Cell cycle and cell division.pptx
- 1. CELL CYCLE ANDCELL DIVISION DR. C. SANKARANARAYANAN
- 2. INTRODUCTION • Living matter– able to reproduce • Unicellular – duplication – cell division – enlarge to get original size • multicellular- - zygote – cell division - growth and development – adult. • History: • Prevost and Dumas – 1824 – cleavage of zygote • Remak – 1841 - new cells from pre-existing cells • Rudolf Virchow – 1855, 1859 – cell lineage theory
- 3. • Strasburger –1873 – two nuclei develop from pre- existing one • Boveri and Flemming – 1879-80 – somatic cell division • Flemming – 1882 – coined the term ‘MITOSIS’ • Braur, Sutton, Van-Benden, Strasburger and Winiwater – 1887-1900 – studied meiosis • Farmer and Moore – 1905 – coined the term ‘MEIOSIS’ • Gregorie – Meiosis I and Meiosis II • Howard and Pele – 1953 – Cell Cycle • Montegomery – coined term ‘synapsis’ • Montose J. Moses – 1955 – Synaptonemal complex
- 4. • Cell cycle-> sequence of growth and division -> formation of cell from parent cell -> division -> daughter cell -> cell cycle • Generation time -> period between two divisions -> depends upon various factors. • Generation time of E.coli - 20 minutes • Epithelial cells – 8 to 10 hours • Onion root tip meristems – 20 hours
- 5. • Phases ofcell cycle: • 1. long undividing state – interphase • 2. short phase of nuclear division, Multiplication or M phase
- 6. • Interphase: • Preparingfor mitosis • Doubling of DNA, studied using autoradiography with labelled thymidine • Synthesis of DNA in synthetic period or S phase • Followed by two “gaps” – G1 and G2 • Not a resting period • Intense metabolism • DNA uncoiled , visible as chromatin • Volume of nucleus increases • Nucleolus maximum size
- 7. • More ATPform from biomolecules • Molecules needed stored • Period 10 – 20 hours – 75 to 90% of generation time • Damaged DNA repaired • FISH can be done – Fluoresccence in situ hybridization. Fluorophore, sulfoindocyasin is used to visualize DNA
- 8. • Starting point– gap1 (G1) active, make cell components needed for cell progeny • Next S phase – synthesis of DNA and proteins like spindle fibres • Next gap 2 (G2) – additional proteins, no DNA synthesis • No DNA synthesis in G1 and G2.
- 9. • M. phase- Multiplication phase – actual cell division • Karyokinesis • Stages – Prophase, metaphase, anaphase, telophase and cytokinesis • Then daughter cell enters G1 or Go phase • G1 phase is also called antephase • Go phase –discovered by Lajtha – 1963 • If DNA is damaged P53 protein block cell cycle leads to cell death or apoptosis
- 12. Cell Division • Formationof daughter cell from parent cell • Continuity from one generation to another • Hereditary materials copied • In sex cells division daughter cell may differ • Prokaryotic cells divide by binary fission • Single DNA – replicate – separated to cells – identical – except mutation • Multi cellular organisms replace worn out cells by cell division • Cell division halts – senescence – deterioration – death • Telomere degeneration – cannot protect chromosomes • Cancer cells – telomere allows continuous division
- 13. • Amitosis: • Directcell division • No equal division of nucleus, no differentiation of chromosomes and spindle • Nuclear membrane does not disappear • Nucleus elongate and divide by constriction • Centripetal constriction of cytoplasm • Examples: Protozoa, mammalian cartilage, bacteria • Unequal distribution may leads to structural and functional abnormalities
- 15. Mitosis • Means –‘thread of fibres’ in Greek • Formation of identical cells, cellular xerox • Fleming coined the term • Phases: • Interphase: period between two mitosis, resting and preparatory phase • Replication of DNA, synthesis of nuclear proteins, formation of centriole, synthesis of ATP, formation of spindle • Duplication of chromosome – monads to dyad
- 17. • Prophase: • Cellbecome spheroid • Increase in viscosity, and refractivity • Chromatin reticulum disappears • Shortening and thickening of chromosome fibers • Outside the nucleus two centrosomes formed by microtubules of tubilin protein • Due to repulsive interaction of tubules centrosomes push to the opposite ends of the cell • In animal cells and fungi fibrils form like a spokes of a wheel called aster
- 18. • Centriole participate– centric mitosis • Plants no centriole – acentric mitosis • Aster formation in animals and fungi –astral mitosis • No aster – an-astral mitosis • Spindle form between two poles • Microtubules number 16 in yeast and 5000 in higher plants • Two types of centrioles – central spindle for centrosome division during early prophase and metaphasic centrioles for anaphasic movement of chromosomes
- 19. • Disappearance ofnuclear membrane and no differentiation of cytoplasm and nucleoplasm • Each chromosome form two kinetochore at the centromere, one attached to each chromatid for attachment of microtubules and formation of spindle fibres
- 23. • Metaphase: • Chromosomesreaches central equatorial plane • Smaller chromosomes at the center and larger chromosomes at the periphery • Centriole lie at the poles • Continuous spindle fibres form fro pole to pole, not attached to chromosomes, form stem-korper or pushing body and help in separation • Chromosomal fibres connect kinetochore and pole, pull chromosomes • Inter-zonal fibres connect centromere and pole, pull chromosomes • Bringing chromosomes on the equator of the spindles is called congression • Spindle – 90-95% tubulin, 3-5% RNA and traces of actin, myosin and lipid
