Cell division, a new way.
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Cell division is the process by which a parent cell divides into two or more daughter cells. There are two main types of cell division: mitosis and meiosis. Mitosis produces two identical daughter cells during growth and repair of the body. It ensures the genetic makeup remains the same. Meiosis produces gametes with half the number of chromosomes and involves two cell divisions. It results in genetic diversity that is important for sexual reproduction.
Cell division, a new way.
- 1. By- Dr. Saurabh Anand Thawrani Guided by- Dr. R.S.
- 2. Contents • Introduction • Types of cells • Types of cell divison • Cell cycle • Mitosis • Meiosis Meiosis I Meiosis II • Applied aspects
- 3. Introduction What is it? The process by which a parent cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division: a vegetative division, whereby each daughter cell is genetically identical to the parent cell (mitosis), and a reproductive cell division,
- 4. • Cell division is an inherent property of living organisms. It is a process in which cells reproduce their own kind. • The growth, differentiation, reproduction and repair take place through cell division. • Rudolf Virchow(1858) suggested ”Omnis cellula e cellula” means every cell is derived from pre existing cell. • There are two types of cell division namely Mitosis and Meiosis.
- 5. • The cell (from Latin cella, meaning "small room") • The basic structural, functional, and biological unit of all known living organisms. • A cell is the smallest unit of life that can replicate independently, and cells are often called the "building blocks of life". • Prokaryote is a single-celled organism that lacks a membrane- bound nucleus (karyon), mitochondria, or any other membrane-bound organelle.
- 6. Types of cell division
- 7. Cell cycle •The cell cycle is the sequence of events or changes that occur between the formation of cell and its division into daughter cells. •It has a nondividing, growing phase called Interphase and dividing phase called mitotic or M-phase.
- 9. Interphase ( inter – between, phases – aspect) :metabolically active phase • Longest phase in the cell cycle • Cell grows in size • Doubles cytoplasmic organelles • Duplicates chromosomes • Three stages: G1, S, G2 1. G1: cell growth 2. S: synthesis of DNA (chromosome duplication) 3. G2: cell growth and preparation for division
- 10. G1 Phase or GAP 1 Phase : • Period in the cell cycle during interphase, after cytokinesis and before the S phase. • The cell is metaboically active, • There is great amount of protein and enzymes synthesis, • There is NO DNA REPLICATION S-Phase or Synthesis Phase : • Period in the cell cycle during interphase, between G1 phase and the G2 phase. • DNA synthesis or replication occurs.
- 11. G2 Phase or GAP 2 Phase: • The final, and usually the shortest phase during interphase • Cell undergoes a period of rapid growth to prepare for M phase. • The nucleus is well defined, bound by a nuclear envelope and contains at least one nucleolus. • At the end of this phase is a control checkpoint (G2 checkpoint) to determine if the cell can proceed to enter M phase and divide. • G2 checkpoint prevents cells from entering mitosis with DNA damaged since the last division, providing an opportunity for DNA repair and stopping the proliferation of damaged cells so that the G2 checkpoint helps to maintain genomic stability G 0 Phase (Quiescent stage) : • Period in the cell cycle where cells do not divide further and exist in a quiescent state.
- 12. Mitotic or M-phase • The most dramatic period of the cell cycle, involving a major reorganisation of virtually all components of the cell. • Since the number of chromosomes in the parent and progeny cells is the same, it is also called as equational division. • It includes two important processes that occur simultaneously. Karyokinesis (division of the nucleus) Cytokinesis (division of the cytoplasm) • Resulting in two daughter cells.
- 13. Mitosis (Gr. Mitos – thread, osis – stage) • Walter Flemming (1882) studied mitotic cell division in animal cells and coined the term mitosis. • Mitosis is a type of cell division in which a parental cell produces two similar daughter cells that resemble the parental cell in terms of chromosomal number. Hence called Equational cell division (homotypic cell division). • This maintains constant number of chromosomes in each cell of successive generation. • It occurs in somatic cells of the body. Also called
- 14. Karyokinesis (Karyon – nucleus, kinesis – movement) The division of nuclear material. • Occurs in four stages 1. Prophase (Gr. Pro – before, phases – appearance) 2. Metaphase (Meta – after, phase – appearance) 3. Anaphase (ana – up, phases - appearance) 4. Telophase (Telo – end, phases – appearance)
- 15. Prophase (Gr. Pro – before, phases – appearance) • It is the longest phase. • The chromatin is organized into distinct chromosomes by coiling or spiralization. • The centrioles develop into asters and move towards the opposite poles of the cell to establish the plane of cell division. • Spindle apparatus begin to appear • Nucleolus and Nuclear membrane disintegrate and disappear • The chromosomes are set free in the
- 16. Centromere : It is the primary constriction in chromosome to which the spindle fibres attach during mitotic and meiotic division. It appears as a constriction when chromosomes contract during cell division. After chromosomal duplication, which occurs at the beginning of every mitotic and meiotic division, the two resultant chromatids are joined at the centromere. Spindle fibres : It is a group of microtubules that extend from the centromere of chromosomes to the poles of the spindle or from pole to pole in a dividing cell. Sister chromatids : During S phase of the cell cycle the DNA is replicated and an identical copy of the chromatid is made. These identical copy of chromatids are called sister chromatids.
