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Cell Cycle Phases

o Every cell has a life cycle that begins with the birth of the cell and ends with the death or division of the cell.

Cell Cycle Diagram

Cell Cycle- Cell Cycle Phases, Cell Cycle Diagram, Cell Cycle Stage, and Cell Cycle Checkpoints 1

o M ⇒ Mitosis

o G1 ⇒ Gap 1

o G2 ⇒ Gap 2

o S ⇒ Synthesis

o G0 ⇒ Gap 0/Resting.

Quiescent/ Senescent
Gap 0 / G0

o The cell has left the cycle and has stopped dividing.

o In humans, liver cells spend a great deal of time in G0.

o Mature neurons and muscle cells are in G0 permanently.

o Liver (hepatic) and pancreatic cells are normally in G0 but can reenter normal division

Interphase

Gap / G1 Phase

o G1 is normally the longest phase.

o Cells increase in size in Gap 1, regions of heterochromatin has been unwound and decondensed.

o RNA synthesis and protein synthesis is very active.

o The G1 checkpoint control mechanism ensures that everything is ready for DNA synthesis.

o The main factor in triggering the beginning of S is cell size based upon the ratio of cytoplasm to DNA.

Cell Cycle Checkpoints

Cell Cycle- Cell Cycle Phases, Cell Cycle Diagram, Cell Cycle Stage, and Cell Cycle Checkpoints 2
Synthesis / S Phase

o DNA replication occurs during this phase.

o Organelles and proteins are produced more slowly.

o Each chromosome is exactly duplicated, but the cell is still considered to have the same number of chromosomes, only now, each chromosome is made of two identical sister chromatids.

Gap 2 / G2 Phase

o During the gap between DNA synthesis and mitosis, the cell will continue to grow.

o Cellular organelles continue to duplicate.

o RNA and protein (especially tubulin for microtubules) are actively synthesized.

o The G2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis/meiosis) phase and divide.

o It checks for mitosis promoting factor (MPF), when the level is high enough, mitosis is triggered.

Cell Division / Mitosis / Meiosis / M Phase

o Cell growth stops at this stage and cellular energy is focused on the orderly division into two daughter cells.

o A checkpoint in the middle of mitosis (Metaphase Checkpoint) ensures that the cell is ready to complete cell division.

Cell Cycle Citations

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