The cell cycle and cell division is a series of events that lead to the formation of new cells. It is not an exaggeration to say that continuity of life depends on this cell cycle.
Cell cycle involves three types of cycles which are described below:
Chromosome Cycle: In this cycle, DNA (Deoxyribose Nucleic Acid) gets replicated into two daughter DNA molecules and later they are separated. It is also called the Karyokinesis.
Cytoplasmic Cycle: In this cycle, all the organelles of the cells and other components of cells (except DNA) get double in number, and then the whole cell gets separated into two. It is also known as Cytokinesis.
Centrosome Cycle: Both the above cycles require centrosomes to be inherited and duplicated to form two poles of mitotic spindles.
Note: Spindle apparatus is a structure of eukaryotic cells. It forms during the cell cycle so that the sister chromatids (it is a chromosome that is newly copied where two of them are still attached to each other) can be separated to form daughter cells.
These three types of cell cycle are seen during cell division.
Cell division is the process through which the cell duplicates itself to form daughter cells. There are 3 different types of cell divisions seen both in eukaryotes and prokaryotes which are: amitosis, mitosis, and meiosis.
Amitosis – Cell Division
Amitosis is the division of nucleus without any evidence of chromosomes. It is also called the direct cell division.
It is a type of asexual reproduction seen especially in unicellular organisms like bacteria, protozoans etc. This method of multiplication is also seen in the growth of fetal membranes of few vertebrates.
In amitosis, the nucleus splits first and then cytoplasm constricts.
Nucleus elongates then takes the shape of the dumb-bell. The depression increases further ultimately slicing the nucleus into two.
After the division of nucleus, constriction of cytoplasm takes place which divides the cell into two daughter cells.
Paramecium, cells of mammalian cartilage and degenerating cells of higher plants are some of the examples for amitosis.
However, cell division involving amitosis causes an unequal distribution of chromosomes, or may even lead to abnormalities in reproduction and metabolism.
Mitosis – Cell Division
Cell cycle is divided into four phases. They are – G1 phase, S phase, G2 phase and M phase. G1, S, and G2 phases are combinedly called the interphase.
It starts right after the previous M phase. Here the daughter cells of the previous M phase begin G1 phase.
G1 phase is a resting phase and is called the first gap phase. However, the cells are not at rest. It is called the first gap phase because there is no active DNA synthesis taking place.
G1 is now termed as the first growth phase as it involves the synthesis of other components of the cell such as RNA (Ribose Nucleic Acid), membranes, and proteins which lead to the growth of cytoplasm and nucleus of the daughter cells to get their mature size.
In this phase, the chromatin is fully extended and it cannot be distinguished as separate chromosomes when seen under a light microscope.
Normal cell metabolism takes the center stage in this phase. It involves transcription of three types of RNA – tRNA, mRNA and rRNA.
Proteins which are synthesized in this phase are the regulatory proteins which control different events of mitosis and the enzymes like DNA polymerase important for synthesis of DNA in the next stage are also synthesized in G1 phase.
G1 phase duration is different for different cells. It may take up nearly 30 to 50% of the total time of cell cycle or it may not exist at all for example in rapidly dividing blastomeres of frogs and mammals.
Many checkpoints control this stage. A checkpoint called, Restriction point determines whether a cell continues its journey of cell cycle, dies or enters into G0 phase.
Lack of nutrition, lack of growth factors, and the inability of the cells to undergo metabolic changes are few of the reasons for cells to get arrested in G1 phase.
Proteins like kinases and cyclins are critical for the cell cycle. Cyclins decide whether a cell should divide or not.
Terminally differentiated cells or end cells which do not have the capacity to divide any further like neurons and striated muscle cells or voluntary muscle cells are arrested in this G1 phase. This type of G1 phase is generally called the G0 phase.
It is to be noted that sometimes, these cells do divide but the frequency of the division is way less than normal cells.
It is also called the quiescent stage. It doesn’t mean that the cell doesn’t grow. The cell grows but has a reduced rate of synthesis of RNA and proteins. The cells are not even dormant or inactive.
Did you know the clone Dolly was developed using G0 cells of mammary glands of a sheep? The nucleus of this G0 cell was used to fuse with the recipient egg’s cytoplasm. This led to the development of Dolly, the first-ever cloned animal in this world.
In this phase, synthesis of DNA and histone proteins takes place. Histones are required in large amounts to synthesize nucleosomes of new DNA.
So, at the end of the S phase, the DNA is successfully doubled.
