the nucleus is the organising centre of the cell. The information in the nucleus is contained within structures called chromosomes. These uniquely:
(i) Control activities of the cell.
(ii) Genetic information copied from cell to cell while the cell divides.
Hereditary characters are passed onto new individuals when gametic cells fuse together in sexual reproduction.

One of the proteins synthesized only in the G₂ period is known as Maturation Promoting Factor (MPF). It brings about condensation of interphase chromosomes into the mitotic form.

Whichever division takes place, it is normally followed by division of the cytoplasm to form separate cells, called as cytokinesis.

In meiosis, the daughter cells contain half the number of chromosomes of the parent cell and is known as haploid state (n).

1. In the absence of meiosis the next generation would have double the number of chromosomes after fusion of gametes.
2. This would have resulted in the birth of an altogether new species or produce abnormal offsprings.
3. The maintenance of characters set would have been possible only through asexual reproduction.

1. Genetic stability - Daughter cells are genetically identical to parent cells.
2. Growth - As multicellular organisms grow, the number of cells making up their tissue increases. The new cells must be identical to the existing ones.
3. Repair of tissues - Damaged cells must be replaced by identical new cells by mitosis.

Metaphase: Chromosomes (two sister chromatids) are attached to the spindle fibres by kinetochore of the centromere. The spindle fibres is made up of tubulin. The alignment of chromosome into compact group at the equator of the cell is known as metaphase plate. This is the stage where the chromosome morphology can be easily studied.
Kinetochore is a DNA-Protein complex present in the centromere DNA where the microtubules are attached. It is a trilaminar disc like plate.
The spindle assembly checkpoint which decides the cell to enter anaphase.

Anaphase: Each chromosome split simultaneously and two daughter chromatids begins to migrate towards two opposite poles of a cell.
Each centromere splits longitudinally into two, freeing the two sister chromatids from each other. Shortening of spindle fibre and longitudinal splitting of centromere creates a pull which divides chromosome into two halves.
Each half receive two chromatids (that is sister chromatids are separated). When the sister chromatids separate the actual partitioning of the replicated genome is complete.

S.No Cytokinesis in plant cell Cytokinesis in animal cell 1 Cleavage cytokinesis occurs only in lower plants, higher plant cells show cell plate cytokinesis It occurs through cleavage 2 The middle part of spindle persists during cytokinesis. It forms a complex called phragmoplast Spindle degenerates during cytokinesis 3 A mid body is absent A mid body develops centrally 4 A row of vesicles develops in the equatorial plane in plant cell cytokinesis A row of vesicles does not form in animal cell cytokinesis 5 Vesicles fuse to form cell plate Cell plate is not formed 6 There is little role of microfilaments in plant cell cytokinesis Microfilaments actively involved in animal cell cytokinesis 7 It starts with the formation of cell plate during telophase It starts as a constriction in cell membrane during late anaphase or early telophase 8 The cell plate grows from the centre towards the lateral walls, so centrifugal The furrow depends from periphery towards the centre and meets at the centre so centripetal 9 Wall formation occurs in the region of cytokinesis Wall formation is absent

One of the proteins synthesized only in the G₂ period is known as Maturation Promoting Factor (MPF). It brings about condensation of interphase chromosomes into the mitotic form.

The replication of chromosomes in the absence of nuclear division and cytoplasmic division resulting in numerous copies within each cell is called endomitosis. Eg: Polytene chromosome.

The factors which promote cell cycle proliferation is called mitogen. Plant mitogens include gibberellin ethylene, Indole acetic acid, kinetin. These increase mitotic rate.

1.Chiasmata:
These are the points of crossing of two non-sister chromatids of homologous chromosomes during prophase I of meiosis.
2. Significance of Chiasmata:
The genetic constitution of the daughter cells differs from the parent cells due to crossing over. There is a mixture of paternal and maternal genes in areas of crossing over.

S.No	S-phase	G2 phase
1.	It is called synthetic phase.	It is called premitotic phase.
2.	Growth of the cell continues as Replication of DNA occurs,	Cell growth continues by protein and cell organelle synthesis, mitochondria and chloroplasts divide.
3.	DNA content increases from 2C to 4C	DNA content remains as 4C.

1. Longest part of the cell cycle, but it is of extremely variable length.
2. It consists of three phases, G₁, S, G₂ phase which is followed by cell division.