Almost all cells can reproduce by binary fission. This requires, growth, synthesis of new DNA, and organized division of cell contents. This occurs in both prokarytes (above) and eukaryotes, but in eukaryotes the cell cycle includes a highly orhanized process called mitosis. We’ll talk about gamete production and meiosis in a later class.
Two sister chromatids are connected at the centromere after DNA synthesis. Previously, each chromosome had only a single DNA molecule. Each chromatid has a single kinetichore which serves as a point of attachment for microtubules. This leads to separation of the chromatids.
In animals each centrosome has 2 centrioles. During the transition to prophase, nucleoli disappear and microtubule arrays (asters) form around centrosomes, which move apart. Chromosomes begin to CONDENSE and become visible. Key point: Enzymes can’t copy condensed DNA to make mRNA or new DNA. Condensation stops EVERYTHING except the ongoing process.
Nucleoli have disappeared and asters continue to develop and move to opposite poles. Chromosomes further CONDENSE. Spindle begins to develop. In prometaphase kinetechore and aster microtubles interact, moving chromosomes back and forth.
Metaphase is the longest stage of mitosis, lasting about 20 minutes. Spindle is fully formed, with microtubules attached to kinetichores of sister chromatids attached to opposite poles. During Anaphase, microtubules pull the pairs apart so that one of each pair moves towards each pole.
With the completion of telophase. Mitosis is complete and is immediately followed by the completion of cytokinesis, which begins in late telophase. In animals, a cleavage furrow pinches the cell in two.
In most eukaryotes, the centrioles are outside the nucleus and spindle formation is extranuclear, requiring the breakdown of the nuclear envelope. In organisms such as diatoms, the spindle forms with the nucleus and the envelope doesn’t break down, but divides.