S phase takes up to 35 to 40% of the time of cell cycle.
It is termed as the second gap phase or growth phase or second resting phase of the interphase.
During this phase, activities of G1 phase continues i.e. synthesis of membranes, RNA, proteins etc. which are important for the growth of the cell.
One of the major proteins that is synthesized in G2 phase is Maturation Promoting Factor (MPF). It condenses the chromosomes to the mitotic form.
It takes roughly about 10 to 20% of the total time of cell cycle.
The features which are characteristics of interphase are as follows:
- The nuclear envelope remains the same.
- Chromosomes are present in the form of long, coiled indistinctly visible chromatin fibers.
- DNA amount doubles.
- The size of the nucleolus increases considerably because of the accumulation of rRNA and ribosomal proteins in the nucleolus.
- Centrioles number increases from one pair to two pairs in animal cells.
- Synthesis of membranes increases during the G2 phase. The extra material of the membrane is stored as blebs on the surface of the cells which are about to be divided.
|Parts of Cell Cycle||Phases||Short Description of the Phase||Duration in Human Cells (In Hours)|
|Interphase||G1||Pre-DNA synthesis phase||12|
|Interphase||S||DNA synthesis phase||8 to 10|
|Interphase||G2||Post DNA synthesis phase||3 to 4|
Centrosome Cycle: When a typical animal cell exits mitosis, the cytoplasm consists of a centrosome which has two centrioles placed at right angles to each other.
Before the end of cytokinesis, the centrioles lose their close connection to one another. This event is triggered by separase activated during late prophase of the previous mitosis.
When the cell is in the S phase, DNA starts replicating and creates a small procentriole next to maternal centriole at the right angle. With the elongation of microtubules, the procentriole converts into a daughter centriole.
At the beginning of mitosis, centrosome splits into two adjacent centrosomes. Each of the centrosomes has a pair of mother-daughter centrioles.
Mitosis is a stage where duplicated chromosomes are distributed into two daughter nuclei – thereby forming two identical daughter cells. It is similar in both animals and plants.
This phase is smooth and the division of mitosis into different phases is only for our convenient reference.
The mitosis is divided into 5 stages or phases which are – prophase, prometaphase, metaphase, anaphase, and telophase.
It is the longest phase of the mitosis. The appearance of chromosomes into thin thread like condensing structures is the characteristic of prophase. In Greek language, pro means before and phasis means appearance.
Cells become viscous, more refractile, and spheroid. Each chromosome is composed of two coiled filaments known as chromatids. Chromatids are formed during the replication of DNA in the S phase.
As the prophase stage progresses, the chromatids become shorter and thicker. Chromatids of a chromosome are called sister chromatids and they are held together by centromere or primary constriction which is a special DNA-containing region.
During prophase, kinetochores (disc-shaped complex of proteins, one kinetochore for one chromatid) get accumulated at centromere.
The chromosomes are uniformly distributed in the nuclear cavity in the early prophase. But as prophase progresses, the chromosomes gather around the nuclear envelope leaving the central space of the nucleus empty.
In the cytoplasm, spindle formation or the formation of mitotic apparatus is the characteristic feature of prophase. In early prophase, there are 2 pairs of centrioles with each centriole being surrounded by an aster.
Note: Centrioles are a pair of cylindrical, rod-shaped microtubular structures present near the nucleus. Centrosomes are the cell organelles that contain centrioles.
Note: An aster is a structure of a cell that looks like a star and is composed of microtubules arising from every direction.
The two pairs of centrioles along with their asters move to opposite poles of the cell and get situated in antipodal or diametrically opposite positions. (It is to be noted that centrioles and asters are not present in plant cells.)
The end of the prophase is marked by the degeneration or breaking down of the nuclear envelope. The process of nuclear envelope’s degeneration is not clearly understood to date.
At the beginning of this stage, the compacted chromosomes or chromatids are scattered in a space in the nuclear region (where nucleus, nucleolus, nuclear envelope were present).
The microtubules enter into the central region of the cell. The free ends (also called plus ends, the ends which are attached to centrosome are called minus ends) of the microtubules grow and shrink in dynamic fashion searching for chromosomes.
The microtubules which get connected to the kinetochores of chromosome are called kinetochore microtubule. Kinetochore microtubules are stabilized by kinetochores.
Kinetochore eventually stably associates itself with the plus/free ends of one or few microtubules from one of the spindle poles. The other kinetochore of the sister chromatid catches its own microtubules from opposite spindle pole.
