Interphase



Interphase is the phase of the cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for cell division. In preparation for cell division, it increases its size and makes a copy of its DNA. Interphase is also considered to be the 'living' phase of the cell, in which the cell obtains nutrients, grows, reads its DNA, and conducts other "normal" cell functions. The majority of eukaryotic cells spend most of their time in interphase. Interphase does not describe a cell that is merely resting but is rather an active preparation for cell division. A common misconception is that interphase is the first stage of mitosis. However, since mitosis is the division of the nucleus, prophase is actually the first stage.

In interphase, the cell gets itself ready for mitosis or meiosis. Somatic cells, or normal diploid cells of the body, go through mitosis in order to reproduce themselves through cell division, whereas diploid germ cells (i.e., primary spermatocytes and primary oocytes) go through meiosis in order to create haploid gametes (i.e., sperm and ova) for the purpose of sexual reproduction.

Identification
Under a microscope, interphase can be recognized because the nuclear membrane is still intact, the chromatin has not yet condensed, and chromosomes are not visible, though the nucleolus may be visible as an enlarged dark spot. The centrioles and spindle fibers are also not yet visible, though the centrosome, which contains and organizes them, may be visible near the nucleus.

Stages of interphase
There are three stages of interphase, with each phase ending when a cellular checkpoint checks the accuracy of the stage's completion before proceeding to the next. The stages of interphase are:


 * G1 (Gap 1), in which the cell grows and functions normally. During this time, much protein synthesis occurs and the cell grows (to about double its original size) - more organelles are produced, increasing the volume of the cytoplasm. If the cell is not to divide again, it will remain in this phase.


 * Synthesis (S), in which the cell duplicates its DNA (via semiconservative replication). This is also known as the Swanson phase.


 * G2 (Gap 2), in which the cell resumes its growth in preparation for mitosis.


 * In addition, some cells that do not divide often or ever, enter a stage called G0 (Gap zero), which is either a stage separate from interphase or an extended  G1 phase, which follows the restriction point, a cell cycle checkpoint found at the end of G1.

The duration of time spent in interphase and in each stage of interphase is variable and depends on both the type of cell and the species of organism it belongs to. Most cells of adult mammals spend about 20 hours in interphase, this accounts for about 90% of the total time involved in cell division.

Interphase and the cell cycle
When G2 is completed, the cell enters a relatively brief period of nuclear and cellular division, composed of mitosis and cytokinesis, respectively. After the successful completion of mitosis and cytokinesis, both resulting daughter cells re-enter G1 of interphase.

In the cell cycle, interphase is proceeded by telophase and cytokinesis of the M phase. In alternative fashion, interphase is sometimes interrupted by G0 phase, which, in some circumstances, may then end and be followed by the remaining stages of interphase. After the successful completion of the G2 checkpoint, the final checkpoint in interphase, the cell proceeds to prophase, or in plants to preprophase, which is the first stage of mitosis.

G0 phase is viewed as either an extended G1 phase where the cell is neither dividing nor preparing to divide and or as a distinct quiescent stage which occurs outside of the cell cycle.

Interphase and other cellular processes
In gamete production interphase is succeeded by meiosis. In programmed cell death, interphase is followed or preempted by apoptosis.