CLASS – 11CHAPTER – 10 CELL CYCLE AND CELL DIVISION
- Strasburger similarly proposed that nuclei are formed from pre-existing ones.
- The sequence of events which occur between the formation of a cell and its division into daughter cells is called cell cycle.
- Cell cycle: The sequence of events by which a cell duplicates its genome, synthesis the other constituents of the cell and eventually divides into two daughter cells is termed as cell cycle.
- Cell cycle consists of two main stages:   
1. Interphase        a. G₁ Phase        b. S Phase        c. G₂ Phase   
2. M-Phase (Mitosis Phase)        A. Karyokinesis        B. Cytokinesis

Parts of Cell Cycle	Phase	Events within the cell
1. Interphase (L. inter = between; Gr. phases = aspect) - It is the most active stage in the cell cycle, when the cell prepares for the division phase. - It is the longest period of cell cycle.	G1 Phase (Gap – I phase/ First growth phase/ First gap period/ Post-mitotic phase/ Pre-synthetic phase) - Pre-DNA synthesis phase	1. Cell metabolically active (high metabolic rate) and grows continuously. 2. Intensive synthesis of cellular components, i.e., E.R., chloroplast, mitochondria, lysosome, etc. 3. Structural and functional proteins are synthesized. 4. RNA, ATP and ribosomes are synthesized. 5. Initiation of DNA replication.
	S Phase (Synthetic phase) - DNA synthesis phase	1. Replication of DNA takes place and the amount of nucleic acid doubles. 2. DNA content increases from 2C to 4C.but the number of chromosomes remains some (2N). 3. Histone proteins are synthesized.
	G2 Phase (Gap – II Phase/ Second growth phase/ Second gap period/ Post-synthetic phase/ Pre-mitotic phase) - Post-DNA synthesis phase	1. Cell volume increases. 2. Cell contains two times the amount of DNA present than in the original diploid cell. 3. Mitochondria and chloroplasts divide. 4. Centrioles get doubled. 5. More RNAs, ATP and proteins are synthesized in preparation for mitosis while cell growth continues.
2. M-Phase (Mitotic phase/ D-phase) - This phase represents the phase when the actual cell division/ mitosis occurs. - It is the phase when the replicated DNA is distributed to the daughter nuclei in a complicated series of events. - It involves 2 stages: A. Karyokinesis B. Cytokinesis	A. Karyokinesis (Division of nucleus) - It corresponds to the separation of the daughter chromosomes into two daughter nuclei, i.e., it is the process of division of nucleus.
	B. Cytokinesis (Division of cytoplasm) - It corresponds to the division of cytoplasm and the separation of the two daughter nuclei into two daughter cells.

- Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. - It is the process of formation of new or daughter cells from pre-existing or parent cells. - Cell division is a highly co-ordinated process. Basically, it involves three major steps:      a. Replication of DNA (genome)     b. Division of nucleus (karyokinesis)      c. Division of cytoplasm (cytokinesis) - There are three major types of cell division:A. Amitosis/ Direct cell division/ Incipient cell division (Gr. amitos = without thread; osis = state): - It is simplest type of cell division in which there is no synthesis of spindle, no condensation of chromatic fibres and no disappearing of nuclear membrane. - It is found in:    - Prokaryotes – bacteria    - Cyanobacteria    - Yeast    - Special eukaryotic cells – Amoeba, cartilage cells, meganucleus of Paramecium, foetal membrane cells, endosperm cells of seeds, diseased cells and old tissues.B. Mitosis: Mitosis results in increase in the number of cells without any change in the chromosome number.C. Meiosis: Besides increase in cell number, meiosis results in reduction of chromosome number to half. - The basic stage in both the types of divisions are almost identical but the results are completely different.(Somatic cell division/ Equational division/ Indirect cell division) - Discovered in plant cells by Strasburger (1875) - Discovered in animal cells (in Salamander) by W.Flemming (1879) - Term mitosis was given by W.Flemming (1882) - Mitosis occurs in:    - Somatic cells (during growth)    - Germ cells of the gonads (during multiplicative phase of gametogenesis)    - Plants: In meristematic cells (e.g., root apex & shoot apex) & cambial cells    - Human: In stratum germinativum of skin, bone marrow, during embryonic development, etc.    - Haploid cells (e.g., lower plants and drones of honey bee)    - Diploid cells (e.g., higher plants and most of animals) - Mitosis consists of following main stages:        A. Karyokinesis             a. Prophase             b. Metaphse             c. Anaphase             d. Telophase        B. Cytokinesis

