Cell Division- Mitosis, Meiosis

Cell division is the process of division of parent cells into two or more daughter cells. Let us know about cell division, its types, processes, phases in greater detail.

The main aim of all organisms is to maintain their race generation after generation and to produce their likes.

This event is governed by two processes- the union of cells and the division of cells.

Nageli (1846) first pointed out that new cells are always formed from pre-existing ones. What is the reason for this cell division?

Well, there is a reason for that. There is a definite relationship between the mass of the cytoplasm and the nucleus, known as karyocytoplasmic relation.

When this relationship is disturbed by the optimum growth of protoplasm, the cell begins to divide.

The nucleus takes an active part during cell division. There are two phases in cell division:

  1. Nuclear Division or Karyokinesis (followed by)
  2. The division of cytoplasm or Cytokinesis

Cell division is a continuous and dynamic process.

Types of Cell Division

There are three types of cell division and they are:

  1. Amitosis or direct cell division
  2. Mitosis or indirect cell division
  3. Meiosis or reduction division

Amitosis – Definition and Process

Amitosis is a direct cell division or binary fission that also called karyostenosis.

This type of cell division occurs in acellular animals and the cells of the foetal membrane of some vertebrates.

In this process, the nucleus elongates and constricts in the middle, thereby becoming dumb-bell-shaped in appearance.

The constriction deepens and separates the nucleus into two halves.

After the division of the nucleus, a groove appears in the middle of the cell dividing it into two daughter cells. These daughter cells are approximately equal in size and may or may not contain the same amount of chromatin material.

Mitosis – Definition, Cell Cycle, Process

Mitos = fibre thread

Mitosis is a complicated process of cell division by which the parent cell divides into two similar daughter cells having the same number of chromosomes as in the parent cell.

The body of different multicellular animals is composed of two types of cells:

  1. Somatic cell and
  2. Germ cell

The somatic cells form the different parts of the body while the germ cells are confined only to the reproductive organs like testis and ovary.

The somatic cell divides exclusively by mitotic division. It contributes to the growth and development of organisms. Mitosis was first studied by Walter Flemming (1882).

Cell Cycle

Certain changes occur in a cyclic order in the cell such as growth, preparation, and division. This is known as the cell cycle.

During interphase (the period between the end of one division and the beginning of the following division) the cell prepares itself for the future mitotic division.

Accordingly, three distinct sub-stages can be distinguished in the interphase as follows:

  1. ‘G1’ phase: First growth or Gap period
  2. ‘S’ phase: Synthetic Period
  3. ‘G2’ phase: Second growth or Gap period

During the G1 phase, the nucleus regulates the growth of the cell when proteins and ribonucleic acid are synthesized.

The G1 phase is followed by the synthetic phase (‘S’–Phase) during which replication of DNA takes place, i.e., DNA is synthesized.

This results in the doubling of the number of DNA strands in each chromosome.

G2 phase then follows the synthetic phase. During this phase, the protein material, and energy, associated with the structure and movement of chromosomes are established.

After the completion of the three phases, mitosis (M–phase) takes place during which chromosomes are parcelled into the daughter nuclei.

The time taken by the above-mentioned phases varies in different organisms but is fixed for a given species. The cells which are not going to divide, do not proceed beyond the G1 phase but begin to differentiate.

Interphase (Resting Phase)

It is the period of interval between two successive cell divisions. This is often called the resting stage, but actually during active changes occur for the onset of the cell division as mentioned earlier.

Therefore, the interphase can be described as the preparatory phase.

Although no cell division takes place during the interphase and it is not considered as a stage of mitosis due to the absence of mitotic events, still it is not out of the place to mention the important events occurring during this phase.

It is because it forms a basis to study the subsequent changes during the course of cell division. The events taking place during interphase are as follows:

  • The cell, as well as the nucleus, undergo maximum growth and the nucleus becomes distinct.
  • The nuclear membrane and nucleus also become distinct.
  • There is an active synthesis of RNA and DNA. Synthesis of DNA and duplication of chromosomes are completed.
  • Presence of two centrioles towards the end of interphase.
  • Chromosomes are not clearly visible due to hydration but they may appear as very thin coiling thread or as granules (called chromatin material).

