Cell Cycle and Mitosis Cell Division part 2

The Cell Cycle: life of a cell from first formation (from a dividing parent cell) to its own division into 2 cells. Before a cell divides, it must copy its genome, which is all of its genetic information. 2 types of Cells Somatic Cells : all body cells except gametes These have the diploid number of chromosomes (2n) Somatic cells divide by mitosis Making exact copies Gametes : sperm and egg cells Are haploid (n), have ½ the number of chromosomes Gametic cells are produced by meiosis Make genetically unique haploid cells

Parts of the Cells Cycle There are three main stages during a cell cycle: Parts of the Cells Cycle Interphase G1 - growth S – DNA Replication G2 – replicate organelles, more growth Mitosis Prophase, Metaphase, Anaphase, Telophase Cytokinesis

This figure represents the amount of time that each stage of the cell cycle takes Interphase G1 S Cytokinesis T A M G2 Mitosis P

Stage 1: Interphase (3 parts) During this stage, a cell grows, copies its chromosomes, and prepares to divide. G1: most of cell’s life is spent here in growth and development. S (synthesis): Chromosomes (in the form of chromatin) are copied in the nucleus = DNA replication, centrioles are also copied. G2: Cell continues to grow, copies cell organelles, prepares to divide. This part of cell cycle lasts the longest—cell spends the majority of its life here!

This figure represents the amount of time that each stage of the cell cycle takes Interphase G1 S Cytokinesis T A M G2 Mitosis P

There is also an M phase checkpoint in mitosis There are checkpoints to control the rate that cells go through the cell cycle. 2 of the 3 checkpoints occur during interphase: G1 checkpoint: most important checkpoint; cells are stopped during G1, and must receive chemical signals in order to continue on to the S phase. If the cell does not receive a go-ahead signal, it will exit the cell cycle and go to G0, a non-dividing state (mature nerve cells and muscle cells typically do this!). G2 checkpoint: makes sure that DNA has been replicated correctly during the S phase before allowing the cell to proceed to mitosis. There is also an M phase checkpoint in mitosis

Cyclins and Cdks Protein kinases are enzymes that activate or inactivate proteins by phosphorylating them Kinases that control the cell cycle must be activated by cyclin, a protein = cyclin-dependent kinases (Cdks) Cyclin + Cdk = MPF complex (mitosis or maturation-promoting factors) Cyclin concentration rises during S and G2 creating a lot of MPF

MPF phosphorylates proteins, initiating mitosis Phosphorylation on the nuclear membrane = breakdown of membrane May help to signal chromosomes to condense MPF is switched off in anaphase and the attached cyclin breaks down, inactivating the Cdk Checkpoint Animation play partial animation (G1 and G2 checkpoints only)

This figure represents the area of each checkpoint G1 Checkpoint Interphase G1 S Cytokinesis T A M G2 Mitosis P G2 Checkpoint

Checkpoint Animation play partial animation – 2nd half Stage 2: Mitosis During this stage, a cell’s nucleus divides into 2 new nuclei (4 phases in mitosis). Mitosis checkpoint: makes sure a complete set of chromosomes will go into each daughter cell by checking if the spindle fibers are correctly attached to the chromosomes. Checkpoint Animation play partial animation – 2nd half (M checkpoint only)

This figure represents the area of each checkpoint Interphase G1 Checkpoint G1 S Cytokinesis Cytokinesis T M Checkpoint A M G2 Mitosis P G2 Checkpoint

Cells Alive Animation… http://www.cellsalive.com/cell_cycle.htm

Nuclear membrane starts to break down. 1. Prophase in Animals: Chromatin condenses into chromosomes (with sister chromatids attached by centromeres). Nuclear membrane starts to break down. Centrosomes (containing 2 centrioles in animals) begin to separate and move to opposite ends of the cell and spindle fibers form from the centrosomes. Animal Cells

Plant Cell Prophase (contain centrosomes but no centrioles)

Centrioles v. Centrosomes Centrosomes are in all eukaryotic cells and aid in spindle formation Cells have 1 centrosome but it is copied before mitosis Centrioles are only in animal cells (and a few types of fungi) Made of microtubules Not required for spindle formation – not sure of their purpose, thought to help with the spatial arrangement of the cell by influencing the placement of the nucleus. centrosome centrioles

Prometaphase Condensed chromosomes are starting to organize themselves in the middle but all are not lined up yet

Centrosomes/centrioles are at opposite ends of cell. 2. Metaphase: Centrosomes/centrioles are at opposite ends of cell. Sister chromatids line up in center of cell on the metaphase plate and attach to spindle fibers at their centromeres. Animal Cells

Plant Cell Metaphase (spindle fibers come from centrosomes)

Chromatids are pulled apart at the centromere. 3. Anaphase: Chromatids are pulled apart at the centromere. Spindle fibers pull chromatids toward opposite ends of cell (toward centrosomes/centrioles). Animal Cells

Plant Cell Anaphase (chromatids move toward centrosomes)

4. Telophase: Chromosomes reach opposite ends of the cell New nuclear membranes form around chromosomes at each end of cell. The chromosomes uncoil into chromatin Cleavage furrow begins to form Animal Cells

Plant Cell Telophase Cell plate begins to form in plant cells only Early Telophase Late Telophase

Stage 3: Cytokinesis During this stage, the cytoplasm completely divides to make 2 new identical cells. The cell’s organelles are divided between the 2 new cells. In animal cells, the cleavage furrow completely pinches in to make 2 cells. In plant cells, the cell plate becomes the cell wall and completely separates the 2 cells. At the end of cytokinesis, 2 new identical daughter cells are formed!

Mitosis animation

This figure represents the amount of time that each stage of the cell cycle takes as well the various checkpoints. Interphase G1 Checkpoint G1 S Cytokinesis C T A M G2 Mitosis P M Checkpoint G2 Checkpoint

Cells Alive Animation http://www.cellsalive.com/mitosis.htm