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CELL DIVISION

Introduction

The cell is the structural and functional unit of life. New cells arise from the preexisting ones. The process by which new cells are formed from the pre-existing cells is called cell division.

In unicellular organisms, the cell division directly produces two individuals and thus, represents a type of reproduction (multiplication).

In multicellular organisms, there are two types of cells; the somatic cells or the body cells (which form the body of the organism) and the reproductive cells (such as gamete-producing cells and-spore producing cells).

The somatic cells divide by mitosis (equational division) and the reproductive cells divide by meiosis (reduction division). Mitosis helps in growth and development of an organism. Meiosis produces gametes in sexual reproduction and spores in asexual reproduction.

All eukaryotic organisms, plants as well as animals, show great regularity as well as similarity in the cell divisions. Generally, a cell increases in size before dividing. This is mainly due to the synthesis of proteins, RNA and DNA. This is followed by division of the cell nucleus (karyokinesis) and finally the division of the cell cytoplasm (cytokinesis). All these events collectively form a cell cycle.

The Cell Cycle

Definition : The cell cycle, also called generation time, is the sequence of events in the life of a cell. The cell cycle starts immediately after one cell division and ends with the completion of the next division.

The cell cycle of eukaryotic cells is classified into (1) interphase (2) karyokinesis and (3) cytokinesis (Howard and Pelc, 1953).

(1) Interphase : It is the preparatory phase during which the cell is metabolically very active and prepares itself for the division.

Three important processes occur in interphase, viz. (a) replication of chromosomal DNA, synthesis of RNA and the basic nuclear proteins (histones) (b) synthesis of energy rich compounds which provide energy for mitosis and (c) in animal cells, division of the centriole.

On the basis of DNA synthesis, interphase is subdivided into following three stages.

G1 (Gap1) : It starts immediately after the previous division. Therefore G1 is called gap phase or first growth phase. Synthesis of proteins and RNA takes place. The cell grows in volume.

S phase (Synthesis phase) : It is the period during which DNA synthesis occurs, i.e. replication of chromosomal DNA takes place. This results in doubling of the chromosomal threads.

G2 (Gap 2) : It is the last part of interphase and occurs just before the new cell division. Hence G2 is called pre-division gap phase or second growth phase. It begins after completion of DNA synthesis in the S phase and ends when new division (karyokinesis) commences. During G2, synthesis of proteins and RNA takes place and the nuclear volume increases.

(2) Karyokinesis : It is the division of the parent nucleus into daughter nuclei.

(3) Cytokinesis : This is the division of the cytoplasm. It occurs after karyokinesis and divides the parent cell into daughter cells.

Karyokinesis and cytokinesis together form the M phase (i.e. cell division).

The total duration of a cell cycle varies greatly in different organisms and under different conditions; e.g. it may be as short as 20-30 minutes in the bacterium Escherichia coli or may take 12-24 hours as in higher plants and animals.

The time required for completion of each phase in the cell cycle varies greatly. In general, actual cell division (M-phase) occupies only a short span of the total cycle while the major span is occupied by the interphase. Normally, time duration of S and G2 phases is more or less equal. The duration of G1 is longer in cells, which do not divide frequently, and is very short in cells, which divide repeatedly in close succession.

G0 stage : It is a stage during which cell cycle is arrested for an indefinite period.

Significance of cell cycle

  1. In multicellular organisms, the 'cycling type' of cells (dividing cells) help in reproduction, growth and replacement of dead cells, healing of wounds, etc.
  2. The interphase allows time for synthesis and growth of the dividing cell.
  3. Properly controlled and regulated cell cycle results in normal and proportionate growth of organisms.
  4. Loss of control over the cell cycle can lead to cancerous growth.

Cancerous cells : For some reason, in some cells, if the control over the cell cycle is lost, then they start behaving abnormally. These cells divide repeatedly in an uncontrolled manner at abnormally high rates. As a result, they do not get enough time for growth and differentiation. Such cells mass together and form tumors in the body, which may lead to cancer.

