Mitosis

Objective: Trace chromosome movement without confusing replication with separation.

Mitosis divides a eukaryotic nucleus so that each daughter nucleus normally receives one copy of each chromosome present in the starting nucleus before S phase. Chromosomes condense in prophase; nuclear-envelope breakdown and spindle attachment occur during prometaphase in many descriptions. At metaphase, individual replicated chromosomes align; at anaphase, sister chromatids separate; at telophase, nuclei reform and chromosomes begin to decondense. DNA replication occurs earlier in S phase, not during mitosis. Stage names describe a continuous process, so microscopy snapshots may show transitional features.

Objective: Explain how attachment and movement support accurate division.

The mitotic spindle is a dynamic structure made mainly of microtubules and associated proteins. Spindle fibers attach to kinetochores at centromere regions of chromosomes; correct attachments orient sisters toward opposite poles. Changes in microtubules and motor-protein actions help separate chromatids during anaphase. The spindle assembly checkpoint monitors unattached or improperly attached chromosomes before separation. Errors in segregation can lead to cells with abnormal chromosome numbers, but the outcome depends on the error and cell context. A spindle diagram often simplifies the three-dimensional, changing structure.

Objective: Distinguish nuclear division from physical cell division and qualify genetic similarity.

Cytokinesis partitions the cytoplasm, typically after or overlapping late mitosis. Animal cells often form a contractile ring that pinches the membrane inward; many plant cells assemble a cell plate that develops into a new partition. Mitosis followed by cytokinesis commonly yields two cells with the same chromosome set as the parent cell, but they are not guaranteed to be genetically identical because replication errors, mutations, or unequal chromosome distribution can occur. The daughter cells can later differentiate and show different patterns of gene expression. Some cells undergo nuclear division without immediate complete cytokinesis.