Exceptions include differentiated nerve and skeletal muscle cells, which normally do not divide and remain arrested in a non-division stage called G0. Cell cycles become arrested in the G1 or G2 stages when certain chemical control signals are absent. These chemical control signals would normally stimulate the cell's passage through certain restriction points in each stage, thereby committing the cell to entering the next stage of the cell cycle.
In interphase, the nucleus is clearly visible as a membrane bound organelle containing one or more nucleoli, dense regions of the chromosomes where rRNA is synthesized. However, in their active state, when being transcribed by mRNA, individual chromosomes cannot be distinguished from each other. Instead, they appear as a tangled, granular material termed chromatin. During S stage replication, each chromosome becomes duplicated, possessing two identical chromatids held together at their centers by a centromere. The ends of each chromatid are called telomeres. Although the cell still contains the diploid number (2N) of chromosomes, the double-stranded chromosomes are ready for division into two complete copies of the genome. Two centrioles, each consisting of two small cylindrical bodies oriented at right angles to each other, are located outside the nucleus in an area referred to as the centrosome. During mitosis, the centrioles migrate to opposite poles of the dividing cell and the spindle, a system of microtubules that radiates in all directions
