<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 but may continue till G in case of heterochromatin . </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 compounds 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> <P> G2 phase occurs after DNA replication and is a period of protein synthesis and rapid cell growth to prepare the cell for mitosis . During this phase microtubules begin to reorganize to form a spindle . </P>

When does rna synthesis occur in the cell cycle