The cancerous or malignant cells are those that show continuous and uncontrolled growth through repeated cell divisions at abnormally high rates.

In the cancerous tissue, the metabolism of cells is disturbed and abnormal. The cells continue to move and crawl (grow) over one another, i.e., they do not show contact inhibition. The cancerous growth may remain localized or may spread to other parts of the body.

Mitosis

Mitosis is the characteristic division of the body cells, hence called somatic division. It can be studied in the meristematic cells in root and stem tips of plants.

Interphasic nucleus : The nucleus increases in volume during interphase. At this stage, the nuclear membrane and nucleolus are prominently visible. The chromosomes appear to form a continuous network (nuclear reticulum or chromatin network) of very fine threads. DNA replication has taken place (S-phase) and chromosomes have doubled. Interphase ends as the karyokinesis begins.

Karyokinesis : It involves a series of changes within the nucleus. This is a continuous process but, for convenience, it has been divided into four phases. These are (1) Prophase (2) Metaphase (3) Anaphase and (4) Telophase. The main features of the nuclear division during each phase are summarized below.

(1) Prophase : During early prophase, the chromatin network becomes visible as it condenses into separate threads or chromosomes. At this stage, each chromosome appears as a very fine, long single thread, the chromonema and is it described as the monad. The nucleus envelope and nuclear are prominently visible.

As the prophase progresses, chromosomes become shorter and thicker (due to the condensing of their coils). In each chromosome, the chromonema splits lengthwise into two identical threads or chromonemata (dyads). These are coiled round one another. Chromosomes become more distinct. Each chromonema becomes short, thick and is called a chromatid. At this stage, each chromosome is shorter, thicker and consists of two identical sister chromatids joined together by a spherical body called a centromere (kinetochore).

By the end of prophase, the nuclear envelope and nucleolus disappear completely. The chromosomes remain distributed in the nucleoplasm.

(2) Metaphase : Metaphase begins with the formation of a bipolar spindle body in the region of nucleoplasm (i.e. center of the cell). It consists of numerous spindle fibers. These are fine thread-like structures formed by the organization of proteins called tubulin into microtubules. There are two types of fibers in the spindle (a) Continuous fibers, which extend from pole to pole and (b) Chromosomal fibers, which extend from pole to the center (equator of the spindle).

The chromosomes move and get arranged in a plane along the equator of the spindle. This results in the formation of the equatorial plate (metaphasic plate). The centromere of each chromosome in the plate is connected with both the poles by the chromosomal fibers.

(3) Anaphase : During early anaphase, the centromere of each chromosome divides longitudinally into two. As a result, each chromosome is now completely divided into two identical halves (sister chromatids) called daughter chromosomes. The centromere of each daughter chromosome remains connected to the pole on its respective side by a chromosomal fiber.

During late anaphase, the two groups of daughter chromosomes are pulled away from each other and start moving towards the opposite poles. Most probably, this movement is caused by the shortening of the chromosomal fibers. In each group, chromosomes appear 'V' or 'L' shaped as the centromeres are pulled towards the poles and the chromosomal arms trail behind.

(4) Telophase : This is the last phase in karyokinesis. The two sets of daughter chromosomes reach the opposite poles. The chromosomes again become long and thin. A new nucleolus is organized. A Nuclear envelope is formed around each set of chromosomes. In other words, a daughter nucleus is organized at each pole in the parent cell. Each daughter nucleus has the same number of chromosomes as that of the mother cell. The spindle fibers also dissolve and disappear gradually.

The two daughter nuclei are identical in structure and characters. They are also exact copies of the original parent nucleus.

Cytokinesis : The division of the cell cytoplasm is called cytokinesis. It starts towards the end of telophase.

In plant cells, cytokinesis usually begins with centrifugal formation of a cell plate along the equatorial plane, which is followed, by a new wall formation. This divides the mother cell into two equal daughter cells.

In animal cells, cytokinesis takes place by the cleavage constriction of the cell cytoplasm. It begins peripherally and progresses centripetally.

