Mitosis and Meiosis

Cell life cycle and Mitosis Interphase: the state in which a cell will spend most of its functional life. When preparing to divide interphase can be split into the G1; S and G2 phases. G0 is the stage when cells perform normal cell functions some cells stay in G0 indefinitely, e.g. skeletal muscle cells and neurons. 

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G1: the cell will produce enough cell material for 2 cells, such as endoplasmic reticulum, ribosome, Golgi apparatus and cytosol. Centriole replication begins S: Duplication of chromosomes, involves synthesis of histones. G2: Lasts only 2-5 hours, devoted to last minute protein synthesis and completion of centriole replication before Mitosis.

Stage 1 of mitosis: Prophase 

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The chromosomes condense (coil tightly) and become visible. The chromatids are connected at the centromere. At the centromere 2 kinetochores reside, one for each chromatid. Kinetochores are the protein structures that spindle fibres attach to. The replicated centrioles move to opposite poles. An array of microtubules (the spindle fibres) extends between the centriole pairs. The nuclear envelope disappears The kinetochore of each chromatid attaches to a spindle fibre to make a chromosomal microtubule As the chromosomes appear the nucleoli disappear ( the disappearance happens at late prophase)

Stage2: Metaphase 

The chromatids move to the narrow called the metaphase plate

3: Anaphase

central zone

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The centromere of each chromatid splits separate Daughter chromosomes are pulled to ends of the cell along the chromosomal microtubules

and chromatids opposite

Stage 4: Telophase   

Cells prepare to return to interphase, nuclear membranes form Nuclei enlarge, nucleoli reappear and the chromosomes The fine filaments of the chromatin become visible {Mass of genetic material composed of DNA and proteins that condense to form chromosomes during eukaryotic cell division. Chromatin is located in the nucleus of a cell.}

Cytokinesis: the cytoplasmic division of daughter cells.

Meiosis The outcome of meiosis is the production of 4 daughter cells. 1. Prophase 1: some things are similar to mitosis: the chromosomes duplicate and the chromatids condense to become visible. But now we have synapsis occurring. Synapsis is the pairing of homologous chromosomes, when they synapse genetic recombination (crossing-over) occurs, this is to increase genetic variation. A tetrad is formed when 4 chromatids are linked. At the end of prophase 1 the nuclear envelope will disappear. 2. Metaphase 1: The tetrads line up across the metaphase plate 3. Anaphase 1: The tetrads break up- separating the maternal and paternal chromosomes. 4. Telophase 1: ends with the formation of 2 daughter cells containing unique combinations of maternal and paternal chromosomes. The first meiotic division is called reductional division because the number of chromosomes has been reduced from 46 to 23. Meiosis ii ( basically the same as mitosis): 1. Prophase 2: the nuclear envelope disappears, the centrioles move to polar regions 2. Metaphase 2: the chromatids line up at the metaphase plate, the centrioles attach spindle fibres onto the kinetochores. 3. Anaphase 2: the chromatids are pulled apart and move to opposite regions 4. Telophase 2: ends with the production of 4 daughter cells containing 23 chromosomes (equational division as the no. Of chromosomes is the same as before division)