Process of Mitosis

During mitotic division, the nucleus divides first called Karyokinesis, which is followed by the division of cytoplasm.

At the time of karyokinesis, the nucleus undergoes a series of orderly changes which can be divided into the following stages:

  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase

Prophase (Initial Stage)

The mitosis (nuclear division) begins with the prophase. This phase is characterized by the following features:

  • The chromatin material of the nucleus gradually condenses by losing water (de-hydration) to form distinct chromosomes.
  • The chromosomes begin to coil tightly and become shorter and thicker.
  • The chromosomes become more and more distinct.
  • Each chromosome is seen to consist of two chromatids which are joined by the centromere and lying very close to each other. The chromatids are not formed by the longitudinal splitting of the chromosomes, but by the replication i.e., the formation of new chromosome material by the parent chromosome. At this stage, each chromosome appears as a double-threaded structure.
  • There is the formation of mitotic apparatus which includes the formation of asters and spindle fibers. The centrosome divides into two and the two centrioles tend to move apart towards the opposite poles. Fibers stretch between the centrioles and are called spindle fibers. The spindle so formed is called the nuclear spindle.
  • Astral rays radiate from each daughter centriole.
  • With the advance of prophase, the nuclear membrane and nucleolus gradually disappear.

Prophase is the longest phase, lasting from one to several hours.

Metaphase (Attachment Stage)

The beginning of metaphase is marked by the complete disappearance of the nuclear membrane and the nucleolus.

The chromosomes now remain suspended in the cytoplasm. The metaphase shows the following features:

  • During metaphase, the chromosomes move towards the centre and they are arranged in such a way that their centromeres lie in one plane at the center of the cell.
  • The chromosomes are now thickest and shortest and have acquired smooth outlines. The chromatids of each chromosome remain parallel to each other so that their double structure is clearly observed.
  • Two centrioles finally remain in the opposite poles, stretching the spindle fibers. It is pointed at the ends and thick at the middle. The spindle fibers are formed by the gelation of the nucleoplasm and cytoplasm.
  • The chromosomes now lie at the equator of the spindle with their centromeres attached to the spindle fibers, whereas the arms are oriented towards the poles. Some of the spindle fibers lie between the chromosomes.
  • The large chromosomes remain at the edge of the nuclear spindle and the smaller ones within the equatorial plane.

This stage lasts for a very short time, covering about 20 minutes.

Anaphase (Separation Stage)

The metaphase passes into the anaphase when centromeres become functionally double and the chromatid begin to move towards respective poles.

This stage is characterized by the following changes:

  1. The centromere of each chromosome divides and the two sister chromatids separate as two daughter chromosomes. (The term daughter chromosome is used when the chromatids are completely separated.)
  2. A repulsive force develops between the two daughter chromosomes and they move apart.
  3. Spindle fibers contacts their respective poles along with the daughter chromosomes attached to them. The centromere is pulled first by the spindle fibers towards the pole of the spindle and the chromosomes are dragged behind with their arms pointing towards the equatorial plates.
  4. The daughter chromosomes reach the opposite poles and form two groups.
  5. According to the position of the centromere, the chromosomes assume either J or V-shapes.

It is the shortest of all stages and the duration varies between 6 to 12 minutes.

Telophase (Reconstructional Stage)

The beginning of the telophase is marked by the arrival of daughter chromosomes at their respective poles.

This is the last phase in which two daughter cells are formed by the following changes:

  • The chromosome with its centromere begins to uncoil and lengthen. So, they become thin, long, and granular.
  • Decndesation takes place as a result of which the chromosomes become invisible.
  • Nucleolus begins to reappear in this stage.
  • The nuclear membrane develops around the chromosomes.
  • Astral rays disappear and spindle fibers break down and are absorbed in the cytoplasm.
  • Two nuclei are thus formed at two poles of the cell. Hence, the events of the prophase are reversed in this stage.

Cytokinesis

It is the division of cytoplasm into two separate cells.

Usually, it occurs in the telophase along with the formation of daughter nuclei. After nuclear division, the cytoplasm of the cell also divides.

In this process, the plasma membrane constricts in the middle line, forming a cleavage furrow. The furrow deepens and divides the parent cell into two daughter cells, each having a nucleus.