- 25. • Anaphase: • Separationof centromere • Pulling of chromosomes to poles by spindle • At the end two groups of chromosomes with equal number form at the pole equal to parental cell
- 27. • Telophase: • Reversalof prophase • Two nuclei form and nuclear membrane appear • Spindle fibres and aster disappear • Cytokinesis: • Animal cell cleavage furrow form to become two cells • Plant cell - cellplate – cell wall and middle lamella form to become two cells
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- 30. MEIOSIS • “Meio” meansto lessen • Farmer and Moore 1905 coined the term • First studied by Van Benedin – 1887 • Strasburger – 1888, Sutton - !900, Winiwater – 1900. • Gregoire – Meiosis I and Meioisis II • Diploid cell become haploid cells
- 31. • Haploid gametesform from diploid cell • During sexual reproduction in eukaryotes. • Male and female gametes fuse to form zygote • One of the main reasons for variations • Chromosome number constant • Variations are the basis of evolution
- 32. • Types: • 1.Zygotic life cycle: Initial Meiosis • Haploid organism -> male and female gametes fusion - > Zygote ( diploid) -> meiosis -> four haploid cells -> new organism. Eg: Bryophytes • 2. Gametic life cycle: Terminal Meiosis • Organism diploid -> germ cells -> meiosis -> gametes -> fusion of gametes -> zygote -> organism Eg: Human beings • 3. Sporic Meiosis: Diplohaplontic, intermediate meiosis • Diploid sporophyte -> meiosis -> micro and megaspores -> male and female gametes by mitosis -> Zygote after fusion -> embryo -> that is meiosis between zygote and gamete
- 33. • Phases ofmeiosis: • Meiosis I: • Prophase I: • Leptotene : slender tread of chromatin become chromosomes after condensation in nucleus • It looks like thread of beads – chromomeres • Irregularly arranged or towards centriole forming a “ bouquet”. It is due to chromosome attachment to nuclear membrane. • In plant cells form tangle of threads – “Synezetic knot”
- 34. • Zygotene: Matingthread • Chromosome become shorter and thickened • Pairing of homologous chromosomes “ Synapsis” or “Syndesis” or Synizesis” • Pairing from centromere to ends – Procentric or from end to centre – Proterminal pairing • Combined homologous chromosomes – bivalent. • It is also called tetrad because of four sister chromatids.
- 35. • Synaptonemal complexformed during zygotene. • It is a ribbon like structure formed between pairing homologous chromosomes. • It has central and lateral elements – LC fibres central fibres connects chromosomes and lateral elements connects nuclear membrane
- 36. • Pachytene: Thickthread • Longest stage • It begins when synapsis is completed • Chromosome as bivalent or tetrad, that is each bivalent with four chromatids • Each pair of chromatids attached by a centromere called sister chromatids • Chromatids of different pair called non-sister chromatids. • Breakage and reunion after interchange between them is called crossing over.
- 37. • Diplotene: Doublethread • Beginning of separation of paired homologous chromosomes • Held together at one or two points called chiasmata • Number of chiasmata varies depends on length • Terminal or intertidal chiasma • Chiasma displaced along the length – terminalization due to despiraling of chromosomes – expressed as terminalization coefficent {T} • Synaptimal complex disappear
- 38. • Diakinesis: • Chromosomesmore contracted • Bivalents move towards periphery • Homologous remain in contact by terminal chiasmata. • Nucleolus detached or disappear
- 41. • Metaphase I: • Chromosomes arranged at the centre • Spindle formed and attached to centromere of the two homologous chromosomes • Two centromeres of each bivalent lie on opposite sides of the equatorial plate • It differs from mitosis, that is each bivalent has two centromeres.
- 43. • Anaphase I: • Homologous chromosomes move towards poles • Due to crossing over separating chromosomes are different. • Centromere do not divide like mitosis
- 45. • Telophase I: • Centromere arrives pole • Each cell has half the number of chromosomes, each with two chromatids • Spindle fibres disappear • Nuclear membrane form • Chromosomes uncoil and become chromatin • It is followed by cytokinesis.
- 47. • Meiosis II: • Like mitosis, but DNA do not duplicate • Prophase II : • Nuclear membrane, nucleolus disappear. • Shortening and thickening of chromatids. • Centrioles move to the poles and spindle fibres arranged • New equatorial plane rotated by 90o, that is perpendicular to the previous plane
- 48. Metaphase II : •Like mitosis chromosomes oriented on the equatorial plate and attached with spindle fibres • Anaphase II : • Centromeres cleaved • Allow the kinetochore to pull the sister chromatids apart. • Now become sister chromosomes and pulled toward opposite poles.
- 49. • Telophase II: • Like telophase I uncoiling, lengthening and disappearance of chromosomes • Nuclear membrane form • Finally four haploid cells formed
- 51. • Non-disjunction : •Normal separation of chromosomes called disjunction. • If they are not properly separated leads to non- disjunction • Leads to more or less amount of genetic material • Example – trisomy or monosomy • Downs syndrome - trisomy of 21 chromosome • Klinefelder’s syndrome – one extra X cromosome
- 52. • Significance ofMeiosis: • 1. Sexual reproduction • 2. Genetic variation • 3. Mutation • 4. Genetic information for gametes or gametophytes and sporophytes.
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