The chromatids which were scattered in the cell oscillate back and forth in poleward and anti-poleward direction. This process of movement is called congression.
It is this process through which chromatids move to the center of the cell. Motor proteins provide the energy required for the chromosomal movement. Microtubule dynamics also play a role in moving the chromosomes to the center of the cell.
Longer microtubules attached to the kinetochores become shorter and the shorter ones elongate. Shortening or elongation of these kinetochore microtubules is because of loss or gain of subunits of kinetochore microtubules (loss or gain happens even when they are attached to the kinetochores).
Formation of mitotic spindle in a typical animal cell has few stages which are as follows:
- First stage: Appearance of microtubules in aster shape around each centrosome.
- Second stage: Centrosome splits into two and move towards the opposite ends of the cell.
- Third stage: As the centrosomes separate, microtubules stretching between them increase in number and elongate.
- Fourth stage: Lastly the two centrosomes reach points exactly opposite to one another thereby forming two poles of a bipolar mitotic spindle.
After mitosis, centrosome will be distributed to each daughter cell.
Interaction of spindle and chromosomes is possible only after the disintegration of nuclear membrane (the spindle is assembled in cytoplasm and the chromosomes are present in nuclear envelope).
For nuclear membrane to disintegrate, three main components of the nuclear membrane which are nuclear pore complexes, nuclear lamina, and nuclear envelope have to be disintegrated separately.
The cell enters metaphase when all the chromatids are aligned at the spindle equator. This plane of alignment of the chromatids is referred to as the metaphase plate.
An organized array of microtubules of the mitotic spindle are responsible for the separation of duplicated chromatids which are present at the spindle equator.
The array of microtubules consists of three types of microtubules.
1. Astral microtubules: These microtubules radiate from the centrosome. These microtubules radiate outside the mitotic spindle to help position the mitotic spindle apparatus in the cell.
2. Chromosomal microtubules: These are also called the kinetochore microtubules. They radiate from the centrosome and connect to the kinetochores of the chromosomes. In a typical mammalian cell, a single kinetochore is attached to a bundle of 20 to 30 microtubules. These microtubules form spindle fiber.
During metaphase, these microtubules exert a pulling force on kinetochores. Chromosomes remain balanced in the center of the cell because of the “tug of war” between the chromosomal microtubules of the opposite poles. This type of microtubules is required during anaphase to move chromosomes towards the poles.
3. Polar microtubules: They are also known as the interpolar microtubules. These microtubules arise from the centrosome and they overlap with the polar microtubules from the other or opposite centrosome. Because of this overlap, they form a structural basket to maintain the mechanical integrity of the spindle.
Microtubule Flux in Mitotic Spindle in Metaphase Stage: Subunits of the microtubules are continuously added and lost at both plus and minus ends of the microtubules. Neither kinetochore nor the centrosome acts as a cap and block the microtubules from either gaining or losing subunits. Kinetochores actually act as a site for dynamic activity.
At the plus end i.e., where the microtubules are attached to the kinetochores, the subunits are added in greater number (loss of subunits at plus end is present but it is lesser). At the minus end i.e., where the microtubules are attached to the centrosome, loss of subunits is more than the addition.
All of the chromatids are split in sync as the cell enters anaphase stage. It is to be mentioned that the genetic material from prophase was called the chromatids not chromosomes (if you haven’t noticed it by now).
However, since the sister chromatids are split in anaphase the genetic material is once again referred to as chromosomes. The split chromosomes begin their respective poleward migration or movement.
Centromere of a chromosome leads the chromosome during the movement while the arms of the chromosomes trail behind the centromere.
Movement of the chromosomes towards the poles is extremely slow. The chromosomes move at a speed of 1 μm per minute. This slow rate of movement ensures that the chromosomes separate precisely and without any entanglement.
The poleward movement of the chromosomes is due to the loss of subunits of microtubules from the pole just like in metaphase. But here in anaphase, there is a difference. In metaphase, mostly subunits were joined at plus end and lost at minus end but in anaphase, subunits are lost at both plus and minus ends at a greater pace.
In metaphase, the length of the chromosomal fibers is constant and whereas in anaphase chromosomal fibers are shortened.
The movement of chromosomes towards the poles is referred to as Anaphase A and the movement of the spindle poles away from each other is referred to as Anaphase B.
During Anaphase B, the polar microtubules start getting elongated because of addition of subunits to the plus end of the polar microtubules.
Checkpoint mechanism in a cell monitors the status of events during the cell cycle. One such checkpoint mechanism is called SAC or Spindle Assembly Checkpoint.
