The Cell Cycle

Chromosomes DNA is packaged into chromosomes during cellular division Number of chromosomes varies for every species Humans have 46 in somatic cells; 23 in gametes

Chromosomes Chromosomes made of chromatin (DNA and protein) Each chromosome made of one long linear DNA molecule with hundreds or thousands of genes Proteins help to maintain structure of chromosome and control gene activities

The Cell Cycle The cell cycle consists of: Interphase G1, S, G2 Mitosis Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis Most time is spent in interphase

The Cell Cycle

Interphase Growth occurs and proteins and cytoplasmic organelles are produced throughout interphase The synthesis or replication of DNA only occurs in the S phase

Prophase Chromatin condenses into chromosomes. Each with a sister chromatid joined at the centromere. Nucleoli disappears Mitotic spindle forms Centrosomes begin to move away from each other.

Prometaphase Nuclear envelope fragments Microtubules extending from centrosomes can now enter nuclear area Each chromatid now has a kinetochore at the centromere Some microtubules attach to kinetochores; other nonkinetochore microtubules interact with other spindles

Metaphase Centrosomes at opposite poles Centromeres/chromosomes line up at metaphase plate Each kinetochore is attached to a microtubule (with sister chromatids attached to opposite sides)

Anaphase Shortest stage of mitosis Cohesin proteins in sister chromatids are cleaved Each chromosome begins movement to opposite pole as kinetochore microtubules shorten Nonkinetochore microtubules lengthen causing the cell to elongate

Telophase 2 daughter nuclei form Nuclear envelopes arise Nucleoli reappear Chromosomes become less condensed Mitosis (division of the nucleus) is complete

Cytokinesis Division of cytoplasm begins in telophase In animal cells, cytokinesis involves a cleavage furrow which pinches the cell into two; plant cells use a cell plate which eventually becomes the cell wall

Binary Fission Asexual reproduction of single celled eukaryotes and prokaryotes Eukaryotic binary division involves mitosis, but prokaryotic binary fission does not

Control of the Cell Cycle Checkpoints within the cell cycle control when particular stages will pause or when a cell will move to another phase Checkpoints generally check to make sure that appropriate functions have occurred prior to moving forward

Control of the Cell Cycle Protein kinases are enzymes that activate or inactivate other proteins by phosphorylation Give the go ahead in G1 and G2 checkpoints Cyclin is a protein whose concentration varies throughout the cell cycle Attaches to cyclin dependent kinases to activate them (such as MPF)

Control of the Cell Cycle Signals inside and outside the cell also play a role in cell division Anaphase will not occur until all kinetochores are attached to spindle fibers Cells will not divide in the absence of essential nutrients or, for most mammal cells, growth factors specific to the cell type Density-dependent inhibition – crowded cells stop dividing (usually a single layer) Anchorage dependence – cells must be attached to a substratum to divide

Loss of Control Some cells do not abide by the controls set forth by the cell cycle and become cancer cells. Normal cells divide 20-50 times before apoptosis, but cancer cells continue dividing indefinitely

Loss of Control Normal cells undergo transformation to become cancerous. Normally these are killed by the immune system, but they may escape detection. If these cells remain at the original site, they are considered a benign tumor; however, if the cells begin to spread and metastasize, this is malignant

http://www. biology. arizona http://www.biology.arizona.edu/cell_bio/activities/cell_cycle/assignment.html http://www.cellsalive.com/mitosis.htm