Aster and the mitotic apparatus
in an animal cell

Astral and Anastral mitosis. In the cell of higher animals and lower plants, the centriole is present just outside the nucleus. It plays a definite role in mitosis. During interphase, the centriole divides into two. The two centrioles then move to the opposite poles during prophase and later on help to organize the bipolar spindle body during the early metaphase.

From each centriole at the pole, radiating fibers extend into the cytoplasm. These are called astral rays and form the aster.

Astral mitosis : "The mitosis in which asters are formed from the centrioles is called astral mitosis."

Anastral mitosis : The centriole is absent in the cells of the higher plants and some animals. In such cells, the astral rays and the asters are not formed at the poles of the spindle body during the metaphase. "The mitosis in which asters are not formed at the poles of the spindle body is called anastral mitosis." It is common in the higher plants.

Mitotic apparatus : It is called achromatic figure and is jointly formed by the centrioles, asters and the spindle body.

Significance of Mitosis

(1) It is an equational division, which maintains equal distribution of the chromosomes after each cell cycle. (2) The resulting daughter cells inherit identical chromosomal material (hereditary material) both in quantity (i.e., number) and quality (i.e., genetic make up or characters). (3) Mitosis maintains a constant number of chromosomes in all body cells of an organism. (4) It helps to maintain the equilibrium in the amount of DNA and RNA contents of a cell, as well as the nuclear and cytoplasmic balance in the cell. (5) Dead cells are replaced by newly formed cells through mitosis. It thus helps in the repair of the body. (6) It helps asexual reproduction, growth and development of organisms.

SUMMARY - MITOSIS

(1) It can take place in haploid as well as diploid cells. (2) Both the daughter cells formed through mitosis receive similar characters and number of chromosomes as that of the mother cell. (3) The original structure of the chromosomes remains unchanged in both the daughter nuclei. (4) Hence, it is an equational division and the resulting daughter cells are identical qualitatively and quantitatively.


Meiosis

In the sexually reproducing organisms, two important phenomena regulate the number of chromosomes in the life cycle. These are meiosis and fertilization. Meiosis is the reduction division in which the diploid (2n) number of chromosomes is reduced to haploid (n) during gamete formation (or spore formation). Whereas, in fertilization, the two haploid gametes fuse to form a diploid zygote. In this way, the diploid condition is restored again in the life cycle.

Meiosis-I (M-I, First meiotic division)

Definition : "Meiosis is a special type of division characteristic of reproductive cells in which the diploid number of chromosomes is reduced to haploid in the daughter cells. In meiosis, chromosomes divide once while the nucleus (and in some cases the cytoplasm also) divides twice. Four haploid daughter cells result from one diploid mother cell. These differ from each other as well as from the mother cell."

Homologous chromosomes : Sexually reproducing diploid organism develops from a diploid zygote (2n). The zygote is formed when haploid (n) male and haploid (n) female gametes fuse at the time of fertilization. Thus, the diploid individual receives two sets of chromosomes; one through the male gamete (paternal set) and the other through the female gamete (maternal set). For every chromosome in the paternal set, there is a similar looking chromosome present in the maternal set. Such similar chromosomes from paternal and maternal sets called homologous chromosomes. In this way, in both the sets, all chromosomes have their homologues.

The meiotic cell cycle : It consists of the interphase, karyokinesis and cytokinesis.

Interphase : It consists of G1, S and G2 phases and involves changes as described earlier in this chapter.

The interphase nucleus : The nucleus enlarges during interphase. The chromosomes are not clearly visible being very thin and long threads. However, nucleus envelope and the nuclear are prominent.

Karyokinesis in meiosis : It consists of two complete nuclear divisions : Meiosis-I and Meiosis-II. The time interval between M-I and M-II is called interkinesis.

It is a reduction division during which a diploid nucleus divides into two haploid nuclei through separation of homologous chromosomes. Chromosomes are not duplicated in M-I.

