The Cell Cycle and Mitosis: “You Complete Me” A process where one parent cell gives rise to two daughter cells- exact replicas of the original cell. Genome British Columbia, 2004 www.genomicseducation.ca

The Cell Cycle Every cell goes through a process of growth, this is called the cell cycle. New cells start at G1 or Gap 1, go on to an S phase (where DNA is synthesized in preparation of Mitosis), and then G2 or Gap 2 before Mitosis or Cell division. Some cells will no longer divide and they exit the cell cycle in G1(neurons, muscle cells, fat cells do this) As one cell enters Mitosis, two cells exit which are exact replicas or clones of the original “parent” cell. Purpose: to replenish dead or dying cells, to allow an organism to grow and develop

Interphase This phase encompasses all of the G1,S, and G2 phases of the cell cycle. The chromatin is diffuse. It may not look like much is going on here, but there is a lot of activity because the cell must prepare for Mitosis: protein synthesis, DNA synthesis, replication of other cellular structures too. Onion root tip (on left side), whitefish (on right side)

Mitosis Mitosis occurs in order for organisms to grow and develop. In order to replenish dead or dying cells such as skin cells, cells in the digestive tract and in the fertilized egg. There are 4 main phases: Prophase, Metaphase, Anaphase, Telophase. Cytokinesis (division of the cytoplasm) follows and one cell becomes two.

Mitosis: Prophase Major processes during this phase: Chromosomes condense and form visible bodies (DNA was replicated in S phase) Nuclear envelope breaks down Onion root tip (top photo), whitefish (bottom photo)

Mitosis: Metaphase Chromosomes attached to spindle fibers line up in the middle (the equator) of the cell Spindle fibers attach to the centromeres and other places along the chromosome Onion root tip (top photo), whitefish (bottom photo)

Mitosis: Anaphase The last bit of DNA at the centromere replicates to allow the the centromere to split The sister chromatids separate and are pulled to opposite sides of the cell Onion root tip (top photo), whitefish (bottom photo)

Mitosis: Telophase Chromosomes now uncoil Nuclear envelope reappears and surrounds the chromosomes Cytokinesis The cytoplasm and all its contents are divided between the 2 daughter cells (cytoplasmic division) The red arrow points at the newly developing plasma (cell) membrane that creates the 2 new daughter cells The 2 daughter cells are exact replicas of the original parent cell – they are clones and have the exact same genetic make-up as the parent cell. Onion root tip (top photo), whitefish (bottom photo)

Can you identify these stages? B D C A. Telophase B. Prophase C. Cytokinesis D. Metaphase Answers:

Summary One parent cell begins the journey through the cell cycle and 2 new identical daughter cells emerge from one cycle. Mitosis occurs regularly in tissues that require new cells continuously such as skin cells, hair follicles, the lining of the digestive tract and in a newly fertilized egg. This process is important as it allows for an organism to grow and develop over its lifespan and replenishes dead or dying cells in certain tissues.

Acknowledgements: Images taken from the following sites: http://biog-101-104.bio.cornell.edu/BioG101_104/tutorials/cell_division.html http://www.accessexcellence.org/AB/GG/mitosis2.html http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/cells3.html