Cytokinesis, mediated by a contractile ring (green) divides the cytoplasm. Loss of cohesion, which marks the onset of anaphase, allows the chromatids to be pulled to opposite sides of the spindle. Metaphase is defined as the point when all the chromosomes are correctly aligned at the spindle equator. Chromosomes attach to microtubules via their kinetochores (red circles), with sister kinetochores attaching to opposite spindle poles. In mitosis, the chromosomes condense, the nuclear envelope (purple) breaks down and microtubules (grey) assemble a bipolar spindle apparatus. Throughout S phase and G2, cohesion keeps the replicated sister chromatids together. The microtubule organising centre (grey circle) also duplicates during S phase. DNA replication in S phase initiates at multiple origins and must be completed before progression into mitosis. In G1, each chromosome is represented by a single chromatid. Somatic cell cycles are subject to several quality control mechanisms, known as cell cycle checkpoints (purple arrow heads). (B) Eukaryotes have segmented genomes represented by sets of linear chromosomes (light and dark blue). Under optimal growth conditions, the next round of DNA replication can re-initiate before the completion of cell division. When DNA replication is complete, the cytokinetic ring (green) divides the cell. (A) Archaea and bacteria typically have a single circular chromosome (light blue) whose replication and segregation are coupled once DNA replication initiates at the single origin (orange circle), the two genomes migrate to opposite ends of the cell while the remainder of the chromosome is duplicated.
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