Various stages in Meiosis--I

The various events in M-I are classified into Prophase-I, Metaphase-I, Anaphase-I and Telophase-I. The important features in each phase are described in brief.

Prophase-I : This is the longest phase in meiosis and involves some very important events. Prophase-I is sub-divided into five stages (a) Leptotene (b) Zygotene (c) Pachytene (d) Diplotene and (e) Diakinesis.

(a) Leptotene (Leptonema) : Chromosomes become visible as long slender threads bearing numerous bead-like nucleosomes (chromomeres). These are arranged in a linear fashion along their lengths. The nuclear envelope and the nucleolus are prominently visible. The thin chromosomes are scattered in the nucleus.

(b) Zygotene (Zygonema) : This phase is characterized by the pairing of homologous chromosomes.

The homologous chromosomes (one paternal and the other maternal) from the two sets are attracted towards each other and form pairs. In each pair, the two homologues lie parallel to each other all along their lengths. This pairing is called synapsis (or syndesis or synizesis). During zygotene, chromosomes become shorter, thicker and more distinct.

(c) Pachytene (Pachynema) : This is the most important stage in meiosis in which a recombination of characters (genes) takes place through a phenomenon called crossing over.

The chromosomes become shorter, thicker and more distinct. Each chromosome has two sister chromatids joined by a centromere. Thus, each pair of homologous chromosomes at this stage consists of four chromatids (tetrad) and is called a bivalent. The paternal and maternal chromatids in each homologous pair are non-sister to one another. The non-sister chromatids are twisted round each other in relational coiling and take part in the crossing over.

Crossing over : Crossing over is an important genetic phenomenon. It takes place between any two non-sister chromatids of a homologous pair. Crossing over consists of a mutual exchange of equal quantity (segments) of chromosomal material between two non-sister chromatids. It involves the following events: (a) The relationally coiled non-sister chromatids, which are taking part in crossing over, break simultaneously at the identical points (i.e. at homologous points); (b) the broken segments are of equal lengths; (c) the segments again join with the chromatids; (d) however, there may be an exchange of the segments between the non-sister chromatids, i.e. the maternal segment may join with the paternal chromatid and the paternal segment may join with the maternal chromatid. This is called crossing over; (e) In this process, the genes located on the segments are exchanged between the two chromatids. (f) Thus, crossing over results in the recombination of genes (characters); (g) Crossing over does not take place between sister chromatids.

Significance of recombination (crossing over ) : (i) The gametes produced through meiosis receive a new combination of characters (genes). (ii) Therefore individuals with new combination of characters are produced in each generation. (iii) This forms the genetic basis for variations and plays important role in evolution.

Crossing Over

d) Diplotene (Diplonema) : Two important events begin during diplotene; (I) the repulsion of homologous chromosomes and (II) terminalization.

I. Repulsion : In each pair, the homologous chromosomes start repelling each other. As a result they begin to separate and uncoil. However, the non-sister chromatids involved in the cross-overs are held together at the points of crossing over. Each such points is called a chiasma where the separating chromatids form a cross-like #(x) figure. A homologous pair can show one or more chiasmata.

II. Terminalization : The separation and uncoiling of the homologues begins at the centromeres and proceeds towards the ends. This causes progressive shifting of the chiasmata towards the ends of the chromatids. This is called terminalization of chiasma.

(e) Diakinesis : This is the last phase of Prophase-I. Chromosomes are still in the pairs and in contact with each other by terminal chiasma. The chromosomes become shorter, thicker and more prominent.

By the end of the Prophase-I, the nucleolus and nuclear envelope disappear completely and the pairs of chromosomes are seen distributed in the nucleoplasm.

Metaphase - I : There is formation of a bipolar spindle body. It has two types of spindle fibers; (a) Continuous fibers extending from pole to pole and (b) Chromosomal fibers extending from pole to the equator of the spindle body. The homologous chromosomes, still in pairs, move towards the center of the spindle. These are arranged along the equatorial plane in such a way that the maternal homologues of all the pairs are facing one pole while the paternal homologues are facing the opposite pole. The chromosomal fibers connect the centromeres of all the homologues to the pole on their respective side. Each chromosome has only one centromere.