The above-mentioned stages give an account of the behavior of chromosomes during mitotic cell division.

Moreover, the changes take place so smoothly and continuously that there is no sharp demarcation between the stages. Abnormalities in mitosis occur as a result of placing the cells under unfavorable conditions during different phases.

Physical agents like temperature, radiations, and chemicals like narcotics, enzymes produce mitotic deformities. Cancer results when the mitosis goes wrong.

Significance of Mitosis

  • Mitosis plays a significant role in the growth of tissues and organs of living matter by the accumulation of new cells and the replacement of the damaged cells.
  • The size of the cell remains fixed due to mitosis.
  • Mitosis ensures equal, qualitative, and quantitative distribution of hereditary material amongst the daughter cells so that the daughter cell resembles the parent cell in their genetic constitution.
  • It is mainly a means of reproduction in lower animals like protozoans.
  • It helps to maintain the constancy of the species because each species has the same number of chromosomes in all individuals and from generation to generation.

Difference Between Plant and Animal Cell Division

  1. In most animal cells, each spindle pole contains a centriole surrounded by astral rays which are known as an aster but in plants cells centrioles and asters are absent.
  2. In the animal cell, cytokinesis occurs by the constriction of the plasma membrane almost in the middle of the cell. This constriction or furrow gradually deepens and finally divides the cell into two cells. But in the plant cell, a cell plate is formed at the equatorial plane, which starts first in the center and gradually extends laterally till it divides the cell into two parts. This is due to the presence of a rigid cell wall.

Meiosis Cell Division – Definition, Process

Meiosis or reduction division is confined to germ cells. It occurs in gonads only and helps in the formation of gametes i.e., either sperms or ova.

The cells which undergo meiosis are called meiocytes. In males, these meiocytes are primary spermatocytes found in the testis, and in females, these are primary oocytes found in the ovary.

But mitosis is operative on all cells of the body except primary spermatocytes and the primary oocytes. Meiosis contributes to reproduction and heredity. What initiates meiosis is not exactly known.

According to Sinha (1960), the relative amount of RNA and DNA is the causative factor. It is stated that if the ratio of DNA to RNA is high, the cell will undergo mitosis and if this ratio is low, meiosis will result. The meiosis comprises two successive nuclear and cytoplasmic divisions.

  • Heterotypic division or Meiosis I: It is the first reductional division, during which the cell containing diploid set (2n) of chromosomes undergoes division into two daughter cells, each having haploid (n) set of chromosomes i.e., reducing chromosome number to half of the original.
  • Homotypic division or Meiosis II: It is the second meiotic division and is mitotic in nature i.e., distributing the chromosomes equally to each daughter cell.

Thus from a diploid parent cell, in which the chromosome number is ‘2n’, four haploid daughter cells are formed having ‘n’ number of chromosomes.

Interphase: The interphase which precedes the onset of meiosis is similar to the interphase before the mitosis. There is a growth phase (G,) followed by an S-phase in which replication of DNA occurs. The S-phase is followed by the second growth phase (G₂). But the replication of DNA of chromosomes takes place only once.

Process of Meiosis

  1. Heterotypic or First meiotic division: It consists of four stages like mitosis, namely Prophase I, Metaphase I, Anaphase I, and Telophase I.

Prophase I

The prophase of the first meiotic division is of longer duration and is somewhat modified. It is distinguished into five substages: leptotene, zygotene, pachytene, diplotene, and diakinesis.

Leptotene or Leptonema (Thin threads)

It is the first stage of the meiotic prophase and is characterized by the following features:

  1. The nucleus increases in size and the nucleolus become distinct.
  2. Chromosomes become distinct. They appear as long, thin, and uncoiled threads and possess a series of dark granules or chromosomes along with their length.
  3. The chromosome has a definite polarized orientation with all its ends directed towards one small area, on one side of the nucleus. This peculiar arrangement is called a bouquet.
  4. The chromosomes do not show the double structure although each of them consists of two chromatids that are held together at the centromere.

