The cell cycle Cytokinesis Mitosis Second phase of growth, further synthesis of organelles occurs, chromosomes start condensing Nuclear division Cytokinesis Cytoplasmic division First phase of growth, the cell grows and develops, and additional organelles are synthesised, the nucleus directs protein synthesis. Synthesis of DNA, the DNA of the chromosomes is replicated

Interphase Centriole Chromosome Nucleolus 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

Nuclear division MITOSIS

Prophase Chromosome Nucleolus Shortens and condenses, becomes visible under the light microscope Nucleolus Gradually disappears

Prophase Centrioles move towards opposite poles of the cell Chromatid Centromere

Prophase Centriole Nuclear membrane Sister chromatids Microtubules extend from here towards the equator of the cell to form the spindle Nuclear membrane Begins to break down Sister chromatids

Metaphase Spindle pole Spindle fibre Spindle equator Centromere becomes attached to a spindle fibre and moves to the equatorial plane Spindle equator 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

Telophase Nuclear membrane Surrounds daughter chromosomes Chromosomes uncoil and gradually become invisible under light microscope Nucleolus reforms

CYTOKINESIS 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

Nuclear division 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 Pairing of homologous chromosomes No Yes Crossing over No Yes Anaphase Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I, sister chromatids separate during anaphase II. Number of divisions involved One Two Daughter cells Two diploid (2N) daughter cells, they are identical Four haploid (N) daughter cells, they are non-identical Occurrence During growth, asexual reproduction Production of gametes for sexual reproduction

References 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 Join with broken fragments of non-sister chromatid i.e. exchange of genetic material occurs Non-sister chromatids overlap Once overlap, they break

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