Various stages in Meiosis - II

 Simultaneous cytokinesis by cleavage constriction

Centriole and aster : In animals, a centriole is present outside the nucleus. It divides into two during prophase-I and later on helps in the organization of the bipolar spindle as well as the asters (as in mitosis).

Anaphase-I : The homologous chromosomes are pulled away from each other and finally separate completely (terminalization is completed). In other words, the sets of maternal and paternal chromosomes separate (segregate) and start moving towards opposite poles. This is due to the shortening of the chromosomal fibers.

Telophase-I : The two sets reach the opposite poles. The chromosomes, each with two chromatids and one centromere, become thin and long. A nucleolus is organized. Each nucleus is haploid as it has received only one set of chromosomes. Hence M-I is called reduction division.

Interkinesis : The time interval between M-I and M-II is called interkinesis.

Meiosis-II (M-II or Second meiotic division)

Second meiotic division is similar to mitosis i.e. it is an equational division in which there is duplication of the chromosomes. The two nuclei formed after M-I divide during M-II and produce four haploid nuclei.

The various events in M-II are classified into Prophase-II, Metaphase-II, Anaphase-II and Telophase-II. Both the nuclei divide simultaneously and all the changes during each phase are similar in both.

Prophase-II : The chromosomes again become shorter, thicker and distinct. Each chromosome has two sister chromatids joined by a centromere. The nuclear envelope and the nucleolus disappear by the end of Prophase-II.

Metaphase-II : The bipolar spindle body is formed. Chromosomes are arranged along the equators in such a way that their chromatids are facing the opposite poles. The centromere of each chromosome is connected with both the poles by chromosomal fibers.

In animals, centriole is present outside the nucleus and participates in the formation of bipolar spindle and asters.

Anaphase-II : During early anaphase-II, the centromere of each chromosome divides longitudinally into two. Therefore each chromosome is divided into two halves (chromatids) or daughter chromosomes.

During late anaphase-II, the two sets of daughter chromosomes are pulled away from each other and move towards the opposite poles.

Telophase-II : The sets of chromosomes reach the opposite poles and a new nucleus is organized at each pole. In all, four daughter nuclei are formed. Each nucleus has half the number of chromosomes as compared to the original mother nucleus. These nuclei also differ from each other in the structure and characters of chromosomes. This is because of the crossing over during prophase-I.

Cytokinesis : This is the division of the cell cytoplasm. It follows the nuclear division and may be successive or simultaneous.

Comparison Between Mitosis and Meiosis

MITOSIS

MEIOSIS

1.

Occurs in somatic cells.

1.

Occurs in reproductive cells.

2.

Consists of only one nuclear division.

2.

Consists of two nuclear divisions M-I and M-II.

3.

Cytokinesis takes place only once.

3.

May take place only once (simultaneous type) or twice (successive type).

4.

Involves division of chromosomes.

4.

Involves separation of homologous chromosomes in M-I and division of chromosomes in M-II.

5.

Dividing cells can be haploid or diploid.

5.

Dividing cells are diploid.

6.

Does not involve either pairing of homologous chromosomes or crossing over.

6.

Pairing of homologous chromosomes and crossing over occur during Prophase-I.

7.

Two daughter cells are formed.

7.

Four daughter cells are formed.

8.

Number of chromosomes present in the mother cell is maintained in both the daughter cells. Therefore it is an equational division.

8.

Diploid number of chromosomes is reduced to haploid in each daughter cell. Therefore it is a reduction division.

9.

Original characters of the chromosomes are maintained in the daughter cells.

9.

Chromosomal characters are altered due to "crossing over" causing recombination of genes.

10.

Daughter cells are similar to each other and also to the original mother cell.

10.

Daughter cells differ from each other as well as from the original mother cell.

11.

Helps in growth and body repairs.

11.

Helps in the sexual reproduction and regulation of chromosome number in the life cycle of sexually reproducing organism.