Mitosis Meiosis

Mitosismitosis-meiosis
– Parent cell divides into two identical diploid daughter cell. Interphase- cell spends most of its time in this phase. G1– cell grows to full adult size. G0– state of quiescence or regular functioning of cell; some cells are differentiating as well while others are terminally differentiated such as muscle and nerve cells. S (synthesis)- DNA replication occurs and centrosomes duplication. G2– Final preparation for mitosis- site of DNA repair, accumulation of ATP to use for mitosis

Mitosis– prophase, metaphase, anaphase, telophase, cytokinesis
Prophase– Nucleolus disappears, chromosomes condense, nuclear envelope becomes phosphorylated and breaks up, centrosomes begin moving to opposite poles. Centrosome– usually one per cell hangs around nucleus until cell division when it duplicates. Made of 2 perpendicular sets of 9 triplet microtubules. Known as microtuble-organzing centers (MTOCs). Spindle fibers begin to form and attach to kinetochore of centromere and to centrosomes. Centromere is connection where chromatids of chromosome connect, kinetochore is the center of the centromere
Metaphase- Chromosomes attached to microtubules align at equatorial plate of cell (the middle)
Anaphase- sister chromatids separate and move to opposite poles
Telophase- Now called chromosomes, reach end of pole and start to decondense, nuclear envelope begins to develop around each set of daughter chromosomes
Cytokinesis- End of telophase- actin and myosin act as contractile belt and cleave two portions of cytoplasm to form two identical daughter cells
Mitosis Checkpoints- highly conserved proteins that regulate the cell cycle progression
Cyclin-dependent kinases (Cdk)- phosphorylate serine and threonine on other proteins and bind to cyclins in the cell. This forms Cdk-cyclin complexes- need enough present to continue through checkpoints. Cdk inhibiting proteins can bind to complex and inhibit activity. This will prevent the cell from moving to the next stage of cell replication. The checkpoints are important because a cell should not continue through mitosis if DNA has not finished replicating for example. Also, if there are enough cells in a particular tissue, then excess cells in the area might cause damage because of spatial issue so a cell will not pass through checkpoints. G1 Checkpoint- Longest stage of checkpoints: ensures cell has grown to the proper size and has enough energy to pass through to the S phase.  This is the checkpoint G0 cells must also pass through to return to cell replication. p53– tumor suppressor protein accumulates when DNA is damaged- can stop the cell during G1 and stimulate apoptosis rather than proceeding to S phase and duplicating damaged DNA . Common protein that gets turned off in many cancers causing cells to continue on through cell replication when they should have been stopped. . G2 Checkpoint- Before entering mitosis- MPF (maturating promoting factor) must be present in high enough concentrations to continue. Last checkpoint before mitosis begins- ensure replicated DNA is ready for mitosis

Metaphase Checkpoint- Makes sure cell degrades cyclin complexes and is stable enough to complete mitosis and become two cells. Cyclin degradation inactivates MPF and G1 CDK-cycling complexes- thus preventing further mitosis for the time being

Meiosis
Cell division resulting in four haploid cells with 23 chromosomes each. Goes through two stages of division- similar to mitosis. Prophase I- same as mitosis, however homologous chromosomes (one father, one mother) pair up during synapsis- form tetrad- 4 chromatid pieces .Crossing over occurs! Creating new genetic material for the offspring. Spermatogenesis can remain in this phase for 3 weeks while during Oogenesis, eggs remain in this phase until ovulation, so up to 50+ years. Metaphase, anaphase and telophase I all occur as in mitosis. Note- homologous chromosomes separate but are composed of two chromatids still. No interphase, so no S phase for DNA replication. Goes through prophase, ana, meta and telophase II as in mitosis- but chromatids are separated and are now own chromosomes.  Forms 4 haploid cells with a unique genetic combination.

Nondisjunction Failure of homologous chromosomes to separate in first meiotic division of anaphase I or sister chromatids not separating during anaphase II. Causes extra copy or portion of chromosome to remain in a cell. One daughter cell might get all while another gets none.