SAC operates during the transition between metaphase and anaphase. SAC postpones the onset of anaphase if the chromosomes fail to align properly at spindle equator or metaphase plate. SAC delays the cell cycle till the chromosomes get aligned properly.
If a cell cannot postpone the segregation of chromosomes, the daughter cells have a high risk of getting an abnormal number of chromosomes which in turn may lead to many health problems like cancer (in children of humans).
End of polar migration of daughter chromosomes marks the start of telophase. The events which took place in prophase occur in reverse sequence in telophase.
Nuclear envelope reassembles around the daughter chromosomes. Mitotic apparatus except for the centrioles disappear. Cytoplasm’s viscosity decreases. Chromosomes get back to their long, slender, and extended form. RNA synthesis starts again as nucleolus reappears again.
Telophase ends with reorganization of nuclei and the entry of the two daughter cells into G1 phase of the interphase.
Cytokinesis is the cytoplasmic division of the cell. Cytokinesis or cytoplasmic division results in the splitting of the cytoplasm in two daughter cells. The division of cytoplasm takes place by a process called cleavage.
Mitotic spindle plays a very important role in determining where and when the cleavage (Not the one that you are thinking. Dirty minds!) occurs.
For most of the cells, cleavage starts in anaphase stage and goes into interphase of the next cell cycle. Puckering and furrowing of the plasma membrane is the first sign of cleavage (take place in anaphase stage).
Furrowing begins in the plane of the metaphase plate (right angles to the mitotic spindle). A ring is formed which is attached to the plasma membrane towards the cytoplasm. The ring called contractile ring is made up of bundle of actin and myosin filaments. It is formed in early anaphase stage.
The contraction of actin and myosin filaments pull the plasma membrane into the furrow. Contractile ring doesn’t thicken as the furrow deepens and continuously loses filaments.
After the end of cleavage, contractile ring gets degenerated. After the end of cleavage, plasma membrane forms a midbody which holds on to the two daughter cells. Midbody consists of two sets of polar microtubules and dense matrix material.
Cytokinesis increases the surface area of the cell as two cells are formed from one cell.
Partition of Cell Organelles
The partition of Golgi complex and endoplasmic reticulum is debatable. One theory state that Golgi complex incorporates in endoplasmic reticulum and stops existing as a separate cell organelle. The second theory state that Golgi complex is divided into multiple vesicles that get distributed to two daughter cells.
Third theory states that (based on studies done on algae and protists) Golgi complex gets equally split into two parts and each daughter cell gets half of the original Golgi complex. Recent studies on cultured mammalian cells have proved that endoplasmic reticulum remains intact throughout mitosis.
Earlier studies on embryos and eggs proved that endoplasmic reticulum undergoes extensive fragmentation during mitosis. It can be understood that different types of cells or organisms use different mechanism of Golgi complex and endoplasmic reticulum inheritance.
Significance of Mitosis:
- Helps in maintaining the size of the cell.
- Helps in maintaining the amount of DNA and RNA in the cell.
- Helps by providing the opportunity to grow for organs and body of the organism
- Old and decaying cells are replaced with new and young cells.
- In a few organisms, mitosis is also involved in asexual reproduction.
- Sex cells also depend on mitosis for the increase in their numbers.
Meiosis – Cell Cycle
The term Meiosis was coined by J.B. Farmer in the year 1905. Meiosis means ‘to diminish’ or ‘to reduce’. Meiosis produces four haploid cells from a diploid cell.
These haploid cells either become or give rise to gametes (present in gonads).
The gametes (of two organisms) fertilize and support sexual reproduction and eventually produce a generation of diploid cells.
Meiosis is an extremely important process to run the reproductive cycle efficiently in plants, animals, bryophytes, microorganisms like Neurospora and Chlamydomonas etc.
Note: Meiocytes are the cells in which meiosis takes place. Cells of gonads in which meiosis takes place are called gonocytes (spermatocytes in males and oocytes in females). In plants, meiocytes are called sporocytes (microsporocyte for males and megasporocyte for females).
Types of Meiosis
There are 3 types of meiosis based on the time at which meiosis takes place. The three types are briefly described below.
It is also known as gametic meiosis. It is seen in animals and some lower plants. Meiosis takes place immediately before the gametogenesis or formation of gametes.
It is also known as zygotic meiosis. It is seen in diatoms, fungi and some algae. Meiosis takes place immediately after fertilization. The organism spends most of its life as a haploid. This is the only stage where the organism is diploid.
