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Second phase of growth, further synthesis of organelles occurs, chromosomes start condensing Synthesis of DNA, the DNA of the chromosomes is replicated Mitosis Nuclear division Cytokinesis Cytoplasmic division First phase of growth, the cell grows and develops, and additional organelles are synthesised, the nucleus directs protein synthesis. Interphase Centriole Chromosome is long and thin, cannot be seen under the light microscope Cell synthesizes more cytoplasm and organelles, chromosomes replicate, cell builds up large store of energy Nucleolus MITOSIS Prophase Chromosome Shortens and condenses, becomes visible under the light microscope Nucleolus Gradually disappears Centrioles move towards opposite poles of the cell Prophase Chromatid Centromere Prophase Centriole Microtubules extend from here towards the equator of the cell to form the spindle Nuclear membrane Begins to break down Sister chromatids Spindle pole Metaphase Spindle fibre Spindle equator Centromere becomes attached to a spindle fibre and moves to the equatorial plane Chromosomes line up at the equator of the spindle Anaphase Daughter chromosome Centromere divides and the daughter chromosmes move to opposite poles with centromere leading Nuclear membrane Telophase Surrounds daughter chromosomes Chromosomes uncoil and gradually become invisible under light microscope Nucleolus reforms Division of the cytoplasm Animal cell Cell membrane invaginates at the mid point of the cell dividing the cell into two, cells are diploid (2N) and are identical to each other Plant cell Tiny vesicles containing cell wall materials coalesce to form the cell plate Cell plate extends outward to the cell wall and separates cell into two MEIOSIS Meiosis I Prophase I Pairing of homologous chromosomes maternal and paternal chromosomes come together and lie close to each other Bivalent Prophase I Sister chromatids separate and become visible, they are held together by the centromere Prophase I Crossing over occurs between chromatids of homologous chromosomes Chiasma Metaphase I Centromere becomes attached to individual spindle fibre Chromosomes lie at the equator of the spindle with members of each homologous pair facing opposite poles Anaphase I Members of each homologous pair move towards opposite poles Telophase I Nuclear membrane reforms around the chromosomes Cell divides into two Prophase II Nuclear membrane disappearing Metaphase II Chromosomes attached to spindle fibres by their centromere, line up at the equator of the spindle Anaphase II Centromeres divide and separate, pulling the sister chromatids to the opposite poles Telophase II Nuclear membrane forms around the chromosomes Interphase Four haploid (N) daughter cells are produced Comparison between mitosis and meiosis Mitosis Pairing of homologous chromosomes No Crossing over No Anaphase Number of divisions involved Daughter cells Occurrence Sister chromatids separate during anaphase One Two diploid (2N) daughter cells, they are identical During growth, asexual reproduction Meiosis Yes Yes Homologous chromosomes separate during anaphase I, sister chromatids separate during anaphase II. Two Four haploid (N) daughter cells, they are non-identical Production of gametes for sexual reproduction • Clegg C.J., Mackean D.G., Advanced Biology: principles and applications • Enger/Ross, Concepts in Biology CROSSING OVER - exchange of genetic material between non-sister chromatids Non-sister chromatids overlap Once overlap, they break Join with broken fragments of nonsister chromatid i.e. exchange of genetic material occurs Crossing over - Chiasma Chiasmata have two functions: (1) hold the homologous chromosomes together as they move to the equator of the spindle (2) allow exchange of genetic materials between homologous chromosomes giving rise to genetic variations in the offspring