- 18. Metaphase (Meta – after, phase – appearance) • Spindle fibres are completely formed • The chromosome become short and thick with two distinct chromatids each • All the chromosomes move towards the centre of the cell and arrange in the equatorial plane, right angles to the position of asters to form metaphasic plate • Chromosomes are attached to spindle fibres at their centromeres
- 19. Metaphase plate : The plane of the equator (a plane that is equally distant from the two spindle poles) of the spindle into which chromosomes are positioned during metaphase
- 21. Anaphase (ana – up, phases - appearance) • The centromere of all the chromosomes undergo longitudinal splitting and the chromatids of each chromosome separate to form daughter chromosomes • The daughter chromosomes move toward the opposite poles from the equator by the activity of spindle fibres
- 23. Telophase (Telo – end, phases – appearance) • During this, the events of prophase will be reversed • The daughter chromosomes reach the opposite poles • The chromosomes undergo despiralization to form long, thin thread like structures called chromatin • Nucleolus and nuclear membrane reappears • The spindle fibres disappear
- 25. Cytokinesis (cyto – cell, kinesis – movement) The Division of cytoplasm • A cleavage furrow develops in the middle of the cell in centripetal direction due to the contraction of microtubules. • It occurs till the edges of the plasma membrane meet. • They fuse to form a separate membrane.
- 26. Overview
- 27. Significance of Mitosis: • It maintains genetic stability within the population of cells derived from same parental cell • It helps the growth and tissue repair • It helps in the replacement of dead and worn out cells • It is a means of reproduction in
- 28. SUMMARY • Type of cell division which takes places during growth, repair and replacement. • Takes place in somatic cells. • Basis of asexual reproduction. • Daughter cells identical in chromosome number and genetic make-up at the parent cell. • Ensures consistency of genetic information in nuclei of cells of one individual. • Chromosomes don’t arrange themselves along equator in homologous pairs during metaphase.
- 30. • Derived from the Greek word meaning 'lessening'. • The term meiosis (originally spelled "maiosis") was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905 • This is a special method of cell division, occurring in maturation of the sex cells, by means of which each daughter nucleus receives half the number of chromosomes characteristic of the somatic cells of the species.
- 31. • Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase. • Meiosis involves pairing of homologous chromosomes and recombination between them. • Four haploid cells are formed at the end of meiosis II. • Homologous Chromosomes : Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during meiosis and contain the
- 32. Sequence Of Events Meiosis I (reductional division) Karyokinesis I Prophase I a) Leptotene b)Zygotene c)Pachytene d)Diplotene e)Diakinesis Metaphase I Anaphase I Telophase I Cytokinesis I Meiosis II (mitotic meiosis) Karyokinesis II Prophase II Metaphase II Anaphase II Telophase II Cytokinesis II
- 33. MEIOSIS I It is reductional division in which a diploid parental cell produces two haploid daughter cells. Hence it is called reductional division
- 34. Prophase I; longest phase (5 stages) a) Leptotene: (bouquet stage) • The chromatin condenses to form chromosomes • The chromosomes appear as long, thin and thread like structures. They undergo coiling and become short and thick. • Each chromosome has two chromatids that are not distinctly visible. • Each chromosome shows bead like structures called chromomeres. • The telomeric ends of all the chromosomes converge towards one side of nuclear membrane;
- 35. b) Zygotene (Zipper stage): • Pairing of homologous chromosomes takes place called synapsis. The pair is called bivalent. • The chromosome continues to undergo condensation and asters keep moving towards opposite poles. c) Pachytene (Tetrad stage): • The chromosomes become more short and thick • Each bivalent shows four chromatids called tetrad • In this stage the exchange of genetic material takes place between the non sister chromatids of homologous chromosomes. • This process is called genetic crossing over. • This results in genetic recombination which is
- 36. d) Diplotene: • The chiasma move towards the tips of chromosomes as the homologous chromosomes of bivalent start moving apart. This event is called Terminalisation e) Diakinesis: • The chromosomes at this stage appear thick, short and distinct • The tips of some chromosome show chiasma • Nucleolus and Nuclear membrane disappear
- 38. Synapsis: Pairing of the homologous chromosomes is called Synapsis Bivalent: A paired unit formed of homologous chromosomes consisting of a paternal and a maternal chromosome is called Bivalent Tetrad: Each homologous chromosome pair (bivalent) shows four chromatids called tetrad Crossing over: Exchange of identical parts between the nonsister chromatids of homologous chromosomes is called Crossing over. Chiasma: These are the regions on homologous
- 39. Metaphase-I • Chromosomes are arranged in the equatorial region with their centromeres towards the poles and arms towards the equator.