Zygotene or Zygonema (Yolked thread or Pairing)

It is the most important stages of the meiotic prophase and is characterized by the following features:

  1. The homologous or similar chromosomes begin to pair. In a pair, one of the chromosomes is paternal, and the other is of maternal origin. The two chromosomes lie side by side through their length, i.e., centromere to centromere and chromomere to chromomere. This process of pairing is called Synapsis.
  2. The chromosomes do not show the double structure yet, i.e., no longitudinal splitting.
  3. Nucleolus increases in size and the centrioles move apart, initiating the formation of the spindle.

Pachytene or Pachynema (Thick Thread)

This stage begins as soon as synapsis is complete and it has the following main features :

  • After pairing, the chromosomes become shorter in length and increase in thickness due to more degree of condensation. The pairs are now called bivalents or dyads. Each bivalent has two centromeres.
  • The chromosomes of the bivalent become much coiled.
  • Each chromosome becomes split longitudinally into two chromatids so that each bivalent consists of four chromatids. Hence the total structure is no more called the bivalent but as a tetrad. The chromatids of each homologous chromosome are known as sister chromatids.
  • The two chromatids of a chromosome coil tightly around the chromatids of the homologous chromsomes.
  • Vigorous coiling between the pairs of homologous chromosomes exerts considerable strain on themselves as a result of which the weaker chromatids break down at one or more points. This strain is due to the synaptic force.
  • The chromatid of one chromosome which breaks at one point unites with the chromatid of the other chromosome of a pair and vice versa. This results in the exchange of fragments of chromatids between the two chromosomes of a pair. This process by which the exchange of chromatid segments takes place is known as crossing over. Only two of the four chromatids are involved in the exchange.
  • At this stage, the nucleolus is also distinct.

Diplotene or Diplonema (Double thread or duplication)

The separation of homologous chromosomes initiates the diplotne.

  1. The homologous chromosomes now uncoil and start separating from each other. The separation is not complete and the chromatids remain in contact with each other at certain points forming chiasmata (sing, chiasma = each point of contact for the exchange of chromatid segments).
  2. Chiasmata become more distinct and are easily visible as cross-shaped structures. This is of importance in the distribution of genes in heredity, i.e., at the chiasmata, exchange of hereditary characters takes place between the homologous pairs of chromosomes.
  3. At the end of diplotene, the chiasmata appear to move away towards the end along the length of the chromosomes. This is known as terminalization, which may continue up to the metaphase – I stage.
  4. The nuclear membrane and nucleolus begin to disappear.

Diakinesis (Moving apart)

The following events take place in this stage.

  1. Bivalent become still shorter and thicker.
  2. They repel each other and migrate towards the periphery of the nucleus.
  3. The nuclear membrane and nucleolus gradually disappear towards the end of this stage.
  4. The centrioles reach the opposite poles and a nuclear spindle is formed between the two centrioles.

Metaphase I

The metaphase of meiosis is mostly similar to that of mitosis. The beginning of this stage is characterized by:

  1. The complete disappearance of the nuclear membrane and nucleolus and there is the formation of spindle fibers.
  2. The bivalents move towards the equator of the spindle and arrange themselves in the middle, i.e., along the equator of the spindle.
  3. The arrangement is such that their two centromeres lie towards the opposite poles, while the arms of the chromosomes are directed towards the equator. Each bivalent has two functionally undivided centromeres.
  4. Each bivalent is arranged independently and paternal and maternal chromosomes point towards the opposite poles.
  5. Spindle fibers extend between the poles and are attached only to the centromere region of the bivalents.

Anaphase I

During this stage

  1. The bivalents move towards the opposite poles of the spindle.
  2. The tetrads consisting of four chromatids become separated into dyads having two chromatids each.
  3. In this process, the homologous chromosomes of a pair become separated from each other and move towards the opposite poles and the remaining chiasmata are also terminalized.
  4. The chromatids of each dyad are attached by a common centromere and out of two chromatids of a chromosome, one is the original chromatid and the other is the mixture of original chromatid and homologous chromatid. Thus, it consists of a mixture of maternal and paternal chromosome parts. In this way, the two homologous chromosomes are brought together during the formation of the zygote, separate from each other.

Telophase I

The arrival of chromosomes at their respective poles demarcates the beginning of telophase.

