesis is an essential process for the growth, development and repair of multicellular organisms, as well as for the replication of unicellular organisms. Defects in cytokinesis can lead to abnormal cell division and contribute to the development of diseases, including cancer.
- In animal cells cytokinesis is by cell furrow formation and in plants it is by cell plate formation.
- Phragmoplast is responsible for the formation of cell-plate in plant cells during cytokinesis.
Previous year NEET questions from mitosis:
1) Which one of the following never occurs during mitotic division?
(NEET 2022)
a) Movement of centrioles towards opposite poles
b) Pairing of homologous chromosomes
c)Coiling and condensation of the chromatids
d)Spindle fibers attach to kinetochores of chromosomes
2) select the incorrect statement with reference to mitosis
(NEET 2022)
a) Spindle fibers attach to centromere of chromosomes
b) Chromosomes decondense at telophase
c) Splitting of centromere occurs at anaphase
d) All the chromosomes lie at the equator at metaphase
3) The fruit fly has 8 chromosomes (2n) in each cell. During interphase of mitosis, if the number of chromosomes at G1-phase is 8, what would be the number of chromosomes after S-phase?
(NEET 2022)
a) 8
b) 16
c) 4
d) 32
4) In a mitotic cycle, the correct sequence of phases is:
NEET (oct.) 2020
a) S, G1, G2, M
b) G1, S, G2, M
c) M, G1, G2, S
d) G1, G2, S, M
5) Anaphase Promoting Complex (APC) is a protein degradation machinery necessary for proper mitosis of animal cells. If APC is defective in a human cell, which of the following is expected to occur?
NEET 2017
a) Chromosomes will not condense
b) Chromosomes will be fragmented
c) Chromosomes will not segregate
d) Recombination of chromosome arms will occur
6) Which of the following options gives the correct sequence of events during mitosis
a) Condensation----Nuclear membrane disassembly----Crossing over----Segregation----Telophase
b) Condensation----Nuclear membrane disassembly----arrangement at equator----centromere division----segregation----telophase
c) Condensation----Crossing over----Nuclear membrane disassembly----Segregation----Telophase
d) Condensation----arrangement at equator----centromere division---Segregation----Telophase
7) Which of the following is not a characteristic feature during mitosis in somatic cells?
a) disappearance of nucleolus
b) Chromosome movement
c) Synapsis
d) Spindle fibres
8) Spindle fibres attach onto
a) kinetochore of the chromosome
b) centromere of the chromosome
c) Kinetosome of the chromosome
d) telomere of the chromosome
9) The complex formed by a pair of synapsed homologous chromosomes is called
NEET 2013
a) equatorial plate
b) kinetochore
c) bivalent
d) axoneme
Answers:
1) b
2) a
3) a
4) b
5) c
6) b
7) c
8) a
9) c
This link will help you for easy understanding of this topic
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