Chapter 5

Cell Division

Chapter 5 Cell Division 5.1Mitosis-Cell Cycle-Stages of Mitosis-Cytokinesis animal cell, plant cell-Effect of Uncontrolled Mitosis- Applications of mitosis:Cloning

5.2Meiosis- Significance of Meiosis-Stages of Meiosis I, Meiosis II-Compare and Contrast

Cell cycle= life cycle of a celldescribes a series of events that occur from the time a cell is formed, until it divides.

Cell cycle24 hoursInterphase 23 hoursMitotic phase 1 hour(M phase)

G1 G1 = Gap 1 =Growth phase 1 Duplication of organelles

Synthesis of cytoplasm for growthG2 = Gap 2=Growth phase 2S phase= Synthesis phase Replication of DNA occurs

Replicated chromosomes consists of 2 sister chromatids

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CELL CYLE

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CELL CYLE

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G1 G1 = Gap 1 =Growth phase 1 Duplication of organelles

Synthesis of cytoplasm for growthG2 = Gap 2=Growth phase 2S phase= Synthesis phase Replication of DNA occurs

Replicated chromosomes consists of 2 sister chromatids Synthesis of enzymes & proteins required for cell division.Cell accumulates energy for cell division.

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CELL CYLE

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Cell Cycle

Cell DivisionM Phase / Mitotic PhaseNuclear divisionCytoplasmic divisionMitosis Meiosis 2 stages2 typesCytokinesis orSomatic cells / Body cellsReproductive cells / Gametes 1cell 2 cells1cell 4 cells

Definitions Nucleic acids, DNAChromosome, Homologous chromosomesChromatid, Sister chromatidChromatin, ChromatidCentromere, Centrioles

Chromosomal NumberChromosomal number refers to the number of chromosomes present in the nucleus of each cell of the species concerned.

e.g. onion cell has 16 chromosomes fruit fly has 8 chromosomes humans have 46 chromosomes

Human cell has 46 chromosomes (23 pairs )

Chromosomal Number

Diploid (2n)Haploid (n)= 2 sets of homologouschromosomes in a nucleus of a cell.= 1 set of unpaired chromosomes.Somatic cell / Body cell- 1 set inherited from each parent.Reproductive cell / Gamete- egg, spermExample: Human2n = 23 pairs = 46Example: Humann = 23

Chromosome Nucleus contains chromosomes

Each chromosome consists of DNA that carries the genes.

Genes determine the individual characteristics of an organism.

Structure of Chromosome

Before duplicationAfter duplicationCentromere

Chromatid

Chromatin = DNA + Histone proteinTextbook pg87 - Photograph 5.4

Sister chromatids

Egg cell (n)Sperm (n)Fertilization Zygote (2n)MitosisFoetus(2n)Somatic cell / Body cell(2n)Gamete / Reproductive cell (n)MitosisMeiosis24Life Cycle of a Diploid (2n) Organism2 sets of chromosomesHaploid = 1 set of chromosomes

5.1Mitosis

Importance / Significanceof MitosisTo increase the number of cells for growthTo replace the dead cells & repair damaged tissuesRegeneration of lost organ

4 stages of MITOSISProphaseMetaphaseAnaphaseTelophase

Prophase Chromatins become shortened, thickened, condensed, tightly coiled.Centrioles move to the opposite pole & spindle fibers are formed in the cytoplasm.Chromatids attach to the spindle fibers by their centromeres. *in plant cell, spindle fibers formed without the presence of centrioles.Nuclear membrane & nucleolus disappear.

Metaphase Spindle fibers attach to the centromeres of the chromosomes.

Chromosomes line up on the metaphase plate.

Anaphase Spindle fibers shorten / contract & pull the sister chromatids apart.Sister chromatids separate at the centromere.Chromatids / daughter chromosomes move to the opposite poles of the cell.

Telophase Two identical sets of chromosomes arrive the opposite poles of the cell.Chromosomes start to uncoil & revert to their extended state (chromatin).Spindle fibers disappear.Nuclear membrane forms around each set of chromosome.Nucleolus reform in each nucleus.

Cytokinesis Animal cell

Actin filaments in the cytoplasm contracts to pull a ring of plasma membrane inwards, forming a cleavage furrow.

Cleavage furrow pinches at the equator & deepens progressively until the cell is separated into two daughter cells.Plant cellTextbook pg 89 - Draw Figure 5.3

Vesicles gather at the equator between 2 nuclei & fuse to form cell plate.

