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cell division cell division Document Transcript

  • Cell division is theprocess by which aparent cell divides intotwo or more daughtercells.Cell division is usually asmall segment of alarger cell cycle.
  • Cell cycle is the periodfrom the time a newcell is produced untilthe cell completescell division.Cell cycle starts withinterphase and isfollowed by mitosisand cytokinesis.
  • Interphase accounts for about 90% of the cell cycle. This interphase is subdivided into three phases , that is i) G1 (growth phase 1) ii) S (synthesis phase)iii) G2 (growth phase 2)
  • 1. Proteins and new cytoplasm organelles such as mitochondria and chloroplasts are synthesised during this stage.2. The metabolic rate of the cell is high.3. The chromosomes are not condensed and appear as thread –like structure called chromatin. Interphase
  • 1. Synthesis of DNA occurs during S phase.2. DNA undergoes replication.3. Each duplicated chromosome consists of two identical sister chromatids. Interphase
  • 1. The cell continues to grow and remain metabolically active.2. The cell accomulate energy and completes its final preparation for cell division. Interphase
  • Cell division starts with nuclear division (karyokinesis) and is followed by cytoplasm division (cytokinesis). Nuclear division can be divided into two major parts:a) mitosisb) meiosis
  • Is cell division that produces daughter cells with the same number of chromosomes as the parent cell. Occurs in:a) meristems of plantsb) most somatic cells of animals
  • a) Produces new cells to replace cells that are worn out and damagedb) Produces new cells for growth and maintenance in living organisms
  • c) Mitosis is a form of asexual reproduction for Amoeba sp. and Hydra sp.d) Preserves the diploid chromosome numbers in somatic cells
  • Mitosis is divided into 4 phases:a) Prophaseb) Metaphasec) Anaphased) Telophase
  • 1. The chromosomes condense , become shorter and thicker , become visible clear under light microscope.2. Each chromosome now consists of pair of sister chromatids joined at centromere.
  • 3. Spindle fibre begin to form.4. At the end of the prophase , nucleus and nuclear membrane begin to disappear.
  • 1. Metaphase begin when the centromeres of all the chrmosomes lined up on the metaphase plate.2. The two sister chromatids are still attached to one another at the centromere.3. Metaphase end when the centromeres divide.
  • 1. During anaphase , the two sister chromatids of each chromosomes separate at the centromere.2. The two sister chromatids are pulled apart to the opposite poles by the shortening of the spindle fibres.
  • 3. Once separated , the chromatids are referred to as daughter chromosomes.4. At the end of anaphase , the two poles of the cell have complete and equivalent sets of chromosomes.
  • 1. Telophase begin when the sets of chromosomes reach the opposite poles of the cells.2. The chromosomes start to uncoil and appear as chromatines and become less visible under light microscope..
  • 3. The spindle fibres begin to disappear.4. The nuclear membrane and the nucleolus reforms in each nucleus.
  • Cytokinesis is the division ofthe cytoplasm occurs towardsthe end of the telophase.Through cytokinesis , thedaughter cells have all theorganelles , nutrients andother components needed tosurvive and maintainthemselves.
  • Actine filaments in thecytoplasm contracts topull a ring of the plasmamembrane inwards ,forming a groove calledcleavage furrow until thecell is separated in twodaughter cells.
  • In plant cell , membraneenclosed vesicles gather atplant cells equator betweenthe nuclei.
  • The vesicles fuse to form cell plate which divide the cell into two daughter cells.After cytokinesis , the new cellenter G1 stage of interphase ,complete the cell cycle.
  • Is the process of nucleardivision that reduces thenumber of chromosomes indaughter cells to half that ofthe parent cell.
  • Meiosis consists of two separate nuclear divisions:a) Meiosis I , which consists of prophase I , metaphase I , anaphase I and telophase Ib) Meiosis II , which consists of prophase II , metaphase II, anaphase II and telophase II
  • The chromosomes begin tocondense . They becomeshorter , thicker and clearlyvisible .Homologous chromosomescome together to form pairs ofbivalents through synapsis .
  • Each bivalent consists of atetrad . A tetrad consists oftwo homologouschromosomes , each of whichis made up of two sisterchromatids .Non-sister chromatidsexchange segments of DNA ina process of crossing over .Crossing over causes newcombination of genes on achromosomes .
  • The points at which segmentsof chromatids cross over arecalled chiasmata .At the end of Prophase I , thenucleolus and nuclearmembrane disappear .
  • The two pairs of centriolesmigrate to the oppositepoles of the cell . Each pairof centrioles acts as acentral point from whichthe spindle fibes will radiate.
  • The spindle fibres pull thetetrads to the middle of thecell.Pairs of homologouschromosomes alignthemselves at the metaphaseplate of the cell.
  • One chromosome of eachhomologous pair is attachedto fibres from one pole whileits homologue is attached tofibres from the opposite pole.
  • The spindle fibres pull thehomologous chromosomesapart from one another andmove them to the oppositepoles of the cell.Each chromosome stillconsists of two sisterchromatids.
  • Each number of thehomologous chromosomes isattached to spindle fibres.At the end of anaphase I ,each pole has only twochromosomes (each with twosister chromatids)
  • The chromosomes arrive atthe poles.Each pole now has haploiddaughter nucleus because itcontains only one set ofchromosomes.The spindle fibres disappear.
  • The nuclear membranereappears to surroundeach group ofchromosomes.The nucleolus thenreappears in eachnucleus .
  • The nuclear membranedisintegrates.The spindle fibres re-form in each daughtercell .
  • The chromosomes arepositioned randomly at themetaphase plate with thesister chromatids of eachchromosome pointingtowards opposite poles.Each sister chromatids isattached to the spindle fibresat the centromere.
  • The centromeres of the sisterchromatids separate .The sister chromatids of eachchromosome are nowindividual chromosomes .Each individual chromosomemoves towards the oppositepoles of the cell .
  • Finally , the nucleoli andnuclear membranes re-form .The spindle fibres break downCytokinesis follows and fourhaploid daughter cells areformed. Each haploid cellcontains half the number ofchromosomes and isgenetically different from theparent diploid cell . Thesehaploid cells becomegametes.