Figure 2.

Replication timing of the human chromosomal bands. Upper panel. Scheme of the cell cycle with the S phase and its different temporal sections highlighted. S phase is the period when the DNA is replicated and could be divided into two parts taking into consideration the block of the cell cycle that happens when the cells grow in a medium with a high concentration of bromodeoxyuridine (BrdU). In this way, the early S phase corresponds to the period in which DNA from the GC-rich pale G-bands is replicated, and the late S-phase is the period in which the GC-poor DNA from the dark G-bands is replicated. Moreover, the gene-richest bands are replicated at the beginning of the S phase (very early replicated DNA), and the gene-poorest bands at the end (very late replicated DNA). The remaining bands, endowed with an intermediate gene density, are replicated in the middle part of the S phase, first the G-pale bands and then the G-dark bands. Bottom panel. Human chromosome ideograms with G-bands showing the GC-richest and the GC-poorest bands [35,36] highlighted by red and blue colors, respectively. Note that the GC-richest bands (red colored) and the GC-poorest bands (blue colored), namely the two band sub-sets replicated in two very different subperiod of the S phase, are generally not adjacent. The other bands, with intermediate compositional properties, are replicated in the central part of the S phase.