Mitosis / Equational division/ Indirect cell division) - It is also known as equational division as the number of chromosomes in the parent cell and the daughter cells is the same. - It is the phase when the replicated DNA is distributed to the daughter nuclei in a complicated series of events. - It involves 2 stages: A. Karyokinesis B. Cytokinesis
A. Karyokinesis (Division of nucleus) - Karyokinesis in mitosis is completed in four stages.
	a. Prophase (Gr. pro= first; phasis = stage)	- Replicated chromosomes, each consisting of 2 chromatids, condense and become visible. (Uncoiling of chromatid) - Nucleolus and nuclear envelope disappear. - Formation of spindle. - Centriole moves to opposite poles. - Microtubuless are assembled into mitotic spindle. - It is the stage of longest duration.
	b. Metaphse (Gr. meta = after; phasis = stage)	- Spindle fibres attached to kinetochores (small disc-shaped structures at the surface of centromers) of chromosomes. - Chromosomes line up at the equator of the spindle to form metaphase plate (Arrangement of chromosomes on equatorial plate.).
	c. Anaphase (Gr. ana = up; phasis = stage)	- Centromeres split and chromatids separate. - Chromatids move to opposite poles.
	d. Telophase (Gr. telo = end; phasis = stage)	- Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. - Nuclear envelope assembles around the chromosome clusters (Formation of daughter nuclei). - Nucleolus, Golgi complex and ER reform.
B. Cytokinesis (Division of cytoplasm) - It is the process of division of cytoplasm, where the two daughter nuclei become separated into two daughter cells. - The process differs in animal and plant cell.	- Equal distribution of organelles and cytoplasm into each of the two daughter cells. - Formation of cell plate (In plant cell) or furrow (In animal cell). - Animal cytokinesis: Appearance of furrow in plasma membrane which deepens and joins in the centre dividing cell cytoplasm into two. - Plant cytokinesis: Formation of new cell wall begins with the formation of a simple precursor − cell plate which represents the middle lamella between the walls of two adjacent cells.

 Differences between Mitosis in Plant and Animal Cells

	Plant Cell	Animal Cell
1	Occurs mostly in the meristematic region.	It occurs in tissues throughout the body.
2	Centrioles generally absent.	Centrioles are generally absent.
3	Asters are not formed (anastral mitosis).	Asters are formed (astral or amphiastral mitosis).
4	Cytokinesis by cell plate formation.	Cytokinesis by furrowing.

 Significance of Mitosis:- The growth (addition of cells) of multicellular organisms is due to mitosis. - In unicellular organisms, mitosis is involved in asexual reproduction (multiplication of cells). - In plants, vegetative propagation involves only mitotic divisions and genetically identical individuals are produced. - Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life. - In multicellular organisms, body growth is by mitotic divisions of the cells. - Replacement of worn out tissues/ cells (e.g., blood cells, skin cells) and repair of injured tissues is by mitosis. - Uncontrolled cell divisions in certain tissues/ organs result in tumors (tumours) or cancer. - Maintenance of surface/volume ratio. - Maintenance of chromosome number. - Regeneration. - Mitosis usually results in the production of diploid daughter cells with identical genetic complement.(Gk. Meioum or meio = to lessen/ reduce, osis = state) - It is a special type of cell division in which the chromosomes duplicate only once, but cell divides twice. So one parental cell produces 4 daughter cells; each having half the chromosome number and DNA amount than normal parental cell. So meiosis is also called reductional division. - It is found in special cell types and at specific period. - It is reported in diploid germ cells of sex organs of animals:      - Primary spermatocytes of testes to form male gamete called spermatozoa      - Primary oocytes of ovaries to form female gamete called ova - It is reported in flowers of plants:      - Pollen mother cells (microsporocytes) of anther      - Megasporocyte of ovule of ovary to form the haploid spores. - It occurs during gametogenesis in plants and animals. - Involves  two  sequential  cycles  of  nuclear cell  division  called Meiosis I and Meiosis II. - Interphase occurs prior to meiosis which is similar to interphase of mitosis except the S phase is prolonged. - 4 haploid daughter cells are formed. - Meiosis consists of following main stages:1. Meiosis I           A. Karyokinesis             a. Prophase I                 i. Leptotene                 ii. Zygotene                 iii. Pachytene                 iv. Diplotene                 v. Diakinesis             b. Metaphse I             c. Anaphase I             d. Telophase I     B. Cytokinesis2. Interkinesis3. Meiosis II          A. Karyokinesis         a. Prophase II         b. Metaphse II         c. Anaphase II         d. Telophase II     B. Cytokinesis