  1. The dyads, after reaching the respective poles, uncoil and become long.
  2. A nuclear membrane is formed around each group of chromosomes.
  3. Nucleolus also reappears.
  4. The cell cytoplasm divides into two.
  5. Thus, two cells are formed, each with a haploid number of chromosomes.

Interphase

The interphase between meiosis I and II is either very short or entirely absent.

Homeotypic or Second Meiotic Division

The second meiotic division is essentially similar to mitosis which occurs independently in both the haploid cells.

Prophase II

This stage is short-lived. During the second prophase, the nucleus and nuclear membrane gradually disappear again in both the daughter haploid cells.

The formation of the spindle starts. The chromatids remain coiled and the dyads have an X-shaped appearance. The two chromatids are joined by centromeres with radiating arms.

Metaphase II

It is of short duration and its beginning is demarcated by the complete disappearance of nuclear membrane and nucleolus. The spindle fibres are formed.

The chromatids move towards the center of the spindle and arrange themselves on the equator. Their centromeres touch the equator but the arms are extended outwards.

Later on, the centromere in each dyad divides into two. Thus, two sister chromatids of a chromosome get separated.

Anaphase II

The sister chromatids along with their individual centromeres move towards the opposite poles. Now they are called daughter chromosomes.

The chromatids are not only short and compact but also very similar to the chromosomes of anaphase in the mitotic division.

Telophase II

The daughter chromosomes at each pole elongate and uncoil and each group gets surrounded by a reappearing nuclear membrane. Nucleolus reappears.

Thus, two nuclei are formed each having a haploid set of chromosomes.

This is soon followed by cytokinesis and two cells are formed from each haploid daughter cell.

Thus, four haploid daughter cells are formed from one parent cell by two successive divisions. The salient feature of meiosis is two successive nuclear divisions with only one duplication of chromosomes.

Significance/ Importance of Meiosis

The importance of meiosis is as follows:

  1. Through meiotic cell division, one germ cell having a diploid set of chromosomes divides into four cells, each containing a haploid set of chromosomes. These cells develop into male or female gametes. It is a logical and necessary part of the life cycle of sexually reproducing animals, as it leads to the formation of gametes.
  2. The male and female gametes unite with each other in the process of fertilization. As a result, a zygote is formed, containing a diploid set of chromosomes. The zygote develops into an organism. Thus, it is the only means of restoring the original number of chromosomes, characteristics of species. In this way, the possibility of doubling the chromosomes of a species in the next generation is avoided.
  3. During crossing over, there is a recombination of maternal and paternal hereditary factors due to breakage and exchange of chromatids. This new combination of genetic material through synapsis, chiasmata formation, and crossing over of chromatids ensures genetic variation in the population and leads to the evolution of organisms.
  4. It provides a basis for the precise transmission of genetic material from one generation to the other.

Comparison between Mitosis and Meiosis

When two types of cell division are compared with each other, some similarities and differences are noticed. These are as follows:

Similarities Between Mitosis and Meiosis

  • Both mitosis and meiosis comprise four stages namely prophase, metaphase, anaphase, and telophase.
  • The process of mitotic cell division and second meiotic cell division is almost similar to each other in distributing chromosomes equally to the daughter cells.
  • During prophase, centrioles move apart forming astral rays and formation of spindle fibres also takes place. Chromosomes become distinct; each having two chromatids due to condensation.
  • The nucleolus and nuclear membrane completely disappear in metaphase and also chromosomes are arranged in the equatorial plate.
  • The migration of chromosomes towards the opposite poles during anaphase and the reorganization of the nuclear membrane, nucleolus, and uncoiling of chromosomes during telophase are similar in both divisions.
  • There is only one replication of chromosomes in both divisions.

Difference Between Mitosis and Meiosis

Mitosis

  • This division occurs in both somatic cells and germinal cells.
  • It is concerned with the growth of the body.
  • The process of mitosis competes in one sequence.
  • It is of a shorter duration.
  • As a result of mitosis two daughter cells are formed from the single mother cells.
  • The daughter cells produced are similar to the parent cell containing the same number (diploid=2n) of chromosomes.
  • Each DNA replication is followed by one nuclear division.