It is also known as sporic meiosis. It is characteristic of flowering plants. Meiosis takes place at any time between fertilization and formation of the gametes.
Microspores in anthers (male in flowering plants), megaspores in ovary or pistil (female in flowering plants) are produced.
Microspores and megaspores are haploid. Production of microspores and megaspores is called microsporogenesis and megasporogenesis respectively.
Process of Meiosis
Meiosis appears as two mitotic divisions without giving time for DNA replication.
The first meiotic division has a long prophase where the homogenous chromosomes get associated with each other and genetic material is interchanged between them. In first meiotic division, reduction of chromosomes takes place and two haploid cells are formed.
First meiotic division is heterotypic division and second meiotic division is homotypic division. In second meiotic division, two haploid cells divide mitotically to produce four haploid cells. Second meiotic division is similar to mitotic division.
First Meiotic Division or Meiosis I – Cell Division
It is also known as heterotypic division. This division (just like mitosis) starts after interphase (similar to the interphase of the mitosis).
DNA replication takes place at S phase but in G2 phase, a change takes place which is important to drive cell towards meiosis instead of mitosis.
Before meiosis takes place, nuclei of the meiocytes swell by absorbing water from the cytoplasm. This results in an increase in the volume of the nucleus three folds. Once the cell passes this stage, meiosis takes place.
It is the longest stage of first meiotic division. It is again divided into 6 substages.
1. Proleptotene Stage: It is also known as Proleptonema. In Greek, pro means before, leptas means thin and nema means thread. Proleptotene sub stage is similar to the early prophase of mitosis. Chromosomes of this substage are thin, long, uncoiled, and slender thread-like structures.
2. Leptotene Stage: It is also known as Leptonema. Chromosomes of this stage become further uncoiled and thread-like structures. Chromosomes take a specific orientation inside the nucleus – the ends of the chromosomes converge where the centrosome lies in the nucleus. This stage/phase is called bouquet stage/phase.
Centriole duplicates and forms two daughter centrioles. These two centrioles move towards the opposite poles of the cell. Once each centriole reaches the pole, the centriole duplicates again and hence there are two centrioles near each pole.
Studies conducted by Nancy Kleckner and et al. at Harvard University on yeast cells talked about the basis of recognition of homologous chromosomes to form the bouquet stage/phase.
According to the studies the homologous regions of DNA of homologous chromosomes get associated in leptotene stage only.
The chromosomes are visible under the microscope in zygotene stage.
DNA is seen to break in leptotene stage.
Note: Homologous chromosomes are a pair of chromosomes that have same chromosomal length, same gene sequence, same centromere location, etc.
Clustering of telomere:
It is seen in yeast cells that homologous chromosomes are paired even before the prophase I starts. In leptotene stage, telomeres of chromosomes or ends of chromosomes are distributed around the nucleus.
But near the end of leptotene, these chromosomes reorganize themselves in such a way that the telomeres get localized at inner side of the nuclear envelope at a side of nucleus. This type of clustering of telomeres is seen in many organisms and the chromosomes appear as stems of flower bouquet.
3. Zygotene Stage: It is also known as Zygonema. In Greek, zygon means adjoining. Homologous chromosomes of mother (by ova) and father (by sperm) get attracted to each other and crossing over takes place.
Crossing over of homologous chromosomes is called synapsis (meaning union in Greek). Synapsis takes place at one or multiple points on the homologous chromosomes.
It is important to note that the alignment of homologous chromosomes is exactly gene to gene while pairing homologous chromosomes.
Synapsis is again of three types which are as follows:
- Proterminal Synapsis: Pairing of homologous chromosomes takes place at the ends first and then continues to the centromere.
- Procentric Synapsis: Pairing of homologous chromosomes starts at the centromere and continues towards the end.
- Localized Pairing: It is also known as random pairing. The pairing of homologous chromosomes starts at random points.
The framework formed after the pairing of homologous chromosomes is called the synaptonemal complex. It completely covers paired chromosomes and is anchored to an end of the nuclear envelope.
Synaptonemal complex has a structure of a ladder. For many years it was thought that synaptonemal complex held each pair of the chromosomes in position so that genetic recombination can start between strands of homologous DNA. It is now proved that the synaptonemal complex doesn’t help in starting genetic recombination.
It is believed now that synaptonemal complex acts as scaffold (a temporary stage where the work is done) which allows interacting of the chromosomes to finish the crossover.
Synaptonemal complex which is formed by pair of synapsed homologous chromosome is called tetrad or quadrivalent or dyad or bivalent.
