Interphase is collectively the gap 1 (G1), synthesis (S), and gap 2 (G2) phases of mitosis, in which a cell grows, replicates its DNA, and grows again. Most cells spend nearly all of their time in this part of cell division, growing in size and carrying out the normal functions of the cell. The length of this particular phase is the most variable among different cell types. When the cell receives signals for division, it moves to the S phase.
During the S phase, the replication of the cell's genetic material, DNA, occurs. In eukaryotic cells, DNA is found in structures called chromosomes. Before replication, these chromosomes exist as long, thin chromatin fibers. Each chromosome is condensed into a structure of two sister chromatids during prophase of mitosis. A chromatid is one of two identical halves of a replicated chromosome. Sister chromatids are identical copies of DNA that remain connected until they are separated during mitosis. A centromere, the point on a chromosome that attaches to the spindle fibers with a kinetochore during cell division, attaches the sister chromatids. The centromere is aided in binding sister chromatids together by several proteins called cohesins and condensins. Once the DNA has been replicated, the cell moves to the second gap phase.During the G2 phase, a structure in the cytoplasm of animal cells that coordinates the formation of microtubules, called a centrosome, allows cell division to proceed during reproduction. The centrosome will organize a complex structure of microtubules, the mitotic spindle, involved in mitosis. Other cellular structures are duplicated during G2, such that each replicated daughter cell produced during mitosis will have all necessary organelles (such as mitochondria, endoplasmic reticulum, etc.). For most cells the G2 phase is relatively short; once complete, the cell is ready to divide. Interphase takes most of the time in the cell cycle, comprising more than 95% of the duration of the cell cycle in most eukaryotes.
Mitosis is a form of nuclear division in which replicated chromosomes are carefully organized and separated in preparation for cytoplasmic division. This process results in two cells that are genetic clones of the parent cell. Mitosis occurs in somatic (nonreproductive) cells that are growing or repairing a multicellular organism. Mitosis begins after the G2 phase. Mitosis consists of five distinct steps, followed by cytokinesis, which is the pinching off of the cytoplasm to form two new cells at the end of mitosis.During mitosis, the genetic material that has been replicated during the synthesis phase is affected by the actions of two types of proteins called cohesins and condensins. Prior to mitosis, the DNA is found spread out inside the nucleus. A cohesin protein helps bind sister chromatids together at the centromere until they separate during anaphase. A condensin is a protein that helps condense DNA into chromosomes during prophase of mitosis. Condensin reorganizes chromosomes when they get compacted during prophase. This reduces the amount of space the DNA takes up. Both of these proteins have been shown to regulate gene expression by controlling the entire chromosome or a single gene. They have also been shown to play roles in DNA repair and in organizing the centromere so that the spindle fibers properly attach during mitosis.