The Cell Cycle, Mitosis and Binary Fission

During interphase a dividing eukaryotic cell grows and replicates its DNA. After replication, each chromosome consists of two sister chromatids joined at a centromere. Counting chromosomes by their centromeres means DNA content can double without chromosome number doubling at this stage.

Not every specialised cell retains the ability to divide. A cell-cycle description applies to cells that do enter the cycle; it is not evidence that all mature cells divide regularly. DNA replication is part of interphase, not a process that occurs for the first time during anaphase.

In prophase chromosomes condense and become visible, and the division apparatus develops. At metaphase chromosomes align at the equator with spindle fibres attached at their centromeric regions. At anaphase sister chromatids separate and move towards opposite poles; each separated chromatid is now a daughter chromosome.

At telophase chromosomes reach the poles, decondense and new nuclei form. Cytokinesis divides the cytoplasm, usually producing two daughter cells. In ordinary mitotic division the daughter nuclei retain the parental chromosome number and, apart from mutation, the same genetic information. Division requires control; dysregulated proliferation can contribute to tumours.

In bacterial binary fission the main circular DNA is replicated, copies move apart and the cytoplasm divides to form daughter cells. Plasmids also replicate but the number inherited by each cell can vary. There is no eukaryotic nucleus or mitotic spindle to divide.

Viruses do not undergo either mitosis or binary fission. Their genetic material directs host-dependent synthesis and assembly of viral components. RP2 uses stained root-tip cells to identify mitotic stages and calculate a mitotic index; observing more cells in a stage may indicate a longer stage only under suitable steady-population assumptions.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.