2 Cell division by mitosis Needed for:reproduction in unicellular organismsgrowth in multicellular organismsreplacement of damaged or dying cellsrepair of damaged organs or tissuesInvolves:replication of DNAcontrolled division of the nuclear material so new each cell has the same DNAdivision of cytoplasm and organelles so each new cell has similar contents
5 Interphase Phase of growth and normal metabolism Nucleus and nucleolus visibleDNA replication occurs just before cell division occurs
6 Prophase First stage of division Nucleus and nucleolus disappear Spindle starts to formDNA condenses and becomes visible as chromosomesCentrioles move to opposite poles (animal cells)
7 MetaphaseChromosomes line up on spindle at the equator of the cell
8 Anaphase Chromatids pull apart at the centromere One chromatid from each chromosome moves to each end of the cell
9 Telophase Chromosomes uncondense and a new nucleus forms around them The cytoplasm splits (cytokinesis) to form two new cells identical to the original except smaller
10 Interphase Phase of growth and normal metabolism Nucleus and nucleolus visible
11 Gametogenesis – cell division by meiosis Sexual reproduction involve the formation of gametes, which are then fertilised to produce a new individual.Gametes are made by a process called gametogenesis.Gametogenesis involves a special type of division called meiosis.In meiosis, the number of chromosomes is halved, so that when gametes unite in fertilisation, they have the correct number of chromosomes.Gametes are called haploid cell as they have only half the number of chromosomes as normal (diploid) cells.Female gametes are made in the ovaries and are called ova.Male gametes are made in the testes and are called sperm.In most species gametes then undergo a period of maturation before they are capable of fertilisation.
13 Meiosis 1 – the first division prophase Ithe chromosomes become visible - each chromosome contains two chromatidshomologous chromosomes move together & a spindle formsmetaphase Ihomologous pairs of chromosomes line up on the equator, attached by their centromeresanaphase Ione of each pair of homologous chromosomes is pulled to either end of the celltelophase Ithe chromosomes condensecytoplasm is split of and cell divides into two daughter cells, each with 23 chromosomeseach chromosome consists of two chromatids
14 Meiosis 2 – the second division prophase IIthe chromosomes become visible - each chromosome contains two chromatidsa new spindle forms, at right angles to the old onemetaphase IIchromosomes line up on the equator, attached by their centromeresanaphase IIone of each chromatid in each chromosome is pulled to either end of the celltelophase IIthe chromosomes condensecytoplasm is split of and each daughter cell divides into two (ie total of 4 daughter cells), each with 23 chromosomes - each chromosome consists of one chromatidGametes
15 Comparing mitosis and meiosis Where it occursBody cellsGonads (reproductive organs)Why it occursCell repair, growth, asexual divisionSexual reproductionNumber of cells produced24Number of divisions1Number of chromosomes in daughter cellsSame as parent (diploid)Half that of parent (haploid)Amount of variation in daughter cellsNoneLots
16 Comparing mitosis and meiosis 2 Advantages for reproductionSimpleRapid divisionAllows variationDisadvantages for reproductionNo variationMore complexSlower reproduction
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