Prophase of Mitosis

  1. It is comparatively simple and of short duration.
  2. There is only one phase of prophase.
  3. The chromosomes are a double structure, each consisting of two chromatids.
  4. All chromosomes behave independently of each other, i.e., no synapsis.
  5. As there is no synapsis, so no tetrad formation.
  6. No chiasmata formation and crossing over. So, there is no exchange of chromosomal material.

Metaphase of Mitosis

  1. Metaphase chromosomes possess one functionally undivided centromere which divides afterward to become double along with the separation of chromatids.
  2. Centromeres lie towards the equator of the spindle and the arms are directed towards the poles.

Anaphase of Mitosis

  1. As a result of the division of the centromere, the sister chromatids pass to the pole.
  2. The composition of the daughter chromosomes is similar to the original one.

Telophase of Mitosis

  • Cytokinesis occurs always after each division.

Meiosis

  • It occurs only in the germinal cells of the gonads (testis and ovary).
  • It is related to the formation of gametes (sperms in testis and ova in the ovary).
  • The process of meiosis is completed in two divisions, i.e., the first division is the education division and the second is the simple mitotic division.
  • It is of a longer duration.
  • As a result of meiosis, four daughter cells are formed from single mother cells.
  • Daughter cells are produced in this division, are not similar to the parent cells as they contain a haploid number (n) of chromosomes.
  • Each DNA replication is followed by two successive nuclear divisions.

Prophase of Meiosis

  1. It is complex and of long duration.
  2. There are two phases namely prophase-I and prophase-II. Prophase-I is of longer duration comprising five substages as leptotene, zygotene, pachytene, diplotene, and diakinesis.
  3. The chromosomes are single thread-like structures, in the beginning, showing distinctly chromomeres (appear as granules). They do not show their chromatids.
  4. Homologous chromosomes get paired and behave combinedly i.e., synapsis occurs.
  5. Tetrad formation takes place.
  6. Chiasmata formation and crossing over occur during diplotene. As a result, there is an exchange of chromatids between paternal and maternal chromosomes.

Metaphase of Meiosis

  1. In the first division, bivalent has two functionally undivided centromeres. So centromeres undergo no division and chromatids do not separate. But centromeres divide only during the second metaphase.
  2. Centromeres remain towards the poles and their arms are directed towards the equatorial region.

Anaphase of Meiosis

  1. As the centromeres remain undivided, so the whole chromosome becomes segregated during anaphase-I.
  2. The composition of daughter chromosomes is different from that of original ones, as homologous paternal and maternal chromosomes separate.

Telophase of Meiosis

Cytokinesis may not occur always in telophase-I but extended to the second division of meiosis. So, daughter nuclei pass immediately into prophase or metaphase of the next division.

Key Terms Related to Cell Division

These are some of the terms which are frequently used when learning and reading about cell division.

AmitosisDivision of the cell by direct nuclear division and constriction.
AnaphaseThird stage of the cell division following metaphase (separation of chromosome)
BivalentTwo paired similar chromosomes in meiotic division.
ChiasmaThe point where crossing over takes place
ChromomereSmallest beaded particles present in the chromosomal thread.
Crossing overAn exchange of chromatid fragments of homologous chromosomes.
CondesationShortening and thickening of chromosomes through spiralization during prophase.
DiakinesisMoving apart stage of chromosomes during prophase-I.
Diplotene Duplication stage of meiotic prophase-I.
DiploidTwo sets of chromosome forming pair as in somatic cells, for example: 46 chromosomes in the cell of human being remain in 23 pairs.
HaploidSingle set of chromosomes but no pairing, for example: only 23 chromosome in gamete.
InterphaseOtherwise known as interkinesis. Resting stage before cell divides.
KaryokinesisDivision of the nucleus.
LeptoteneThin thread stage of meiotic prophase-I.
MeiosisReductional Division.
MetaphaseSecond phase of cell division following prophase.
MitosisEquational division.
PachyteneThickening stage of meiotic prophase-I.
ProphaseBeginning stage of cell division (meiosis and mitosis).
SynapsisTemporary pairing of chromosomes at meiotic division-I (zygotene).
TelophaseLast stage of cell division.
TetradEach pair of homologous chromosomes consisting of four chromatids.
ZygotenePairing stage of meiotic prophase-I.

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