<P> Before a cell can enter cell division, it needs to take in nutrients . All of the preparations are done during interphase . Interphase is a series of changes that takes place in a newly formed cell and its nucleus, before it becomes capable of division again . It is also called preparatory phase or intermitosis . Previously it was called resting stage because there is no apparent activity related to cell division. Typically interphase lasts for at least 90% of the total time required for the cell cycle . </P> <P> Interphase proceeds in three stages, G, S, and G, followed by the cycle of mitosis and cytokinesis . The cell's nuclear DNA contents are duplicated during S phase . </P> <P> The first phase within interphase, from the end of the previous M phase until the beginning of DNA synthesis, is called G (G indicating gap). It is also called the growth phase . During this phase, the biosynthetic activities of the cell, which are considerably slowed down during M phase, resume at a high rate . The duration of G is highly variable, even among different cells of the same species . In this phase, the cell increases its supply of proteins, increases the number of organelles (such as mitochondria, ribosomes), and grows in size . In G phase, a cell has three options . (1) To continue cell cycle and enter S phase (2) Stop cell cycle and enter G phase for undergoing differentiation. (3) Get arrested in G phase hence it may enter G phase or re-enter cell cycle . The deciding factor is availability of nitrogens and storage of energy rich compunds at the deciding point called check point . This check point is called G cyclin or C. It causes transition of G to S phase . Once the check point of G phase is crossed, cell cycle will go uninterrupted till it is completed . </P> <P> The ensuing S phase starts when DNA synthesis commences; when it is complete, all of the chromosomes have been replicated, i.e., each chromosome has two (sister) chromatids . Thus, during this phase, the amount of DNA in the cell has effectively doubled, though the ploidy of the cell remains the same . Rates of RNA transcription and protein synthesis are very low during this phase . An exception to this is histone production, most of which occurs during the S phase . </P>

When does the mitochondria replicate in the cell cycle
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