Definition and meaning of Interphase

  An illustration of interphase in Gray's Anatomy. The chromatin has not yet condensed, and the cell is undergoing its normal functions.

  An image of the nucleus of a cell (HT1080) currently in interphase (probably G1). Note: Cytoplasm of this cell or the neighboring cell is not visible (top-left), which is currently in the telophase of mitosis. Image taken using an optical microscope and DAPI staining of DNA.

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^{[citation needed]}. 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.^[1]

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. Chromosomes are copied.

  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. Interphase has 3 parts: growth, DNA replication, and preparation for division.

  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 division.

• 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.^[2] Interphase includes G1, S, and G2 phases. Mitosis and cytokinesis however, are separate from interphase.

  Interphase within sequences of cellular processes

  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.^[3]

  Interphase and other cellular processes

In gamete production interphase is succeeded by meiosis. In programmed cell death, interphase is followed or preempted by apoptosis.

  References