The cell cycle is the process of cell replication where two daughter cells are created from one parent cell. The cell cycle is made up of interphase & mitosis. Mitosis is the phase of actual cell splitting (duplication), while Interphase is made up of the G₀, G₁, S, and G₂ phases which prepare the cell for mitosis. The S phase involves “Synthesizing” new DNA and doubling the amount of DNA present so there is enough for both daughter cells. G stands for “Gap” as these are the gaps in time between mitosis and the S Phase. Cells in G₀ are no longer in the cell cycle. Cells in G₀ are “resting” & no longer actively going through the phases of the cell cycle. Some tissue can easily transition between G₀ & G₁ with the right signals while other cell types are “stuck” in G₀.

Anti-metabolites chemotherapy drugs like 5-flourouracil & sulfa drugs (antibiotic) inhibit DNA synthesis and stall the S phase of the cell cycle. Chemotherapy Paclitaxel inhibits microtubule action involved in Mitosis. Therefore, cells being treated with this medication are stuck in M phase.

Permanent tissues are “stuck” in the G₀ phase and cannot undergo further mitosis. Permanent tissues include muscle (cardiac & skeletal) and the CNS. These tissues can only undergo hypertrophy (not hyperplasia) to deal with increased stress and form fibrous nonfunctioning scars during the healing process.

Stable tissues are those that primarily reside in the G₀ phase, but given the right signals can reenter the cell cycle. For example, lymphocytes are most often in a resting period, but during infection they are triggered to divide. Liver cells are another example of a stable tissue that can undergo replication following injury to the organ to facilitate healing.

Labile Tissues are cells which are constantly replicating with no time in the G₀ phase. These cells act as the stem cells for tissues such as the bone marrow and skin.

Picture Used:

Derivative of “Major events in mitosis” by NIH available at http://commons.wikimedia.org/wiki/File:Major_events_in_mitosis.svg by Public Domain

Now that you have finished this video you should check out the next video in the Cell Injury, Cell Death & Cancer sections which covers Oncogenes & Tumor Suppressors.