Virchow (1885) pointed out that new cells are always formed through the division of pre–existing cells and investigated the process of cell division.
: The sequence of phases in cell cycle is G1, S, G2 and M phase. It means the life cycle of a cell involves two distinct phases
(1) Interphase (non–dividing period) : Biologists divide interphase into three distinct periods on the basis of synthetic activities
(a) Post–mitotic or first growth period (G1–period) : The young daughter cell grows in size during this period. Its duration is most variable and the non–dividing cells remain permanently in this stage.
(b) Synthetic period (S–period) : It is characterised by the replication of DNA. A diploid cell during this phase has double the amount of DNA (i.e. 4n). Histones are also synthesized so that two chromatids are formed in each chromosome. Time taken in 30-50% of cell cycle.
(c) Premitotic or second growth period (G2 period) : It is characterised by increased nuclear volume. During its certain metabolic activities occur as a prerequisite of cell division. Time taken is 10-20%.
(2) Mitotic Phase : Separated into two categories – The nuclear division or karyokinesis, and the division of the cytoplasm or cytokinesis.
Type of Cell division :
Cell division is following three types –
(1) Amitosis (Gr. Amitos = without thread) – In this type of cell division the nucleus elongates and is constricted into two equal daughter. Discovered by Remak and also called direct cell division eg. Protozoa, bacteria.
(2) Mitosis (Gr. mitos = thread) The study of mitosis is done in the root tip of onion. The time taken in active mitosis is from ten minutes to some hours. Mitotic division or mitosis is the division of a somatic cell into two, maintaining the same number of chromosomes. It is divided into Karyokinesis and Cytokinesis.
(a) Karyokinesis : The process of karyokinesis includes the division of nucleus into two daughter nuclei. The nucleus undergoes a number of complex but regular and well organized steps, so that the process is separated into phases or stages – prophase, metaphase, anaphase and telophase.
(b) Cytokinesis in animal cells : In case of animal cells the cell cytoplasm divides by constriction. Cytokinesis by constriction also occurs in certain plant cells and in some protists.
(c) Cytokinesis in plant cells : In plant cells, cytokinesis is accomplished by the formation of phragmoplast from carbohydrate and lipid containing vesicles of Golgi and ER vesicles form Cell plate at the equator of the dividing cell is formed.
(3) Meiosis : Meiosis is a specialised and much complicated type of cell division, occurring only in the diploid reproductive cells and results in the formation of haploid sex cells or gametes. The term meiosis was coined by Moore and Farmer (1905). The gametes formed as a result of meiosis possess half the number of chromosome as found in the parent cells. The cells undergoing meiosis are known as meiocytes. In animals, the meiocytes are the primary spermatocytes and primary oocytes present in the gonads while in plants these are represented by sporocytes found in sporogonia.
Process of Meiosis
In meiosis two complete cell divisions follow in close sequence with or without a short interphase between them. The first meiotic division is known as reduction division or heterotypic division. The second division is known as homotypic division and is a simple mitotic division in which the two haploid cells formed as a result of heterotypic division, divide again forming four daughter cells.
First prophase : This is the longest phase of meiosis, it is divided into five stages –
- Chromosomes are long thread like with chromomeres on it.
- Volume of nucleus increases.
- Chromatin network has half chromosomes from male and half from female parent.
- Chromosome with similar structure are known as homologous chromosomes.
- Leptonemal chromosomes have a definite polarization and form loops whose ends are attached to the nuclear envelope at points near the centrioles, contained within an aster. Such peculiar arrangement is termed as bouquet.
- M. (Electron microscope) reveals that chromosomes are composed of paired chromatids. A dense proteinaceous filament or axial core lies within the groove between the sister chromatids of each chromosome.
- Chromosomes condense and get shortened.
- Homologous chromosomes make pairs.
- The pairing is called synapsis.
- The main component of synaptonemal complex is protein.This synaptonemal complex was discovered by Moses.
- Chromosomes become thick and short.
- Each chromosome pair splits longitudinally into 4 chromatids. This is called a bivalent or tetrad.
- Each tetrad has four kinetochore (two sister and two homologous).
- Non sister chromatids of bivalent show exchange or segments at molecular level.
- Synaptonemal complex helps in keeping the pairs stable.,
- Exchange of segments is called crossing over.
- Electron-Microscopy (EM) reveals a number of electron dense bodies about 100 nm in diameter at irregular intervals within the centre of the synaptonemal complex. These structure are called recombination nodules.
- At this stage the paired chromosomes begin to separate.
- Cross is formed at the place of crossing over between non-sister chromatids.
- Homologous chromosomes move apart they remain attached to one another at specific points called chiasmata.
- At least one chiasma is formed in each bivalent.
- Chromosomes are attached only at the place of chiasmata.
- Chromatin bridges are formed in place of synaptonemal complex on chiasmata.
- This stage remains as such for long time.
- In some spermatocytes and oocytes the diplotene chromosome disperse in a particular configuration.
- Chiasmata moves towards the ends of chromosomes. This is called terminalization.
- Chromatids remain attached at the place of chiasma only.
- Nuclear membrane and nucleolus degenerates.
- Chromosome recondense and tetrad moves to the metaphase plate.
- Chromosomes come on the equator.
- Due to repulsive force the chromosome segments get exchanged at the chiasmata.
- At this stage the arms are directed towards the equator and centromeres towards the poles.
- Homologous chromosomes move towards different poles after exchanging some segments due to crossing over.
- Each chromosome has two chromatids undivided and attached at the centromere.
- Two daughter nuclei are formed but the chromosome number is half than the chromosome number of mother cell.
- Nuclear membrane reappears.
- After telophase I cytokinesis may or may not occur.
- At the end of Meiosis I either two daughter cells will be formed or a cell may have two daughter nuclei.
- Meiosis I is also termed as reduction division.
- After meiosis Ist, the cells in animals are reformed as secondary spermatocytes or secondary oocytes; with haploid number of chromosomes but diploid amount DNA.
- Meiosis II starts just after the end of Telophase I.
- Each daughter cell (nucleus) undergoes mitotic division.
- Meiosis II is similar to mitosis.
- The various stages of meiosis II are prophase II, Metaphase II, Anaphase II and Telophase II.
- At the end of Meiosis II cytokinesis takes place.
- Four daughter cells are formed after the completion of one meiotic division.
- The chromosome number of daughter cells is haploid.
- Meiosis II is termed as equational division.
- In Meiosis I the kinetochores of homologous chromosomes are separated while in Meiosis II the sister kinetochores of one chromosome are separated.
- The four daughter cells receive one chromatid each of the tetravalent.
Significance of Meiosis
- Gametes have half the number of chromosomes than the mother cell.
- This stage is important for sexual reproduction as two gametes fuse to form a zygote.
- If the chromosome number is not reduced than after every fusion the number of chromosomes doubled in a zygote. So it is necessary that before the formation of zygote the reduction division should take place to form the gametes.
- Sexual reproduction includes one meiosis and one fusion.
- Exchange of segments give rise to new varieties.
- This process introduce genetic variations.
- The four daughter cells will have different types of chromatids.