G2 Phase

G2 is the second gap phase, occurring after DNA replication is complete; the DNA content stays at the replicated 4C level throughout G2 and into mitosis, since no further copying occurs. Cell growth continues through protein and organelle synthesis, and organelles that have their own DNA, mitochondria and chloroplasts, divide during this window so that the eventual daughter cells inherit an adequate supply of each. A defining event of G2 is the synthesis of tubulin, the protein subunit that polymerises into microtubules; these microtubules go on to organise into the spindle fibres that will physically separate the chromosomes in mitosis, so G2 is effectively when the cell manufactures the hardware for division even before division visibly begins. G2 also produces a protein made only at this stage of the cycle, Maturation Promoting Factor (MPF), whose job is to trigger condensation of the loosely dispersed interphase chromatin into the tightly coiled mitotic chromosomes recognisable under the microscope; MPF activity is therefore the molecular switch that converts a G2 cell into a mitotic one. Like G …