A bivalent indicates that a synaptonemal complex has two chromatids and a tetrad indicates that a synaptonemal complex has four chromatids. Zygotene ends with the end of synapsis.
4. Pachytene Stage: In Greek language, pachus means thick. In this stage, the synaptonemal complex holds together either two or four chromatids closely.
Structures called recombination nodules are seen in the center of synaptonemal complex.
It is at these places where crossing over of the chromosomes takes place.
When genetic material is exchanged between a non-sister chromatid of a homologous chromosome, it is known as chiasmata formation.
Note: Sister chromatids are the chromatids of a single chromosome. Non-sister chromatids are the chromatids of different chromosomes.
Stern and Hotta reported in 1969 that a very small amount of DNA is synthesized. The synthesized DNA is used to repair broken DNA of the chromatids during chiasmata formation.
Note: Nucleolus is prominent until this stage and is associated with the Nucleolar Organizer Region (NOR) of the chromosome.
5. Diplotene Stage: The end of genetic recombination marks the beginning of diplotene. Dissolution of synaptonemal complex takes place and the chromosomes are attached to each other at certain points by X shaped structures called chiasmata.
These points of attachment visually show the extent of genetic recombination. Chiasmata are more visible because the chromatids develop a tendency to move away from each other in diplotene stage. Diplotene stage is characterized by intense metabolic activity.
6. Diakinesis Stage: This is the last sub-stage of Prophase I. Meiotic spindle is ready and the chromosomes are ready to get separated. The nucleolus disappears, Nuclear envelope breaks down.
Chiasma moves from centromere to the telomere and intermediate chiasmata disappears. This movement of chiasmata is called the terminalization. Terminal chiasmata still keeps these chromatids connected and exist up to the metaphase. Tetrads move to the metaphasic plate.
Two homologous chromosomes of each bivalent (one pair of chromosomes in a tetrad) get connected to the spindle fibers of the opposite pole. Sister chromatids are connected to the same pole’s microtubules.
For the sister chromatids to get connected to the microtubules of same spindle pole, they are arranged side by side of their kinetochores. Maternal and paternal chromosomes’ orientation has an equal likelihood of facing either pole.
When these chromosomes get separated in anaphase I, each spindle pole gets a random assortment of maternal and paternal chromosomes.
Chiasmata disappears at the transition between metaphase I and anaphase I because the arms of chromatids of each bivalent lose cohesion. Cohesion between centromeres of sister chromatids remains strong.
The chromosomes get reduced and disjointed at anaphase stage. Chromosomes move towards their respective poles.
Chromosomes disperse but not as seen in telophase of mitosis.
There are no big or dramatic changes in telophase I as compared to the telophase of mitosis. Nuclear envelope may or may not reappear.
Once karyokinesis is completed, cytokinesis takes place and two haploid cells are formed.
Interkinesis: It is the stage between meiosis I and meiosis II. It is short-lived. Cells in this stage are called either secondary spermatocytes (males) and secondary oocytes (females). The cells are haploid.
It is to be noted that the two haploid cells after meiosis I have double the amount of DNA as compared to ovum or sperm cells. It is because each chromosome is represented by two attached chromatids.
Second Meiotic Division or Meiosis II – Cell Division
Second meiotic division is also known as the homotypic meiotic division.
As mentioned earlier, it is pretty much similar to mitosis.
In this stage, each centriole gets divided into two thereby forming two pairs of centrioles. Each pair migrates to the opposite pole.
Microtubules arrange themselves in a spindle shape which are placed at right angles of the spindle of first meiosis. Chromosomes (all chromosomes have two chromatids) become short and thick.
In this stage, the chromosomes arrange themselves on metaphasic plate of the spindle. Centromere gets divided into two which results in each chromosome producing two monads or daughter chromosomes. Microtubules of spindle get attached to the centromere of the chromosomes.
Due to the shortening of chromosomal microtubules and stretching of polar microtubules daughter chromosomes move towards their respective poles.
Chromosomes migrate to the poles. The endoplasmic reticulum forms a nuclear envelope. Nucleolus reappears. After Karyokinesis is finished, cytokinesis takes place. As a result, four haploid cells are formed. These cells have different types of chromosomes because of the crossing over that took place in Prophase I.
Significance of Meiosis:
- Meiosis maintains constant and definite number of the chromosomes in every organism.
- Due to crossing over, genes exchange and this causes genetic variations among the species which serve as raw material for evolutionary process.