Steps in Meiosis	Phases and Events in Meiosis Cell Division
1. Meiosis I (Heterotypic/ Reductional division)	A. Karyokinesis - It consists of 4 main stages.	a. Prophases I - It consists of complex events and is divided into five stages.	i. Leptotene/ Leptonema (Gk. leptos – slender, tainia – band, nema - thread) - Condensation and coiling of chromatin fibres lead to formation of distinct chromosomes, which appear as fine single threads of beads. - As they contract and become thicker, their dual nature is seen.
			ii. Zygotene/ Zygonema (Gk. zygon – yoke, tainia - band) - The homologous chromosomes start pairing and this process of association is called synapsis. - Chromosomal synapsis is accompanied by formation of synaptonemal complex. - Complex formed by a pair of synapsed homologous chromosomes is called bivalent or tetrad.
			iii. Pachytene/ Pachynema (Gk. pachys – thick, tainia - band) - The two sister chromatids of each chromosome become visible and the bivalent becomes a tetrad (with 4 chromatids). - Recombination nodules, the sites of crossing over appear. - Crossing over occurs between non-sister chromatids of homologous chromosomes. - Crossing over is catalysed by an enzyme, recombinase.
			iv. Diplotene/ Diplonema/ Double threaded stage (Gk. diplos – double, tainia - band) - It is of longest duration. - Diplotene is marked by the dissolution of synaptonemal complex and the recombined chromosomes start separating from each other except at the sites of crossing over, i.e.,chiasmata. - These X-shaped structures are called chaismata.
			v. Diakinesis (Gk. dia – through, kinesis - movement) - The chiasmata move to the tip (terminalisation) and may slip off or remain at the tips. - Chromosomes are fully condensed and meiotic spindles assembled. - Nucleolus disappears and nuclear envelope breaks down - Diakinesis represents the transition to metaphase I.
		b. Metaphase I - The bivalent (tetrads) are arranged at the equatorial plate. - The centromeres are aligned in two rows. - Microtubules from opposite poles of the spindle attach to the pair of homologous chromosomes.
		c. Anaphase I - The members of every homologous pair of chromosomes separate from each other and start moving to the opposite poles of the spindle while chromatids remain associated at their centromeres. - The centromeres do not split. - The number of chromosomes becomes half the number of chromosomes of the parent cell.
		d. Telophase I - The chromosomes have reached the poles. - A nuclear membrane forms around the chromosome clusters at each pole i.e., nuclear membrane and nucleolus reappear. - Cytokinesis follows (diad of cells).
	B. Cytokinesis - Cytokinesis generally follows the first nuclear division. - Two daughter cells (dyads) are formed.
2. Interkinesis	- It is a stage between two meiotic divisions (Meiosis I and meiosis II). - It is similar to a short interphase between meiosis I and meiosis II. - There is no replication of DNA. - Sometimes, this phase may be absent.
3. Meiosis II (Homotypic/ Equational Division) - It is similar to mitosis and it is also an equational division.	A. Karyokinesis	a. Prophase II - The sister chromatids of each chromosome start condensing and become shorter. - Nuclear membrane disappears.
		b. Metaphase II - The bipolar spindle is completely formed. - The chromosomes become arranged at the equator. - Microtubules from opposite poles of spindle get attached to kinetochores of sister chromatids.
		c. Anaphase II - The centromere of each chromosome divides into two simultaneously. - The sister chromatids separate and move to the opposite poles as daughter chromosomes.
		d. Telophase II - The chromosomes have reached the poles. - They start uncoiling and become thin. - The nuclear membrane and nucleolus start reappearing to form daughter nuclei.
	B. Cytokinesis - The nuclei are separated into two cells by cytokinesis. - Each daughter cell (dyad) of meiosis 1 divides to form two daughter cells and hence at the end of meiosis of a diploid cell (meiocyte) four haploid daughter cells are formed.

 Significance of Meiosis:- Formation of gametes: In sexually reproducing organisms.- Genetic variability: It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution. The crossing over results in variation of genetic characters in the offspring.- Maintenance of chromosomal number: By reducing the chromosome number in gametes. Chromosomal number is restored by fertilisation of gametes. - It is an alkaloid widely used in plant breeding for doubling the chromosome number. - Colchicine is extracted from the corms Autumn Crocus (Colchicum autumnale). - It is called “mitotic poison” because it does not allow the formation of spindle because it prevents assembly of microtubules. - The chemical does not inhibit chromosome replication. - As a result, the colchicine treated meristematic cells show doubling of chromosomes. - This property of increasing the number of chromosome sets or genomes is called polyploidy. - Polyploidy provides:    i- New variety and species. (e.g., Potato)- Growth and reproduction are characteristics of cells, indeed of all living organisms.- Rudolf Virchow (1855, 1859) – omnis cellula e cellula (every cell is derived from a cell).CELL CYCLEQuiescent stage (G₀): State of arrested condition- These cells that do not divide further exit G₁ phase to enter an inactive stage called quiescent stage (G₀) of the cell cycle.- Cells that do not divide and exit G₁ phase to enter an inactive stage called G₀.
- Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.Cell Division (Cell reproduction/ Cell multiplication)
- The process by which new cells are formed from the pre-existing cells, is called cell division.MITOSIS (Gr. mitos = thread; osis = state)
- The apical meristem of root i.e., root tips, especially onion root tips are excellent material for the study of mitotic division.- Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.- The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced.- Asexual reproduction in many plant and animal species involves mitosis.MEIOSIS (Heterotypic/ Reductional division)
- Meiosis is a type of specialized cell division where the number of chromosomes is reduced to half in the daughter cells; hence it is also known as reduction division.Colchicine   ii- Vigorous offspring.