Intermitotic phases

Distinct intermitotic phases are found in eukaryote cells but are not so obvious in prokaryote cells (Lark, 1963) and, especially in the light of the phenomena just discussed, may be considered a prerequisite for cell differentiation. Those organisms described above which do not possess a G1 phase in their division cycles (Bostocket al., 1966 Guttes et al., 1967 Ord, 1968) do not show an elaborate sequence of cell specialization. The presence of the G1 reflects a precise control over the timing of the initiation of nuclear DNA synthesis and suggests that some component of the initiation mechanism may be labile since initiation usually occurs only once in each division cycle. In differentiated tissues it appears the G1 phase provides a time in the division cycle where a cell can be held in a nondividing state in order to perform a specialized metabolic role. 

Azathioprine is a phase-specific drug that is toxic to cells during nucleic acid synthesis. Phase-specific drugs are toxic during a specific phase of the mitotic cycle, usually the S-phase, when DNA synthesis is occurring, as opposed to cycle-specific drugs that kill both cycling and intermitotic cells. 

Deoxyribonucleic acid synthesis is the principal fate of the deoxyribonucleotides, and cells in a nonproliferating state apparently do not contain appreciable pools of these compounds. DNA is made during a particular portion of the intermitotic period (the S phase of the cell cycle) and it appears that the free deoxyribonucleotide precursors of DNA are made mainly at this time and are incorporated into the polynucleotide with little accumulation. This, coupled with the fact that only a small proportion of cells in most animal tissues are undergoing division at any one time, accounts for the low concentration of deoxyribonucleotides in animal tissues. 

---