- 40. Anaphase-I • The Centromeres do not under go longitudinal splitting • The chromosome of each homologous pair move towards opposite poles by the activity of spindle fibres. This is called separation or disjunction of chromosomes.
- 41. Telophase-I • The homologous chromosomes separate and reach the opposite poles • The nuclear membrane reappears around the chromosomes at each pole • The spindle fibres disappear • Cytokinesis occurs
- 42. Cytokinesis 1- It is the division of cytoplasm. Interkinesis- The Interphase after the first meiotic division is called Interkinesis. It may be present or absent between meiosis-I and meiosis-II. If present it may be short or in some cases telophase-I directly enters to prophase-II. It is similar to Interphase except for the absence of replication of DNA.
- 43. Meiosis II Occurs soon after meiosis-I. There is no duplication of chromosomes. Hence called mitotic meiosis
- 44. Prophase-II • The chromosomes start condensing again • Spindle apparatus begin to appear • The nuclear envelope and nucleolus disintegrate and disappear
- 45. Metaphase-II • The Chromosomes arrange in the equatorial region at right angles to the asters • The Spindle fibres connect to the centromere.
- 46. Anaphase-II • The centromeres of all the chromosomes undergo longitudinal splitting. • The chromatids of each chromosome separate and they move towards opposite poles.
- 47. Telophase-II • The chromosomes arrive at the poles and undergo decondensation to become thin and long chromatin fibres. • A nuclear envelope is formed. • Nucleolus also appears • The spindle fibres disappear
- 50. Significance of Meiosis • It helps to restore diploidy and maintain the constant number of chromosomes for a species. • Meiosis produces new combination of chromosomes and genes by crossing over and by a random distribution of paternal and maternal chromosomes to daughter cells.
- 51. SUMMARY • Location - gonads. • Reduction division - results in halving number chromosomes. • Cell division involved in gamete production. • Four daughter cells formed. Male - all four develop. Female - three abort, one develops. • Two divisions occur. • Chiasma formation leading to crossing over occurs prophase I resulting in a mixing of genetic material of the chromosomes. • Metaphase I - bivalents arrange themselves on equator of cell. Results in further variation as random which may position themselves. • Anaphase I - chromosomes separate.
- 53. Applied aspects Defective Mitotic Division • It usually results in the production of uncontrolled growth of cells or defective cells. • Uncontrolled Mitosis • If the genes that regulate the cell cycle are damaged or mutated, cell divides abnormally or may lead to uncontrolled cell growth. • Cause may be due to Harmful ray: nuclear radiation, UV ray, Viruses, Carcinogenic chemicals: food additives, benzo (α) pyrene in
- 54. Micronuclei • Micronuclei are extranuclear cytoplasmic bodies. • They are induced in cells by numerous genotoxic agents that damage the chromosomes. • The damaged chromosomes, in the form of acentric chromatids or chromosome fragments, lag behind in anaphase when centric elements move towards the spindle poles. • After telophase, the undamaged chromosomes, as well as the centric fragments, give rise to regular daughter nuclei. The lagging elements are included in the daughter cells, too, but a considerable proportion is transformed into one or several secondary nuclei, which are, as a rule, much smaller than the principal nucleus and are therefore called micronuclei. • Bigger micronuclei result from exclusion of whole chromosome following damage to the spindle apparatus of the cell, whereas smaller micronuclei result from structural aberrations; causing chromosomal fragments.
- 56. Defective Meiotic Division • Defective meiotic divisions usually result in the production gametes with chromosomal numerical abnormalities • Down Syndrome - trisomy of chromosome 21 • Patau Syndrome - trisomy of chromosome 13 • Edward Syndrome - trisomy of chromosome 18 • Klinefelter Syndrome - extra X chromosomes in males - i.e. XXY, XXXY, XXXXY, etc. • Turner Syndrome - lacking of one X chromosome in females - i.e. X0
- 57. References • http://www.biologyreference.com/Gr-Hi/History-of- Biology-Cell-Theory-and-Cell- Structure.html#ixzz4FChB53a6 • Journal of Oral and Maxillo Facial Pathology Vol. 12 Issue 1 Jan - Jun 2008. Clinico-pathological correlation of micronuclei in oral squamous cell carcinoma by exfoliative cytology .Devendra H Palve, Jagdish V Tupkari. • Vander’s human physiology 8th edition • Kahel-frotcher- atlas of human anatomy 10TH edition. • Color Atlas of Cytology, Histology, and Microscopic Anatomy4th edition, revised and Enlarged
- 58. Thank You