Cell plates fuse to form new cell walls & plasma membranes.

2 daughter cells are formed.*Cellulose fibers are produced by the cell to strengthen the cell walls

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Importance of controlled MitosisTo obtain identical set of chromosomes as the parent cell for proper functioning of an organism.

Rate & timing of cell division in animals & plants are important for normal growth, development & maintenance.

Control system within cell ensures the cell division is complete & the cell divides in a controlled manner.

Effects of uncontrolled MitosisCan produce cancerous cells / malignant cells.Cancerous cells divide uncontrollably to form tumour (abnormal mass of cells which can invade & destroy neighboring cells).

Controlled growthUncontrolled growthA single organised layerMulti-layered &disorganisedCells are differentiated and carry out specialised functionsCells are undifferentiated and do not have specialised functionsThe nuclei and number of chromosomes are normalThe nuclei and number of chromosomes are abnormal

Application of MitosisCloningTissue cultureGenetic engineering (Biotechnology)

1. Cloning Cloning = a process of producing clones (new individual which is genetically identical to the parent) through asexual reproduction.

Many unicellular organisms & plants can reproduce asexually by mitosis to produce clones.

Animal cloning Dolly the sheep1997, the first mammal clone named Dolly was successfully cloned by a team of scientist headed by Ian Wilmut from Scotland.

How to clone Dolly ?Nucleus is removed from a somatic cell from the udder of a white faced sheep. Nucleus is removed from an unfertilised ovum of an egg donor (black-faced sheep).

Electric shock is used to fuse the nucleus & the denucleated ovum.

The cell divides by mitosis, forming an embryo.

The embryo is then implanted into the uterus of a surrogate mother.

The surrogate mother gives birth to a white-faced lamb.

Tissue Culture Technique

2. Tissue Culture = another technique of cloningTissue culture = the growth of tissues of living organisms in a suitable & sterile culture medium, containing nutrients and growth hormones.

Isolate an explant (small pieces of plant tissues) from the parent plant.Use enzymes to digest the cell wall to produce naked cells (cells without cell wall = protoplasts).Transfer the explant to a sterile culture medium and culture environment (incubator without sunlight) for growth. Tissues of explant divide by mitosis to form a callus (a group of undifferentiated mass of tissues).Callus develops into embryos & later into plantlet.Transfer the explant to soil for it to grow into adult plant.

3. Genetic engineering = Modify / Alter & engineer the genes of an organism Purposes: To increase the crop yield.To improve the quality of crops.To produce new species.To ensure uniformity in the traits of the plants.

Transgenic organism=GMO (Genetically modified organism)=organism that carries foreign gene/(s) that bring different / new traits.

e.g. enhanced growth, resistant to herbicides, pests, diseases

Cloning

Advantages Disadvantages Many clones are produced in a short time.No genetic variation & Prevent natural selectionGood qualities of plants / animals can be selected & maintained in clones.Same level of resistance towards pests & diseases all clones may die & extinctCan be carried out any time of the year. (without pollination & not seasonal).Disrupt the natural equilibrium of an ecosystem.Increases the rate of production & quality of products.Long-term side effects of using transgenic organisms.To ensure continuity of hereditary traits from parent to the clones.Cloned animals have shorter life span.

5.2Meiosis

Importance of MeiosisTo produce haploid gametes (reproductive cells)

To maintain the same chromosomal number in offspring from one generation to the next.

To ensure the offspring inherit traits from both parents

To create variation

To ensure the continuation of life (sexual reproduction)

Meiosis Textbook pg94

Meiosis IMeiosis IIProphase IProphase IIMetaphase IMetaphase IIAnaphase IAnaphase IITelophase ITelophase II

Prophase Icrossing-over

Comparison

Mitosis Meiosis Produce 2 diploid daughter cells from 1 parent cellProduce 4 haploid daughter cells from 1 parent cellDaughter cells have the same chromosomal number as the parent cellDaughter cells have half of the chromosomal number of the parent cellDaughter cells are genetically identical to the parent cellDaughter cells have variationInvolves 1 nuclear divisionInvolves 2 nuclear divisionsCytokinesis occurs onceCytokinesis occurs twice

Comparison

Meiosis IMeiosis IIProphase IProphase IIMetaphase IMetaphase IIAnaphase IAnaphase IITelophase ITelophase II

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