The Cell Cycle

There are three stages of interphase (non-dividing phase of life); G1, S, G2.  G1 is classified as the growth phase, S is when chromosomes are replicated, and G2 is when the cell prepares to divide.  

After interphase is mitosis which is when the nucleus divides into 2. Prophase is the first stage of mitosis and during this time chromatin condenses into chromosomes, the nuclear membrane disappears, and miotic spindle fibers forms. Microtubules connect to each kinetochore on the chromosomes.  During metaphase the chromosomes line up on the equator of the cell (metaphase plate, between the two poles). During anaphase the sister chromatids are pulled apart (now each is an individual chromosome) and one of each type chromosomes moves to each pole. Telophase is when the nuclear membrane reforms. 

Cytokinesis is the last part of cell division and is when the cytoplasm splits and two cells are formed.  In animal cells this occurs by a cleavage furrow and in plants the golgi bodies bring material to form a cell plate in the middle of the two new cells and this will become cell membrane for the new cells. A cell wall will form between the cell membranes.

There are 3 checkpoints in the cell cycle that assess whether conditions are favorable to continue with the cycle.  The first of these is the G1 checkpoint which occurs near the end of G1. At this checkpoint DNA is assessed and if it is damaged the cell tries to fix it; if the cell can not fix the damage the cell will die. Some cells stay in a G0 phase where they do not divide and will remain at this point (e.g. nerve cells). At the end of G2 is the G2 checkpoint.  This assesses whether the DNA was replicated correctly; if it was not it tries to fix it, if it can not be fixed the cell dies. The M checkpoint is during metaphase and makes sure the microtubules were attached correctly to the kinetochores before entering anaphase. 

Cyclin-dependent kinases are enzymes that allow the cell to continue through the checkpoints.  Cyclin (protein) attaches to the kinase and causes phosphorylation which unblocks its active site and continues activity.

There are many growth factors (enzymes/hormones) that can cause a cell to divide.  Density-dependent inhibition causes cells to stop dividing when they become too close to one another (density is maxed).  Anchorage dependence is when cells divide only if they are attached to an external surface.