Interphase: Interphase generally lasts at least 12 to 24 hours in mammalian tissue. During this period, the cell is constantly synthesizing RNA, producing protein and growing in size. By studying molecular events in cells, scientists have determined that interphase can be divided into 4 steps: Gap 0 (G0), Gap 1 (G1), S (synthesis) phase, Gap 2 (G2).
Gap 0 (G0): There are times when a cell will leave the cycle and quit dividing. This may be a temporary resting period or more permanent. An example of the latter is a cell that has reached an end stage of development and will no longer divide (e.g. neuron).
Gap 1 (G1): Cells increase in size in Gap 1, produce RNA and synthesize protein. An important cell cycle control mechanism activated during this period (G1 Checkpoint) ensures that everything is ready for DNA synthesis. (Click on the Checkpoints animation, above.)
S Phase: To produce two similar daughter cells, the complete DNA instructions in the cell must be duplicated. DNA replication occurs during this S (synthesis) phase.
Gap 2 (G2): During the gap between DNA synthesis and mitosis, the cell will continue to grow and produce new proteins. At the end of this gap is another control checkpoint (G2 Checkpoint) to determine if the cell can now proceed to enter M (mitosis) and divide.
Mitosis or M Phase: Cell growth and protein production stop at this stage in the cell cycle. All of the cell's energy is focused on the complex and orderly division into two similar daughter cells. Mitosis is much shorter than interphase, lasting perhaps only one to two hours. As in both G1 and G2, there is a Checkpoint in the middle of mitosis (Metaphase Checkpoint) that ensures the cell is ready to complete cell division.
Pumping Mycocytes
We may not immediately think of our heart as a collection of individual cells. But it is the complex interaction of numerous cell types that give the heart its ability to pump blood. Some cells form heart connective tissue, other cells grow into heart valves. And muscle cells give the heart its ability to beat and pump blood throughout the body.
You can dissolve an embryonic heart into its individual cell types with trypsin, an enzyme that destroys the protein glue between the cells. Plate these cells in a dish and you will see some cells - called myocytes - that beat independently
A single cell beats when a complex series of gates - called ion channels open and close in an organized manner. Cell physiologists can measure how these ion channels work using a technique called the patch clamp.